REGIONAL STUDY FOR COASTLINE MONITORING AND DRAWING UP A DEVELOPMENT
SCHEME FOR THE
WEST AFRICAN COASTAL AREA
REGIONAL COASTAL RISK PREVENTION PLAN IN WEST AFRICA
REGIONAL PRE DIAGNOSTIC STUDY
INTERMEDIATE DOCUMENT NO. 2
FEBRUARY 2010
This report is an intermediate working document which should be completed by
contributions from the national teams.
The purpose of this document is to fix general framework items to be able to
apprehend the problem issues related to coastal erosion in West Africa and to place
in context the contributions expected from national consultants.
The next, full version will also be more succinct, as the more detailed, technical items
will be placed in an annex.
The regional study for shoreline monitoring and drawing up a development scheme for the West
African coastal area was launched by UEMOA as part of the regional programme to combat coastal
erosion (PRLEC – UEMOA), the subject of Regulation 02/2007/CM/UEMOA, adopted on 6 April
2007. This decision also follows on from the recommendations from the Conference of Ministers in
charge of the Environment dated 11 April 1997, in Cotonou. The meeting of Ministers in charge of
the environment, held on 25 January 2007, in Cotonou (Benin), approved this regional coastal
erosion programme in its conclusions. This study is implemented by the International Union for the
Conservation of Nature (UICN) as part of the remit of IUCN’s Marine and Coastal Programme (MACO)
for Central and Western Africa, the coordination of which is based in Nouakchott and which is
developed as a thematic component of IUCN’s Programme for Central and Western Africa (PACO),
coordinated from Ouagadougou. UEMOA is the contracting owner of the study, in this instance
through PRLEC – UEMOA coordination of the UEMOA Commission. The work is carried out under the
supervision of:
 The PRLEC-UEMOA Regional Steering Committee set up to improve the orientation of the
different projects and oversee their diligent and efficient execution. This is presided over by
the State, which holds the presidency of the Council of Ministers of UEMOA.
 The PRLEC- UEMOA Regional Scientific Committee, established with a view to assisting the
UEMOA Commission in validating the technical and scientific contents of projects initiated
within the framework of the implementation of PRLEC. This committee also expresses a
technical and scientific opinion on all the reports drawn up within the framework of the
implementation of this programme.
This regional pre-diagnostic was written on completion of phase II of the study launched by UEMOA
and implemented by IUCN. This is an initial document, which will be completed with:
 the results of the national diagnostic studies currently in progress.
 additional elements that will be gathered at regional level.
 the results expected from the case studies.
This pre-diagnostic is accompanied by a provisional cartographic analysis of the geodynamic of the
coast at a scale of 1:250,000 and a typology of the coastal facies of West Africa represented on the
1:250,000 provisional map by segments. This document should also be completed and validated in
the subsequent phases. This 1:250,000 cartographic analysis is first and foremost a working tool for
modelling and structuring the representation of the coastal fronts. It should serve as a basis for an
analysis of the development scheme at a scale of 1:500000, which will cross-reference the observed
vulnerability of the coasts with the coastal issues at stake as observed.
THIS DOCUMENT IS PROVISIONAL AND SHOULD THEREFORE BE COMPLETED AND ENHANCED WITH
DATA ORIGINATING FROM THE NATIONAL DIAGNOSTIC STUDIES. THE FINAL, COMPLETED
DIAGNOSTIC STUDY WILL SERVE AS THE BASIS FOR FORMULATING THE COASTLINE MONITORING
PROGRAMME AND THE RECOMMENDATIONS FOR COMBATING COASTAL EROSION THROUGH THE
REGIONAL DEVELOPMENT SCHEME.
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SUMMARY
“… Several countries in the subregion with coastal zones have already raised the alarm about the adverse effects
saltwater intrusion and storm surges could have on infrastructure and coastal ecosystems if there is a rise in sea
level. The loss of land at an elevation of 0.5 to 1 m above sea level would mainly affect the most useful areas,
agricultural and/or the most populated areas. Generally, the largest cities, or those expanding the most rapidly,
are located in low elevation lagoonal settings that are particularly exposed to the effects of a rise in sea level. Note
that in certain cases the rise in sea level would, in the long term, flood rice fields, land and infrastructure causing
the displacement of coastal populations; small streams, rivers and irrigation channels would be submerged;
mangroves would be destroyed or displaced and there would be adverse effects on oyster farming…"
International Conference for the reduction of the vulnerability of natural social and economic systems
in West Africa faced with climate change. August 2006.
The recent dramatic events that occurred on the Atlantic Coast in France remind us that the coast is a dynamic,
mobile, living interface and that human facilities in this context should be governed by pragmatic risk
management. Areas flooded following a storm surge that exceeded the height of the diking had initially been
“dyked” for exclusively agricultural purposes.
As time passed, the risk was forgotten, and these areas were gradually urbanised. The rise in the price of real
estate was an economic factor, which, against a background of increasing demand on the coast for housing and
leisure activities, overrode a realistic analysis of the limits of these developments, such as the first people to
develop a man-made habitat on this coastline had.
This event also reminds us that storm waters, however rare, are dangerous phenomena of great amplitude. The
first results of the national diagnostic studies tell us that in West Africa such storm surges have been responsible
for serious losses, of human life, goods and assets, which, even if they do not receive the same media coverage,
are all the more serious since they concern whole vulnerable populations living temporarily or permanently in
these coastal zones.
The very notions of vigilance and early warning should be investigated, when a weather forecast turns out to be
right, and all the other concurrent factors are known and that finally the risk analysis systems implemented do not
have the capacity to generate a clear signal for the populations and local decision-makers… The question here is
no longer solely technical or scientific but is directly related to risk management.
CHARACTERISATION OF COASTAL SYSTEMS
This regional pre-diagnostic study allows us to propose a regional characterisation of the coastal zones
considered by the study. This characterisation is based on the following elements:
A COAST THAT IS ESSENTIALLY COMPOSED OF SOFT FORMATIONS
Exposed hard rock is rare and most of the coastline comprises soft, sandy sediment deposits from the
Quaternary Period. The principal rock outcrops not prone to erosion are rare and are a structural part
of the coastal region as a whole:




Basalt on the Cape Verde Peninsula
Rock outcrops at Cap Verga and the Conakry peninsula
Freetown Breakwater
Relict of sandstone or hardpan spared by erosion (sandstone on the Senegalese Petite Côte, the
Bijagos and around the periphery of Accra)
 Granites and metamorphic rocks present on all of Liberia, Western Côte d'Ivoire and the
central part of the coast of Ghana.
The landform is, on the whole, not very rugged. The continental shelf is narrow in the main, around
thirty kilometres on average, except from Guinea Bissau to the Sherbro islands in Sierra Leone, where
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it widens considerably to 200 km. This continental shelf is marked by some deep features: the Khayar
canyons in Senegal to the North of the Cape Verde Peninsula, deep zones in Sierra Leone to the South
of Freetown, and the deep canyon (“Bottomless pit”) that cuts through the shelf to the right of Abidjan
in C‚te d'Ivoire.
5
Indication of the localisation of the principal lithological units of the coastal region of West Africa
6
RELATIVELY LIMITED SEDIMENTATION CAUSED BY WIND AND RIVERS
The narrowness of the continental shelf and the few remarkable features (in particular, “Bottomless
hole”) probably play a role in intercepting and trapping sediment flux.
The zone from the North of Mauritania to the Cape Verde peninsula is served by mainly Aeolian
sediment supply that is redistributed along the coast by the Canary current, combined with a NorthSouth regular coastal drift roughly parallel to the coast.
River sediment supply corresponds to the network of major coastal rivers most of which have dams
built on them which are responsible for part of the continental sediment interception. The extension of
the catchment areas in correspondence with the orographic and geological characteristics lead to the
differentiating of:
 The major river basins, in particular transboundary, composed of zones, in particular
Sudanian, with moderate rainfall.
 The small river basins, in a topography dissected by peneplain on Precambrian formations
covered by a thick mantle of alterite, located in the zone with the highest rainfall (particularly
in Liberia).
In the mangrove and/or lagoon areas, these sediment inputs are largely trapped in the river mouths,
either by sand-mud formations and the roots of the mangroves, or in the coastal lagoons. These
accumulations of sediment are remobilised and put back into circulation in the marine environment
seasonally by flood peaks, or continuously by tidal currents where these are significant.
A STEEP RAINFALL GRADIENT
Rainfall varies from less than 100 mm in the Saharan areas of Mauritania to more than 4,000 mm on
the coast of Guinea, Sierra Leone and Liberia. The climate of West Africa is primarily modulated by
the seasonal movements of the intertropical convergence zone, which separates the continental mass of
hot, dry air with North-Easterly winds (Harmattan) and the mass of moist sea air originating from the
Gulf of Guinea (monsoon).
TWO PRINCIPAL CURRENTS
Coastal currents and water-column density centre around two main currents:
 The Canary current system, which is subject to seasonal variations, but generates a NorthSouth coastal drift parallel to the coast from Mauritania to the south of Senegal (West front).
 The Guinea current system, from Guinea to Nigeria and beyond, globally parallel to the coast,
which generates a West-East flow (South front).
The intermediate zone which is subjected to the direct or indirect influence of these two currents is
largely turbulent. It extends from Guinea Bissau to the Sherbro Islands in Sierra Leone. In this last
zone, there are strong tidal currents, corresponding to the high tidal amplitude (in the order of 5 m
for spring tide), which is itself related to the notable widening of the continental shelf in this zone,
and the presence of numerous small islands, in particular around the Bijagos archipelago in
Guinea Bissau.
The tidal currents remain globally moderate across the remainder of the coast, with spring tides
varying from 1m to 1m40.
The propagation of ocean waves affects the whole of the two major sea fronts along this coastline,
West and South, with an orientation that is generally oblique in relation to the coast.
Note that in the front between Mauritania and Guinea Bissau the orientation of the ocean waves is not
constant all year round and varies particularly in June-July and August, with more marked
meteorological events from August to December. In the Gulf of Guinea, the orientation of the ocean
waves is roughly the same all year round, typically convergent with the coastal drift.
COASTAL FACIES AND PROFILES
The cartographic analysis on a scale of 1:250,000 appended to this report presents a typology of
coastal facies that centres around two complementary readings:
 A classification of the shoreline by segments defined in accordance with the systemic
typology chosen in annex 1 of the report.
 On a coastal rim approximately 20 km wide, zoning carried out in accordance with the
sediment characteristics and methods of potential sediment transfer to the coastal area.
These elements of analysis are accompanied by an initial reflection on the sensitivity of the different
classes to a rise in sea level, human land use and the principal geodynamic characteristics.
There are five distinct major coastal profiles from North to South:
 The straight coastal regions from Mauritania to the Cape Verde peninsula, composed for the
most part of sandy formations subject to the direct action of the coastal drift. In the immediate
proximity of and behind the ridge/sandbar, there are vast expanses of low-lying salt marshes
in places situated below sea level.
 A coastal region with headlands and softened coves from Cape Verde peninsula to Basse
Casamance with the exception of the major estuaries, structured by rocky outcrops of
sandstone and severely weathered, fragile ferruginous cuirass.
 The coastal mangroves, in varying degrees of deterioration, corresponding to the coastal river
mouths: Sine Saloum, Casamance, the “Southern rivers” region, from Guinea Bissau to Sierra
Leone.
 A coastal region highly structured into rocky headlands and sandy coves from Liberia to the
West of C‚te d'Ivoire. This same profile is also to be found in the central part of Ghana.
 From the West of C‚te d'Ivoire to Benin stretch two sediment basins that mark the limits of
the soft coastline (C‚te d'Ivoire and Dahomey basins) nonetheless characterised by important
lagoon and channel systems parallel to the coast and situated behind a sandbank that is very
narrow in places (lidos).
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The major part of the coast in the study area has a high sensitivity to coastal erosion related to
(i) the nature of the materials; (ii) the sediment flows which are limited either by continental or
river mouth trapping, or due to the coastal sediment partitioning that can be observed on coasts
that are more predominantly structured into headlands and coves. The developments and
infrastructures that disturb a coastal drift that is quite typically parallel to the shore create
observable direct impacts: siltation upstream and erosion downstream of the portions of the
coastal region that have undergone human artificialisation.
In the headland and cove coastal regions, the diversity of situations in relation to the dominant
ocean waves and the coastal drift means that the sensitivity to erosion should be analysed on a
case by case basis depending on the local configuration.
In every case, coastal development should be part of a multiple scale plan enabling the
replacement of local erosion management in the wider context of the sedimentary
compartments concerned.
9
Catchment areas, isohyets and modes of sediment transfer in West Africa
CLIMATE CHANGE AND RISING SEA LEVEL
Uncertainty seems to be the main characteristic of long-term climate forecasts for West Africa.
Nonetheless it is considered that climate change should be reflected by increased climate variability.
In particular, a reduction of precipitations is envisaged, which is expected to be accompanied by an
increase in their intensity.
The rising temperature of the surface waters of the oceans may lead to an increase in intensity and
perhaps of the frequency of extreme events, which are responsible for an increased risk of storm
surges also associated with increased risks of flooding in the catchment areas and at river mouths in
estuary and lagoonian areas. Such events were observed in 2007.
A rise in sea level is also expected to cause the advancement of the salt-water wedge which would
have negative consequences on the availability of water in the coastal zones (especially arid ones),
both for human consumption, but also for agriculture, and secondarily for stock raising.
The consequences of the rise in sea level, in particular regarding the “loss of coastal land” should not
be interpreted “mechanistically” or from a purely topographic reasoning, except in the case of humaninstigated developments. For in fact, while the natural coastal systems are often able to adapt and
migrate, artificialised coast is subject to the risk of submersion. This is the case, for example, for the
dyked rice growing systems in Guinea, where the lowest soils, which are also the most productive, are
directly under threat. In many cases, the coastal formations will be able to adapt and the contours “of a
future coast’” will depend, in particular, on the speed differential between sediment deposition
phenomena and the rise in sea level. Migration inland in certain wetland areas is a real possibility (if
the corresponding spaces are available).
Note also the importance of other vertical movements related to tectonics (subsidence/upheaval),
which can be of significant amplitude, and which remain little known and difficult to situate.
COASTAL ZONE OCCUPATION: PROSPECTIVE SKETCH OF STRUCTURAL
TRANSFORMATIONS
GEOGRAPHIC CONCENTRATION
The coastal region has been provisionally described in this study as the territory situated less than 25
km from the coast:
 the total surface area is estimated at 115,000 km„, which is 5% of the total surface area of the
12 coastal countries concerned by the study.
 This coastal region today concentrates 31% of the total population and 51% of the urban
populations of these countries.
 All the capital cities lie in this zone.
 The level of urbanisation here is twice as high as in the hinterland.
 The current average population density is 260 per km„, with maxima of 1,000 persons per km„
in Togo and Benin and zones with less than 10 persons per km„ in Liberia or Guinea Bissau.
 This coastal region today accounts for approximately 56% of the total GDP of the coastal
countries, which is 21% of rural GDP and 76% of the total urban GDP.
These few (estimated) data items show the extent of the strategic importance of this coastal zone,
where the major part of “modern” activity is concentrated, with all the advantages and risks that this
geographic concentration brings with it. Nonetheless, it is useful to remain relative regarding the
signification of average data to typify an entire coastal region which is primarily marked by its
contrasts.
TWO SCENARIOS
The study proposes two scenarios of how population and economic activity could evolve over the long
term (2050) in this coastal region, with an intermediate picture in 2020. These images of the future are
replaced in the more general context of Sub-Saharan Africa, which could (should…) begin to catch up
on the rest of the world economically, and the population of which will continue to migrate from
landlocked countries to coastal countries.
The rate of economic growth in West Africa is expected to rise, to almost 6% in long periods, which
would support the rate of urbanisation of the whole region and strengthen the process of concentration
of economic activity in the coastal area. The population density in the coastal zone could therefore
triple by 2050, to reach an average of 800 persons per square kilometre, and more than 3,000 persons
per square kilometre in Benin and Togo! A quarter to a fifth of the total surface area of the coastal
region would then be urbanised.
Two scenarios have been envisaged:
 The dominant scenario leads to a high, even difficult to manage concentration of activity in
the coastal zone.
 The second scenario, called “controlling disparities” implies a planned policy of development
and facilities able to polarise the growth and concentration of activities in the areas inland of
the coast. Even in this scenario, the land (whether built on or bare) occupied by the towns will
cover almost the entire coastal zone in the case of Benin and Togo, which only have around
one hundred kilometres of coastline and are surrounded by very dense countries such as
Nigeria and Ghana. In Gambia, Ghana and C‚te d'Ivoire, in the coastal zone 40 to 60% of the
total “developable” surface area would be urbanised, which is also considerable.
The concentration of economic activity in the areas near the coast would above all concern the
“modern” sector, with heavy industry, the manufacturing and services industries, the popular economy
of the metropolises and secondary urban centres, the number of which is expected to increase to
around 250 in 2020 and 500 in the long term. It will also affect the rural economy and the primary
sector, with the development of peri-urban agriculture and agro-industry, which will be expected to
serve a market of 70 to 80 million urban consumers, around sixty million of whom will be living in the
12 metropolises. In 2050, the coastal zone could account for two thirds of the total GDP of the 12
coastal countries according to the dominant scenario and at least 60% in the controlling disparities
scenario. The average density of economic activity would then exceed 2 million dollars (in 2000
prices) per km„, which is 11 to 13 times the current level.
These figures confirm, if it were necessary, that the zone located immediately behind the actual
coastline constitutes an area that is of strategic importance for the whole region, where
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competition for the use of land and sites will be increasingly keen. The economic future of the
eleven coastal countries and even of the hinterland will depend in large part on this coastal area,
which, due to the population concentration, contains both a wealth of potential of all kinds and is
subject to growing environmental constraints.
Note, however, that these developments will occur in a very contrasted way depending on the
region in question. Some countries, such as Guinea Bissau or Liberia, will simply continue to
conquer land that is relatively unpopulated and where urban networking is and will remain not
very dense. The availability of water could constitute an essential constraint in how the land use
in the coastal area evolves.
CRITICAL ZONES
This reflection makes it possible to identify some critical zones for the development of the coast, and
the countries where it is most urgent to implement specific urban policies, essentially the countries in
the Gulf of Guinea with centres of density (the major cities and secondary cities in C‚te d'Ivoire and
Ghana, the Nigerian conurbation round to Lom†). The Dakar region would also be concerned.
The long range analysis shows that the West African coastal areas will be highly solicited to
support growth in terms of population and economic activity which will remain primarily
focused on the coastal areas. The history of the development of the urban framework of the
Coastal countries, the opportunities for openings on the world are so many reasons for
envisaging this coastal densification of human land use. Taking this data into account for
strategic purposes can be achieved through a deliberate policy of developing the secondary
towns, infrastructure, and assistance with investment in the areas in the hinterland behind the
coast, the attractiveness of which would be supported by an active, effective decentralisation
policy.
TOWARDS A REGIONAL COASTAL EROSION PROGRAMME
The results of this study should lead to a set of recommendations able to frame and guide the
implementation of UEMOA’s regional programme to combat coastal erosion. In this current stage of
regional pre-diagnostis it is obviously premature to identify the measures this plan will comprise.
Nonetheless, a global architecture could be envisaged that would comply with best practices in terms
of risk prevention.
Associating a shoreline monitoring programme with a coastal development scheme focuses on coastal
erosion, this coastal risk prevention programme would incorporate three major compartments designed
to mutually reinforce each other:
 Protection – attenuation of the impacts related to shoreline dynamics
 Watch – Vigilance – Knowledge of the hazards and experience feedback.
 Preparation of populations and decision-makers.
These three main lines would be underpinned and stimulated by regional provisions regarding coastal
risk management, in application of a regional strategy of which the present study would contribute to
identifying the main elements. This regional plan should contribute to a regional policy of global
security for the population and coastal areas. The provisions and recommendations that will originate
from this should also be harmonised with the provisions that have already emerged from other
13
regional initiatives (Abidjan Convention, GCLME). The points on which the harmonisation of policies
could be given priority should be identified.
The structure of regional proposals emerging from the study would make it possible to centre and
develop synergies with the other components of the UEMOA PRLEC.
General organisation (hypothesis) of the future regional coastal erosion programme
OUTLOOK AND CONTINUATION OF THE WORK
The publication of this pre-diagnostic report brings to a close phase II of the study, which was initially
scheduled (chronogramme validated at the regional workshop in September 2009) to end in mid
February 2010. Phase III of the programme is centred around the same components with an objective
of finalising the work of diagnosis, and drawing up a provisional development scheme. In parallel,
14
networking activities are continuing and given concrete form through the implementation of several
tools designed to be updated and used in the future.
Some delays may have been experienced, in particular in the response times to the call for proposals
concerning the case studies, and in the conducting of the national diagnostic studies in the eleven
countries (the report includes an outline of conditions at end February 2010) which make it
appropriate to adjust the initial completion deadline for the study to the end of 2010.
PROGRESS REPORT AND STRUCTURE OF PHASE III
Phase III will comprise different sections:






Finalisation of the national diagnostic studies and of the regional diagnostic study
The conducting of case studies and the layout of the maps for their presentation.
Validation and finalisation of the 1:250,000 scale map.
Cartographic interpretation, drawing up and drafting of the provisional development scheme.
Production of the cartography of the development scheme on a scale of 1:500,000.
Various communication and technical and scientific networking operations on a regional
scale.
This phase will extend from 10 March 2010 to 1st September 2010. The deliverables expected are the
following:





Final regional diagnostic study.
Final cartography of sensitivity at 1:250,000.
Final case studies
Provisional development scheme and recommendation for shoreline monitoring.
Cartography of the development scheme at 1:500,000, provisional version.
This third phase will be completed at the beginning of September 2010 by a regional seminar to
present the results.
PHASE IV AND FINALISATION OF THE STUDY
The following products will be presented at the regional meeting in September 2010:
 Final regional diagnostic study
 Final 1:250,000 cartography
 Recommendations for combating coastal erosion and the provisional version of the
development scheme accompanied by 1:500,000 maps
 Results of the case studies
 Detailed products of the national diagnostic studies
 Specific studies, in particular socio-economic ones
The validation of these products will make it possible to begin the last phase of the study which
will aim to produce a final version of the recommendations for controlling coastal erosion and the
provisional version of the development scheme accompanied by 1:500,000 maps.
These documents will be presented in the form of an illustrated summary suitable for a wide
distribution.
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TABLE OF CONTENTS
Summary ...........................................................................................................................................4
CHARACTERISATION OF COASTAL SYSTEMS..................................................................4
A coast that is essentially composed of soft formations................................................................4
Relatively limited sedimentation caused by wind and rivers.........................................................7
A steep rainfall gradient ..............................................................................................................7
Two principal currents.................................................................................................................7
Coastal facies and profiles ...........................................................................................................8
CLIMATE CHANGE AND RISING SEA LEVEL...................................................................11
COASTAL ZONE OCCUPATION: PROSPECTIVE SKETCH OF STRUCTURAL
TRANSFORMATIONS..............................................................................................................11
Geographic concentration..........................................................................................................11
Two scenarios ...........................................................................................................................12
Critical zones ............................................................................................................................13
TOWARDS A REGIONAL COASTAL EROSION PROGRAMME ......................................13
OUTLOOK AND CONTINUATION OF THE WORK............................................................14
Progress report and structure of phase III...................................................................................15
Phase IV and finalisation of the study........................................................................................15
ABBREVIATIONS.....................................................................................................................18
1. Introduction ............................................................................................................................19
2. characterisation and vulnerability of coastal systems............................................................22
2.1. CLIMATE CONTEXT....................................................................................................22
2.1.1.
Climate systems .........................................................................................................22
2.1.2.
Temperature regime ...................................................................................................25
2.1.3.
Climate zoning...........................................................................................................26
2.1.4.
Ocean-atmosphere interactions...................................................................................28
2.2. OCEANOGRAPHY ........................................................................................................30
2.2.1.
Principal currents .......................................................................................................30
2.2.2.
Water temperatures ....................................................................................................33
2.2.3.
Ocean waves..............................................................................................................33
2.2.4.
Tides..........................................................................................................................35
2.3. SUMMARY PRESENTATION OF COASTAL SYSTEMS..........................................36
2.3.1.
Lithology of the coastal zone......................................................................................36
2.3.2.
The continental shelf..................................................................................................39
2.3.3.
Cartographic inventory of types of coast and approach to their geodynamics in relation
to coastal erosion.......................................................................................................................40
2.3.4.
Differentiated sediment supplies ................................................................................40
2.3.5.
Natural ecosystems and biodiversity...........................................................................46
3. Sources of pressure and issues at stake: elements of long-range analysis .............................47
3.1. demographic-economic issues AND urban systems........................................................47
3.1.1.
Demographic transition: in the middle of the ford …..................................................47
3.1.2.
Analysis of urban growth ...........................................................................................49
3.1.3.
Urban footprint and space consumption......................................................................52
3.1.4.
Medium and longterm demographic forecasts ............................................................54
3.1.5.
What are the medium and longterm urbanisation scenarios? .......................................55
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4.
5.
6.
7.
8.
9.
3.1.6.
Consumption of space by the towns in the coastal zone in the medium and long term.59
3.2. ECONOMIC outlook.......................................................................................................60
3.3. Transport Infrastructure ................................................................................................63
3.3.1.
Road and rail networks...............................................................................................63
3.3.2.
Harbour infrastructures ..............................................................................................67
Hazards....................................................................................................................................71
4.1. Elements OF climate forecast..........................................................................................71
4.1.1.
Uncertainty of climate models for West Africa...........................................................71
4.1.2.
Climate change scenarios ...........................................................................................71
4.1.3.
Projections to horizon 2050........................................................................................72
4.1.4.
The Southern Oscillation (ENSO) ..............................................................................75
4.1.5.
Frequencies of extreme events ...................................................................................75
4.1.6.
Significant wave height..............................................................................................75
4.1.7.
Trend: rising sea level and storm surges .....................................................................76
issues at stake ..........................................................................................................................81
Responses.................................................................................................................................82
6.1. SUBREGIONAL ORGANISATIOns .............................................................................82
6.1.1.
UEMOA ....................................................................................................................82
6.1.2.
Guinea Current Large Marine Ecosystem (GCLME): .................................................82
6.1.3.
Canary Current Large Marine Ecosystem (CCLME): .................................................83
6.1.4.
The Abidjan convention .............................................................................................83
6.1.5.
Regional projects .......................................................................................................84
TOWARDS A REGIONAL COASTAL EROSION PROGRAMME...................................85
7.1. SECTION 1: Protection AND attenuation OF impacts..................................................86
7.1.1.
Strategic measures (development scheme)..................................................................86
7.1.2.
Priority measures (local erosion management)............................................................87
7.2. Section 2: Watch – Vigilance – Knowledge of the hazards and experience feedback....87
7.2.1.
Towards a strategic regional observatory of growth and settlement in the coastal zones .
87
7.2.2.
The coastline monitoring programme .........................................................................88
7.2.3.
Necessary monitoring and reinforcement of marine protected areas (coastal sentries) .89
7.2.4.
Exceptional events and storm surges: develop monitoring functions and weather alerts .
89
7.3. Preparation of populations and decision-makers TO COMBAT COASTAL EROSION
89
7.4. implementation of regional coastal risk governance bodies ...........................................89
OUTLOOK AND CONTINUATION OF THE WORK ........................................................91
8.1. Progress report and structure of phase III .....................................................................91
8.1.1.
Finalisation of the national diagnostic studies and of the regional diagnostic study .....92
8.1.2.
Launch of the case studies..........................................................................................93
8.1.3.
1:250,000 cartographic analysis .................................................................................95
8.1.4.
1:500,000 cartography of the development scheme.....................................................95
8.1.5.
Conception of the development scheme......................................................................97
8.1.6.
Communication and networking.................................................................................97
8.1.7.
Phase IV and finalisation of the study.........................................................................98
Annexes....................................................................................................................................98
9.1. Cartographic inventory of types of coast and approach to their geodynamics in relation
to coastal erosion.........................................................................................................................98
9.1.1.
Typology of sea fronts ...............................................................................................99
9.1.2.
Sedimentology mapping of the coastal fringe ...........................................................112
17
9.2. ANNEX 2: SOME TERMS AND CONCEPTS RELATED TO COASTAL
DYNAMICS..............................................................................................................................122
ABBREVIATIONS
MEA
Multilateral Environmental Agreements
ANCORIM
Atlantic Network for Coastal Risk Management
ASS
Sub-Saharan Africa
CDC-ST
Coastal drift current – sediment transport
ECOWAS
Economic Community of West African States
CILSS
Comit† Inter Etats pour la Lutte contre la S†cheresse au Sahel – Inter-state committee
for combating drought in the Sahel
SWAC
Sahel and West Africa Club
GCLME
Guinea Current Large Marine Ecosystem
GHG
Greenhouse gas
DEM
Digital elevation model
OMVG
Gambia River Basin Development Organization
OMVS
Senegal River Basin Development Organization
UNEP
United Nations Environment Programme
PRLECRegional Coastal Erosion Control Programme (UEMOA)
SRTM
Shuttle Radar Terrain Model
UEMOA
West African Economic and Monetary Union
IUCN
International Union for the Conservation of Nature
WA
West Africa
WALTPS
West Africa Long-Term Perspective Study
18
1. INTRODUCTION
The myth of a predominantly continental Africa “with its back turned to the sea 1” would appear to
have worn rather thin. While agriculture has probably been one of the main occupations of the coastal
populations and even though the populations of fishermen have inherited real traditions of continental
fishing originating, for example, from the interior delta of the Niger, nonetheless many traces remain
of a very ancient human presence, in Mauritania or in Senegal, for instance, as attested by the shell
piles found in many places.
The migrations consecutive to the droughts in the 1970s and 80s, the development in colonial times of
major urban centres, practically all on the coast, have given the West African coastline all the
appearances of a veritable pioneering front; this ties up with a colonial history the first stages of which
were also the creation of the trading posts and factories which were motivated by the prospect of
“mining” the natural and geological resources of the interior. This history is not completely a thing of
the past, as witnessed by the essentially “utilitarian” nature and the customary usage values which are
the basis for the development and use of the West African coastal areas. The current imperatives of
protecting people and goods in a concept of security and social progress cannot be separated from the
Millennium Goals which today impose the necessity of revisiting the relation between African
societies and their coastal lands.
Pragmatically, the growing threats weighing on the coastal economies also call for new reference
points and new solutions for the development of land and marine areas which must become
transboundary. It is a question of envisaging in a coherent and functional manner the specific
dynamics of a regional coastal interface, which would certainly provide resources and multiple
ecological services, but whose land area which is after all limited must be optimised and managed to
be able to maintain its role as support and development engine for the coastal countries.
The pace of change is accelerating, and climate change is expected, over the coming decades, to be
reflected as profound changes in the natural behaviour of coastal areas, whether it is a question of
integrating into a renewal the development approaches founded on the precautionary principle and a
forward-looking vision of the possible ways the situation could evolve. These approaches should take
into account the reciprocal influences between coastal territories, the transboundary interdependence
of sea fronts and the functional role of natural infrastructures in preserving irreplaceable ecological
services that are necessary for development in a situation where the population is still growing.
This original regional study brings together several dozen technicians and scientists from the South
(from the countries concerned, from Mauritania to Benin) and from the North at the service of a
common goal which is part of UEMOA’s environmental policy.
It should help to build the foundation of shared, renewed management of the common coastal heritage
considered in its entirety this time, at different scales, not simply spatially but also temporally through
a forward-planning exercise that has been initiated. This ambition is consistent with the more global
perspectives of NEPAD and should contribute to the strategic distancing necessary to avoid making
“back to the wall” decisions, which today have largely shown their limited usefulness for the
preservation and sustainable management of the coastal heritage.
This regional pre diagnostic study succeeds the methodology memo issued in September 2009, which
aimed to pose the bases of an overall regional approach, in which this document is only an
intermediate milestone.
1
Pelissier. P. 1990.- Post-scriptum to "Rivages. L’Afrique tourne-t-elle le dos ƒ la mer ?" Cahiers d’Etudes
Africaines.
117, XXX-1, 7-15p.
19
A few facts
The Sub-Saharan West Africa considered here (including Nigeria in addition to the 11 countries
concerned by the study) represents a total coast line of approximately 5,100 kilometres2 (see table I)
for a total surface area of 6,139,952 square kilometres. The ratio between total land area and coastline
is 1 to 0.00083.
In other words, 1 km of coastal region corresponds to 1,200 square kilometres of continental
land area! As an indication, in the United States this value is 345 km„, in France it is 52 km„ (with the
overseas territories), in China 633 km„ and Brazil 1,160 km„. West Africa is therefore one of the
regions in the world with the least extensive coastal front compared to its continental land area, despite
the fact that the mangrove systems are typified by a coastline that is largely “fractal” and therefore
obviously longer than a straight coastline. It could be said that to a certain extent some West African
continental lands are subject to major ecoclimatic constraints that restrict their occupation and use, but
the more favourable conditions on the coast could also be construed as an incentive to the
concentration of economic activity and population there.
2
This type of data is subject to caution, as the shoreline is a fractal object, linear dimensions depend on the scale
of measurement.
20
Table I – Ratio of coastline to inland surface area in the countries concerned by the study.
Country
Mauritania
Senegal
Gambia
Guinea Bissau
Guinea
Sierra Leone
Liberia
Cote d'ivoire
Ghana
Benin
Togo
Coastline in Perimeter
km
in km
754
531
80
350
320
402
579
515
539
121
56
5 828
3 171
820
1 074
3 719
1 360
2 164
3 110
2 632
2 110
1 703
Surface area
1 030 700
196 600
113 00
36 120
245 857
717 40
111 370
322 432
239 460
112 620
56 785
Estimated
surface
%
Km² for 1
area of the coastline/perime
km of
25km
ter
coast
coastal strip
14 550
15 330
1 725
12 590
10 760
10 990
14 090
14 260
12 370
2 664
1 854
12.9%
16.7%
9.8%
32.6%
8.6%
29.6%
26.8%
16.6%
20.5%
5.7%
3.3%
1 367
370
141
103
768
178
192
626
444
931
1 014
Km² surface area
of country for 1
km² of the 25 km
coastal strip
70.8
12.8
6.6
2.9
22.8
6.5
7.9
22.6
19.4
42.3
30.6
In conclusion, even if it is not always acknowledged, the African coastline is, more than anywhere else
in the world (except Australia), an area that is supremely sensitive. With or without the movement
towards the coast, it is to be expected that the coastal region will be subject to strong pressure from the
hinterland that is escalating at a faster pace than almost anywhere else in the world. Hence the
importance and relevance of the strategic concern expressed in this study on the dynamics of the
coastal areas of West Africa.
21
2. CHARACTERISATION AND VULNERABILITY OF COASTAL
SYSTEMS
2.1. CLIMATE CONTEXT
Shoreline dynamics are produced by complex phenomena resulting from the interaction between the continental
sediment load (from rivers or the wind) and morphogenic agents (ocean waves, currents, tides, winds and
precipitations), which transport and redistribute sediment supply. The force and way these morphogenic agents
act are, to a large extent, governed by meteorological phenomena.
2.1.1. CLIMATE SYSTEMS
2.1.1.1. PLUVIOMETRIC REGIME
Climate varies enormously from the North to the South of the study zone and to a lesser extent from
East to West. The climate of West Africa is primarily modulated by the seasonal movements of the
intertropical convergence zone (Erreur ! Source du renvoi introuvable.1, CEDEAO-SWAC/OECD
2008), which separates the continental mass of hot, dry air with North-Easterly winds (Harmattan) and
the mass of moist sea air originating from the Gulf of Guinea (monsoon).
Precipitations constitute one of the most determining factors of the different types of climates in this
region. Their distribution reflects a gradient that decreases from the south to the north. The annual
total precipitation rate is used to distinguish between several different pluviometric regions (Leroux,
2001):





Regions with very low precipitations (0 to 100 mm)
Regions with low precipitations with cumulative totals between 100 and 500 mm
Regions with moderate precipitations with cumulative totals between 500 and 1000 mm
Regions with annual cumulative total precipitations of between 1000 and 1500 mm
Regions with high precipitation rates with annual cumulative totals of over 1500 mm.
The cumulative total precipitations are in a gradual scale from North to South from 100 mm at the
latitude of Nouakchott to more than 4,000 mm in Conakry, Monrovia or Robertsfield (see figure 3).
This gradient differs, however, in the eastern part of the study zone, with cumulative totals of
precipitations being more limited in the South of Togo and Benin (1200 to 1500 mm). The annual total
is not enough, however, to characterise the pluviometric system. However, the analysis of monthly
totals is used to distinguish two types of pluviometric systems:
 Unimodal regimes with an annual maximum (Erreur ! Source du renvoi introuvable.2a)
on the coastal zones from Sierra Leone to Mauritania.
 Bimodal regimes, with two maxima respectively in April-May-June and SeptemberOctober-November, separated by a minimum in July-August known as “small dry
season”, although it is not a true dry season (Erreur ! Source du renvoi introuvable.2b)
on the coastal zones from Benin to Liberia.
Temperature ranges are typically small at less than 10°C.
22
Figure 1: Monsoon cycle in West Africa (CEDEAO-SWAC/OECD, 2008)
Figure 2: The two types of pluviometric regimes on the coastal region a) unimodal regime in Conakry, Guinea and
b) bimodal regime in Porto Novo in Benin with the "small dry season" in July.
23
Figure 3. Distribution of precipitation in West Africa (annual averages) – Source: ORSTOM
24
1.
Figure 4: Average monthly precipitations of a few coastal stations in West Africa for the period 1961-1990.
2.1.2. TEMPERATURE REGIME
The coastal areas in West Africa have small temperature ranges with diurnal ranges typically lower
than 10°C and annual temperature ranges of often less than 8°C (Leroux, 2001), as the thermal inertia
of oceanic waters has a regulating effect.
Figure 5. Map of average annual temperatures
(unbroken line on the continent) and average annual amplitudes in degrees (dotted lines) Celsius.
25
Figure 6: Monthly average temperatures of a few coastal stations in West Africa for the period 1961-1990.
2.1.3. CLIMATE ZONING
There are many ways of classifying the climate regions in West Africa (Hayward and Oguntoyinbo,
1987; Leroux, 2001). Often based on the Koppen and Thornthwaite classification (Gentilli, 1958), or
other, less well-known systems.
Some of the methods are complex and require the use of observation data. In the case of West Africa, more subjective
methods, based on the amalgam of regions which have the same climatic conditions, have also been used. Grandidier
(1934) divided Africa into 14 climate regions, based on temperature. Aubréville3 (1949) uses the number of rainy months,
the number of dry months, the humidity rate, average temperatures and temperature ranges to produce a complex map that
divides the region into four major zones: Saharan, Sahelian, Sudanian and Guinean. These major zones are in turn divided
into smaller climatic regions. Harrison Church (1961) created a map of the climatic regions of West Africa based on the
seasonal precipitation regimes.
Leroux (2001) produced a map of the climate zones in Africa based on six classes of humidity, seven
classes of temperatures and seven classes of precipitations. The coastal domain is divided into three
major zones (5):
1. Maritime trade wind type I, comprising the Cape Verde archipelago and the coastal
regions of Mauritania and North Senegal;
2. Libero-Guinean encompassing the coastal regions in southern Senegal, Gambia,
Guinea, Sierra Leone and Liberia;
3. Permanent Atlantic monsoon comprising the coastal regions extending from the
south of Liberia to Benin.
This division approximately matches the one proposed by Mahé et. al (2001) the corresponding zones
of which are called North West Sahel, Guinea and North Coast.
3
Aubréville's classification, although old, is still current and rallies to the same cause climatologists and other
specialists, in agricultural systems, forests, etc. (Aubréville. 1949.- Climats, For•ts et D‚sertification de l’Afrique
tropicale. 351p.)
26
Figure 7: Climate domains and regions of West Africa (Adapted from Leroux, 2001).
Hayward (1987) proposes a classification based on pluviometrics including, among other things, the
seasonal cycle of temperatures. This classification is based on the length and start date of the rainy
season. A further subdivision was made based on annual pluviometric totals. Analysis of the Climate
Research Unit (CRU)'s database of temperatures and precipitations4 can also be used to suggest
climate zoning. The study zone can be divided into five major climate zones based on the average
annual total precipitations (figure 7) during the reference period 1961-1990:
1. Desert-type climate with annual precipitations of under 100 mm encompassing the whole
Mauritanian coast north of 18 degrees North latitude.
2. Sahelian type climate with precipitations between 100 and 500 mm encompassing the
south of Mauritania and the coast of Senegal north of 15 degrees latitude.
3. Sudanian type climate with precipitations between 500 and 1,000 mm covering the
Senegalese coast and Gambia between 13 and 15 degrees North.
4. Tropical humid type of climate covering the coastal regions of southern Senegal, Guinea
Bissau, Ghana, Côte d'Ivoire, Togo and Benin where precipitations are between 1,000
and 2,000 mm.
5. Equatorial type climate with annual precipitations exceeding 2,000 mm on the coastal
zone of Guinea, Sierra Leone and Liberia. This breakdown and zone naming is not
absolute and differs depending on the author.
This breakdown can be examined in depth by considering the pluviometric regimes in order to
differentiate between:
1. a zone where there is no real dry season, with a typically bimodal regime covering the
coastal region from Benin to Liberia;
2. a zone with a single, very clear dry season concerning the coastal region from Sierra
Leone to Mauritania.
4
This data is supplied for emerged land on a regular grid from 2.5°latitude by 3.75° longitude (Mitchell and Jones,
2005).
27
Figure 8: Average annual total precipitations (green)
and average diurnal temperature range for the period 1961-1990, according to interpolated data from the CRU.
The diurnal temperature range can be used to distinguish between two zones in the coastal fringe of
West Africa:
1. The whole coastal zone from Benin to Guinea, where the average diurnal temperature range is
less than 10°C.
2. The coastal zones of Guinea Bissau, Senegal, Gambia and Mauritania where the diurnal
temperature range is between 10°C and 15°C.
On the basis of these different studies we can select, as did Mahé et al. (2001) and Leroux (2001), the
division into three major climate zones given in figure 7.
2.1.4. OCEAN-ATMOSPHERE INTERACTIONS
2.1.4.1. WIND REGIME
Seasonal variations in the wind regime are governed by the movements of the intertropical
convergence zone that separates the north-westerly Harmattan from the south-westerly monsoon
circulation. The south coast of Benin to the South of Senegal is continuously under the influence of the
Atlantic monsoon circulation and the prevailing winds are south to south-westerly in all seasons.
In summer, the coastal areas of North Senegal and Mauritania are subject to prevailing North-Northwesterly winds. The rest of the year the prevailing winds come from the North (Dakar) and North-east
(Nouakchott). In Conakry, the winds are mainly westerly. This component is also remarkable in
Abidjan where the prevailing South-Westerly wind is more marked than in the zones further to the
East of the Gulf of Guinea.
28
Figure 9. Wind roses for the principal cities on the West African coast, from top to bottom: Nouakchott, Dakar,
Conakry, Abidjan and Lomé. 4 periods for each city: January-February-March, April-May-June, July-AugustSeptember, October-November-December. Source: ACMAD.
29
The seasonal variations of the trade wind regime have a strong effect on those of the ocean. Along the
coasts of Western Africa, the prevailing trade winds are south-westerly, generating the phenomenon of
African monsoon during the northern summer. The seasonal variability of the temperature of the ocean
surface is particularly important due to the thinness of the homogeneous surface layer and the rising of
cold water along the coasts (principal upwellings of the coast of Mauritania and Senegal and to a
lesser extent, Benin).
The Gulf of Guinea is the principal source of water vapour that feeds a large part of the precipitations
on the continent. The surface temperature of the sea governs the heat exchanges between the ocean
and the atmosphere, and therefore the evaporation, temperatures and humidity of the lower layers of
the atmosphere.
2.2. OCEANOGRAPHY
2.2.1. PRINCIPAL CURRENTS
Two major currents correspond to major marine ecosystems that concern the region: the Canary
current and the Gulf of Guinea current.
The Canary current (figure 11) is situated at an average depth of 500 metres, with a speed in the
order of 10 to 15 cm per second. The current includes coastal upwellings. It is classified as a class I
current, with highly productive waters (more than 300g C/m3/year) conditions that are particularly
favourable for the development of small pelagic fish species. It follows the West African coast from
North to South between 30°N and 10°N. In winter, the current is at its strongest, with the force of the
trade winds.
 10a: It moves in parallel to the coast to 20°N. At latitude 15°N, it turns westward under the
influence of the equatorial current. In spring, the current weakens with the trade winds, while
the equatorial counter-current grows stronger.
 10b: In summer, the trade winds grow weaker still, reducing the supply of water mass
emanating from the North.
 10c: The Canary Current weakens further. The equatorial countercurrent, which is then at its
maximum, forks Northward, away from the Canary Current on the coast.
 Figure 10d: In autumn, the Canary Current is at its weakest level.
The pilot charts for the North and South Atlantic confirm this data5.
The Guinea currant (figure 12) is a West-East superficial current situated at an average depth of 25
metres6. The speed varies and can exceed 50 cm per second. It constitutes a prolongation of the North
Equatorial Countercurrent. This current, reinforced by the winds, comprises a zone of important
upwellings from July to October, which have a cooling effect and therefore stabilise the lower layers
of the atmosphere and attenuate the convection and quantity of water vapour.
5
These pilot charts require proper graphic processing and will be proposed in an annex of the final diagnostic
report.
6
Abe. J. & al. Guinea Current – GIWA Regional Assessment. 42.
30
Figure 10: Surface ocean circulation for the North-West coast of Africa
(a) in northern winter, (b) spring, (c) summer and (d) autumn (Mittelstaedt, 1991)
31
Figure 11: Boundaries of the Canary Current region (source: GIWA Regional Assessment 41)
Figure12. Boundaries of the Guinea Current region
32
2.2.2. WATER TEMPERATURES
The waters of the Guinea current are highly stratified, with a surface layer temperature varying from
25 to 29°C covering a layer of very salty subtropical water whose temperature varies between 19 and
28°C. The waters of the Canary current are also highly stratified and combine masses of water with
different temperatures and salinity levels (warm tropical waters, cold waters, warm Guinean water
with low salinity in the rainy season).
The increase in temperature of surface waters observed in the tropical Atlantic contributes, in
particular, to increasing the frequency and intensity of tropical cyclones in the Caribbean and the Gulf
of Mexico (an anomaly of 0.5°C can lead to an increase in cyclone activity of around 40%)7. This
increase in water temperature is due to a number of factors including the density of aerosols8.
The Southern Oscillation: The equatorial Pacific is the seat of a coastal upwelling which drives cold, deep water
up to the surface. However, each year, when the month of December approaches, the South Westerly winds that
blow on the coast decrease, which weakens this rising of cold water and therefore heats up the surface waters.
This phenomenon lasts for a few months and the cycle resumes. But it sometimes happens that the warm waters
accumulate to an abnormal extent on a large surface to the East and Centre of the Pacific Ocean. This is the El
Niño phenomenon which brings destructive precipitations to regions that are usually dry such as the North of
Peru, or droughts to regions that are usually wet, such as West Africa. In fact, El Nino is the warm phase of the El
Nino Southern Oscillation (ENSO) which is a cyclical variation of the global atmospheric pressure between the
East and West of the Pacific Ocean. The cold phase, called La Nina, also causes extreme events in the world.
Several studies have shown that El Nino is an important factor for inter annual variability in low latitudes. Recent
climatology studies show that strong El Niño episodes are related to a low cyclonic activity in the North Atlantic
and vice versa. (Bove et al., 1998; Donnelly and Woodruff, 2007). The influence of El Nino in Africa is still
disputed, although several studies have highlighted the relation between the very powerful el Nino episodes which
are accompanied by a weakening of the African monsoon (Donnelly and Woodruff, 2007; Janicot et al., 1996;
Otto-Bliesner, 1999). The southern oscillation probably contributes to a large extent to the uncertainty that
characterises the climate simulations in West Africa.
2.2.3. OCEAN WAVES
The ocean waves are essentially oriented depending on the prevailing wind regime, and are therefore
oblique for a large part of the coast, outside the divergence zone between the Canary current and the
current of the Gulf of Guinea (Guinea Bissau particularly), where it is difficult to pinpoint general
trends, given the importance of the tidal flows related to the depth contours of the continental shelf,
which widens considerably at this location.
2.2.3.1. WAVE HEIGHTS
7
Saunders. A. & A.S. Lea. 2008 - Large contribution of sea surface warming to recent increase in atlantic
hurricane activity. Nature. 451: 557-561p.
8
Amato T.A & al.. The role of aerosols in the evolution of tropical North Atlantic Ocean Temperature Anomalies.
Science. 324: 778-781p.
33
Figure 13. Map of the average value of significant wave height for the month of January (analysis of SSHA Topex
st
Pos…idon data for the period between 1 January 1992 and 15 Mars 2005 at the nodes of a 1† x 1†mesh):
January, April, August, November. According to THOMAS. Y-F. 2005.- Climatologie de la hauteur significative
(h1/3) des vagues sur le littoral d'Afrique de l'ouest 8ƒ to 22ƒ n– 12ƒ to 26ƒ w. fascicule 1: Topex – Poseidon data.
34
Figure 14. Estimation of significant wave height
of decennial occurrence. According to THOMAS. Y-F. 2005.
The maximal height of annually occurring significant wave is in the order of 3 metres to the North of
Dakar, 2.8 m to the South. The significant wave of decennial occurrence varies from 4 m-4.7 m to the
North of Dakar and 3.5 metres to the South. The significant wave of centennial occurrence is
estimated at 4.7 to 5.7 m to the North of Dakar, 3.7 m to 4 m off the coast of Guinea Bissau and in the
order of 5 m off the coast of Guinea and Sierra Leone.
2.2.4. TIDES
The tides are mainly semi-diurnal for the whole of the zone, with two daily peaks and minima.
The average tidal currents are very variable, essentially depending on the morphology of the
continental shelf: in the order of 1.2 m from Mauritania to Senegal, reaching 5 metres in Guinea
Bissau, decreasing steeply in C‚te d’Ivoire (1m). They are slightly higher in Togo and Benin (1.4 m).
Where tidal ranges are wide, tidal currents can reach considerable speeds, which still depend on the
local morphology of the shelf.
35
2.3. SUMMARY PRESENTATION OF COASTAL SYSTEMS
“There is no other milieu in Africa as sensitive and mobile on every scale of place and time as the interface
represented by the coastal fringes. This is proven by the widespread schema of coastal complexes that bar the
rias that are being filled up: low-lying valleys, marshes, lakes, witnesses to quaternary submergings, ancient rims
and mudflats where mangroves and grassy salt-marshes mingle with coastal lagoons deep in places but often a
thin film of water, spits and coastal rims battered and reshapen by the ocean waves and the winds from the sea,
the whole solidarily and alternatively swept by the spates of coastal rivers and invasions of seawater constitute the
natural framework the most accidentally changing and the most vulnerable to human manipulation".9
2.3.1. LITHOLOGY OF THE COASTAL ZONE
The major part of the present day coastal region is typified by the contact of the ocean with
sandy formations offering little resistance to the action of the coastal drift currents. Even the
granite or sandstone rocks along the shore edge from Liberia to Ghana, since they are profoundly
altered, only emerge locally in small rocky headlands.
2.3.1.1. RECENT SOFT FORMATIONS (PLIOQUATERNARY)
 Sandy, dune formations and clay depressions: these extend over the whole coast from
Mauritania to Dakar, with a coastal rim and practically straight beaches. Ancient and current
dune formations extend continuously in transition with the sandy coastal rim. Narrow in
places, they border the saline depressions occasionally flooded by the sea or salt flats
(sabkhras).
 Coastal rim and sandy terraces: the most extensive parts are located at the level of the
Sherbro Islands, from the East of Cote d'Ivoire from the Volta delta down to Benin. However,
there are other, similar deposits of lesser dimensions from the South of Senegal to Liberia.
These deposits of variable thickness and continental extension correspond to several phases of
fluviomarine siltation, often in continuity with the alteration coverages in place.
 Sandy-mud sediments: these deposits result from the interaction between the tidal currents
and the trapping of sediments by the mangroves, leading to a flood-prone coastline, dotted in
places with sandy spits. They are largely presents at Siné Saloum in lower Casamance and on
the whole of Guinea Bissau and Guinea, round to the Sherbro islands in Sierra Leone. The
coast in contact with the ocean is doubled with an "interior coast" reached by the tides, but not
exposed to ocean waves.
2.3.1.2. SEDIMENT FORMATIONS (CRETACEOUS AND TERTIARY)
These are profoundly altered, subject to a long evolution of the soil that included the formation of
ferruginous cuirass of a variety of ages and dimensions. The relict rocks that have not been altered and
the ferruginous cuirasses offer a certain amount of resistance to marine erosion, determining small
headlands and rocky shallows, thereby structuring the coast into sandy coves bounded by these
headlands.
 Limestone, marl-limestone and relicts of ferruginous cuirasses: these are present close to
the coastal area of Dakar at Joal Fadiout, or at the level of the marine continental shelf. Often
masked by sandy coverage or relicts of cuirasses, they are only rarely visible on the coast.
36
Deposits in the marine environment, they contain significant reserves of phosphates and of
small deposits of hydrocarbons.
 Sandstone and sandy-clay sediments: Contrary to the previous types, these correspond to
deposits on the continental milieu, “continental terminal”, tertiary sands in C‚te d'Ivoire,
argillaceous soils ("terres de barre") in Togo and Benin, and border the coastal zone from
Southern Senegal to Liberia.
2.3.1.3. SANDSTONE FORMATIONS FROM THE PRIMARY AND PRECAMBRIAN ERAS
Except in the Accra region of Ghana and in a small number of places in Guinea Bissau, they are not in
contact with the ocean. They appear as coastlines with alternating sandy coves and small rocky
headlands.
2.3.1.4. INTRUSIVE AND METAMORPHIC ROCK FROM THE PRECAMBRIAN ERA
Present in the coastal zone of almost the whole of Liberia, to the west of Cote d'Ivoire and east of
Ghana. Whatever type they are, these rocks have been subject to a long period of geological alteration
and the soils and thick alterites that result offer little resistance to marine erosion. Some veins of
harder rock escape this situation, however (dolerite, pegmatites, quartzites) or weathered granite in
preserved, separate "balls". The coast is structured in small rocky headlands, portions of a few hundred
metres of rocky coast, with alternate sandy coves and creeks.
 Granite and metamorphic rocks: these dominate from Monrovia to Fresco in Cote d'Ivoire,
with the frequent presence of blocks of rock on the foreshore or reefs in the sea.
 Metamorphic rock and basic rock: These characterise the coast of Ghana at the Cape Three
Points, with true portions of rocky coast.
2.3.1.5. INTRUSIVE ROCK FORM VARIOUS PERIODS (BASALT, DOLERITE, GABBROS)
These are rare on the coastal area with a rocky cost and the occasional sandy creek (Cape Verde
peninsula at Dakar, Cape Verga in Guinea, Conakry peninsula in Guinea, rocky breakwater in
Freetown, Sierra Leone and Robertsport in Liberia). Despite their slight extension, these rock outcrops
largely protruding into the sea play a very important role in the integral coastal currents.
37
Figure 15: Indication of the localisation of the principal lithological units of the coastal region of West Africa.
38
2.3.2. THE CONTINENTAL SHELF
The morphology of the continental shelf varies from one sector of the coast to another. It is narrow
and deeply dissected by canyons that cut into the shelf edge from Mauritania to the latitude of Dakar
in Senegal.
It then widens and constitutes a vast reservoir of sediment opposite Guinea Bissau and Guinea, and
becomes less important down to Sierra Leone facing the Sherbro islands. Related to this extension of
the shelf, the tidal ranges are extreme in this zone, and may exceed 5 metres in Guinea Bissau.
It then narrows down and remains narrow extending round to Benin. This shelf presents some notable
features, including Khayar canyon in Senegal (South of the Grande C‚te) and the “Bottomless Hole”
situated exactly to the right of Abidjan in Cote d'Ivoire. These features constitute zones that trap the
sediment transported by longshore drift.
Figure 16. Continental shelf from Mauritania to Benin (blue shading from 0 to -300 metres)
39
2.3.3. CARTOGRAPHIC INVENTORY OF TYPES OF COAST AND APPROACH TO THEIR
GEODYNAMICS IN RELATION TO COASTAL EROSION
This cartographic inventory has four objectives:
 Define a uniform reference framework based on a typology applicable to the whole of the
West African coast, in order to place in context and compare the various local manifestations
of coastal erosion.
 In the selected typology, take into account criteria of exposure to potential natural risks of
human settlements on the edge of the coastal area or likely to be built there in the future.
 Diagnose the methods of sediment transport from the continent to the coastal area and their
contributions to the conditions of equilibrium or disequilibrium in interaction with the coastal
drift and sediment transport currents.
 Highlight elements of reflection on the possible impact on coastal geodynamics resulting from
hypotheses about rising sea levels.
The work of cartography has been presented on a scale of 1:250,000 (which means, for the study zone,
approximately 16 metres of map) and a summary work on a scale of 1:500,000 is planned. Aware that
a scale of 1:250,000 does not provide the accuracy of satellite image interpretation required on a
number of sites to present the context of erosion, the diagnostic study was completed (as it was
impossible to locate all the information on the map) by means of a systematic “zoom” of the coast at a
scale of +/- 25,000 (using tools such as Google Earth, among others).
The layers of information used for this inventory were stipulated in the methodology memo presented
in September 2009.
The proposed typology of the coastal systems centres around two complementary readings expressed
on the 1:250,000 provisional geodynamic analysis map accompanying this report:
 A classification of the shoreline by segments defined in accordance with the typology chosen.
 On a coastal rim approximately 20 km deep, zoning carried out in accordance with the
sediment characteristics and methods of potential transfer of sediment to the coastal area.
The two typologies implemented (map instructions) are presented in annex 1.
2.3.4. DIFFERENTIATED SEDIMENT SUPPLIES
Sediment flows determine not only how coastal systems evolve, but many other aspects as well, in particular the
10
productivity of the waters. It was demonstrated that humanity has increased global continental sediment flows by
a volume in the order of 2.3 +/- 0.6 billion tonnes through erosion, agriculture and deforestation. On the other
hand, the multiplication of reservoirs has decreased these flows by a volume in the order of 1.4 +/- 0.3 billion
tonnes. An estimated 100 million tonnes (including 3 billion tonnes of carbon) are sequestered in these reservoirs
today.
10
Syvitski. J.P. & al. 2005.- Impact of Humans on the Flux of Terrestrial Sediment to the Global Coastal Ocean.
Science. 308, 376-380p.
40
2.3.4.1. AEOLIAN SEDIMENT INPUT ZONES
The coastal zone stretching from Nouadhibou to Dakar, with the exception of the mouth of the
Senegal river, is typified by a large range of sandy, predominantly dune coverage. The continentcoastal interface is highly subject to erosion and Aeolian transport under the action of winds from
substantially different directions:
 The North-South to North/East– South/West Harmattan which blows continuously during
more than six months of the year.
 The West-East sea wind and occasional monsoon winds during the short rainy season which
are South/West - North/East.
The Harmattan is the driving force behind the coastal drift currents (CDC) and sediment transport
from continent to coast, but the role of winds in the opposite direction is far from negligible in
reconfiguring the dune formations along the edge of the littoral and sediment transfers towards the
continent. Starting from June in particular a South-North current cell forms opposite the Senegalese
Grande C‚te.
This global schema should be nuanced however with the increasing importance of sand transfer from
the coast to the continent gradually descending towards the South, in particular from the mouth of the
Senegal river.
With the exception of the Senegal river and the very occasional coastal wetland outlets, no
watercourses, even temporary, reach the coast in this area.
Note also the existence of the Khayar canyon to the north of the Cape Verde peninsula, whose role in
trapping the sediment transported by the coastal drift should be taken into account.
2.3.4.2. FLUVIAL SEDIMENT INPUT ZONES
Fluvial sediment input is conditioned by (i) pluviometrics; (ii) the extent of the catchment areas; (iii)
morphology and their lithological and pedological types. Figure 17 schematises the principal
catchment areas in relation with the rainfall.
Principal rivers in West Africa11.
(Transboundary catchment areas and shared rivers are shown in bold).
Country
Mauritania
Senegal
Gambia
Guinea Bissau
Guinea
Sierra Leone
11
12
Name
Senegal
Siné-Saloum
Casamance
Gambia
Konkouré
Konkouré
Corubal
Kolente
Moa
Surface area drained
in km²
Water supplied to the
12
sea from rivers (Qsp)
440 000
774
200 000
2000
UNEP. 1980.- River inputs in West and Central African marine environment. Regional seas programme. 62p.
Specific flow rate: number of litres of water flowing per second on 1km² of river basin.
41
Liberia
Cote d'ivoire
Ghana
Togo
Benin
Sassandra
St Paul
St. John
Cestos
Senghuen
Cavalli
Sassandra
Davo
Bandama
Comoé
Tano
Pra
Volta
Densu
Ancobra
Argensu
Maho
Mono
Mono
Ouémé
Couffo
Zou
200 000
6750
295 000
1465
394 000
1260
135 000
1 080
The biggest, most extensive catchment areas (figures 17 and 18), are in areas with low rainfall.
Reciprocally, in the regions with the highest rainfall, the catchment areas are smaller. This observation
holds true for Liberia particularly, where the weathered, basement topography is rugged, even
dissected into multiple small catchment areas. This remark goes hand in hand with the observation of
coastal sediment compartmentalisation which is obviously accentuated in those coastal facies with
headlands and sandy coves, which characterise rocky coasts (of limited extension in the case of West
Africa). A large proportion of the catchment areas of the Niger, and globally in the Fouta Djallon part
with the heaviest rainfall, does not contribute to sediment input in the zone considered by this study.
For the regions with high pluviometrics, the importance of flood peaks is noted, which are reflected in
(i) marked desalination of coastal waters (Guinean waters); (ii) the expulsion and remobilisation of the
sandbar at the estuary mouth, in particular in the mangrove areas. This removal effect is also essential
in the dynamics of lagoon outlets.
It would obviously be extremely contingent to produce figures regarding sediment transport in this
hydrographic network. The existing data, even for the most extensively studied rivers, remains quite
unreliable, and the inter-annual variability of the precipitations regime is also major. The droughts
in the 1970s marked a break in the pluviometric series, with a reduction in average flow rates in the
order of 15 to 20%, sometimes even higher, as in the case of the Senegal river. Severe low water flows
have become frequent, with occasional drying up of flows in certain streams in the Sudan-Sahelian
zone.
It should, however, be observed that these sediment transports are largely trapped (i) by dams; (ii)
through the multiple, more or less functional developments of low-lying ground for agriculture, which
occupy a high proportion of fluvial valleys, including the perimeters of irrigated agriculture. The
marked regression of gallery forests in the Sudanian zone and crop growing on the banks also alter the
river profiles and in the long term contribute to filling up the river beds.
42
Figure 17. Distribution of the hydrographic network and principal catchment areas
(according to Hydrosheds data)
43
Figure 18. Catchment areas, isohyets and modes of sediment transfer in West Africa
44
These hydrologic constraints have led to the building of dams on the majority of large rivers, often for
hydroelectric power (50% of dams), but also for agricultural purposes. In certain cases, there are several
purposes; the Senegal River Basin Development Organization for example tries to reconcile agricultural
production goals with the production of hydro-electric power and navigation. There are approximately 150
dams in West Africa, with several more scheduled. This number is relatively low when compared to
Southern Africa, however, which has the majority of dams (there is a total of 1,300 dams in Africa and
45,000 worldwide). The two largest dams in West Africa are the Akosombo on the Volta in Ghana, built in
1964, which stands 134 metres high (4th highest in Africa) and has a capacity of 150 billion cubic metres (3rd
largest in Africa) and the Kossou on the Bandama in C‚te d’Ivoire, which has a capacity of 28 billion cubic
metres (6th largest in Africa).
The consequences of these developments are multiple, in particular in terms of conserving biodiversity, but
also in reducing sediment load and the speed of flows particularly during flood peaks. The consequences in
the coastal zone and the deltas are often major: salinisation of soil and waters, erosion related to sediment
deficits, accretion and delta formation related to energy deficits for the annual expulsion of silt plugs.
Figure 19. Localisation of major dams (over 15 metres) in West Africa. Source: SWAC/OECD.
With the exception of special cases such as the Gambia and Senegal, which have regional river development
organisations (OMVG, OMVS), the majority of these dams were designed at national level, and therefore
often without taking into account in depth the remote impacts of the developments, which should be
considered on a sub-regional scale.
45
2.3.5. NATURAL ECOSYSTEMS AND BIODIVERSITY
This chapter will be dealt with in phase III on the basis of data taken from the national diagnostic studies.
Figure 20. Land use in West Africa (2000).
Source: Global land Cover 2000 project. Joint Research Center. European Commission.
46
3. SOURCES OF PRESSURE AND ISSUES AT STAKE: ELEMENTS
OF LONG-RANGE ANALYSIS
There are multiple sources of pressure on the coastal systems, which are very often related to key socioeconomic issues, from which they are sometimes inseparable.
The pressure on natural resources (fishing and other activities that convert biodiversity, agricultural
production, tourism and coastal constructions outside of urban areas, etc.) will be analysed in phase III, on
the basis of the results from the national diagnostic studies.
3.1. DEMOGRAPHIC-ECONOMIC ISSUES AND URBAN SYSTEMS
For the purposes of this study, the coastal zone has been somewhat arbitrarily defined as a 25 km strip from the
shoreline, which explains the rather alarming rates of land use sometimes observed. In certain countries, such as Benin,
the coastal zone is considered on a width of 70 km.
Shoreline dynamics are a completely natural phenomenon which is in no way new, except perhaps in its
possible intensification and acceleration, and so the organising of a social response to this phenomenon can
only be justified by (i) threats caused by these dynamics to the security of people and goods installed in the
coastal zone; (ii) the fact that these human installations and activities sometimes reinforce the dynamics in a
way that is not conducive to reducing the aforementioned threats.
These key socio-economic issues are clearly concentrated primarily in the urban zones for obvious
reasons: the concentration of population, infrastructure and investments. In addition, the development of
these urban zones cannot be dissociated from demographic considerations related to the geographic
distribution of population settlements. Land settlement is therefore one of the key variables to be taken
into account, but it is clearly not the only one. A second variable concerns economic activity and its related
consequences in terms of infrastructure, services and space consumption. It should be clearly noted that
reflections on these two subjects should be placed the global context of the West African region and more
generally that of Sub-Saharan Africa (ASS), because the future of the coastal area and fringe is related to
that of the whole region the coastal area is a part of.
3.1.1. DEMOGRAPHIC TRANSITION: IN THE MIDDLE OF THE FORD ….
Sub-Saharan Africa is the last region in the world to undergo demographic transition. This process
implies a population multiplication of a factor of almost ten between 1950 (approximately 180 million) and
2050 (more than 1.7 billion according to United Nations forecasts). The total rate of population growth has
risen from 2.3% in 1950 to 2.6% in 2000. The forecasts indicate a rate of 2.2% in 2025 followed by a dip
leading to 1.7% in 2050.
Such a tenfold increase in the total population of sub-Saharan Africa should not be expected to be uniform
across all the regions that it comprises, from the sub-desert zones on the edge of the Sahara or even the
Sahel, to certain already densely populated rural zones where there is pressure on land, land-locked
countries with limited potential such as Niger – despite its mining potential – or sub-groups of countries
which do not strictly speaking constitute settlement spaces like the CILSS or ECOWAS.
With natural growth rates in the order of 2 or 3% per annum, the adjustment of population to physical constraints and
potentials but also and especially to market forces implies net migration rates that depend on the size of the entities
47
considered. On the scale of the countries in sub-Saharan Africa, with their artificial borders inherited from decolonialisation, these migration rates are in the order of one for one thousand or one for a hundred per annum of the total
population. But these migrations between Nation States are only a small fraction of the migratory flows between smaller
territorial entities such as districts or communes, between rural milieu and urban milieu, between land-locked areas,
outside the market, and the growth centres, which are often associated with the points of contact between Africa and the
rest of the world and are therefore often very close to the coast: “development does not take place everywhere at the
same time or at the same speed”.
3.1.1.1. REDISTRIBUTION OF POPULATION BETWEEN COASTAL AND LANDLOCKED COUNTRIES
Over the past half-century, the proportion of the total population of West Africa living in the twelve coastal
countries (including Nigeria) increased from 27% to 31%, while the natural growth rate (births minus
deaths) of these coastal countries is typically lower than in the landlocked, less advanced countries. The past
growth of this ratio is therefore a result of net migration from the interior towards the coastal countries.
Despite the troubles which have affected several coastal countries such as C‚te d’Ivoire, Liberia and Sierra
Leone, this migration has continued over the past two decades. The ratio of total population in the coastal
region to total population of the 19 countries in the WALTPS study represented 27% in 1950 compared to
31% in 2006.
Migration within the region has had a considerable effect on the distribution of the population between countries and
ecological zones. For example, between 1930 and 1990, the population of what is now Burkina Faso tripled, from 2.8 to
8.7 million, while the population of Cˆte d'Ivoire increased eightfold, from 1.4 to 11.4 million. With half the population of
the area that is today Burkina Faso in 1930, Cote d'Ivoire now has a population a third larger. If intra-regional migration
had not occurred, the economic and social situation and environmental problems of the Sahel countries would no doubt
be considerably worse than they are today…
From this example we could draw the conclusion that the population of West Africa is highly mobile. To
assess the real significance of these net inter-state migration flows and compare population mobility in subSaharan Africa with the other regions in the world, the size of the entities considered should be taken into
account. Net migration between the countries in a region is all the more significant the smaller the country,
which is the case in Africa. When this factor is taken into account, it emerges that mobility between
countries in West Africa has been and remains much lower than in other regions of the world13.
Local mobility: 30 to 40% of the population of West Africa no longer live in the place they were born
in. International migration is only one aspect of population redistribution in the area within the region. The
WALTPS study showed that after a lapse of time of a generation, an estimated 30 to 40% of the population
of West Africa no longer resides in the district or commune they were born in. It can be noted from the map
below that highly populated rural areas (with a density greater than 50 persons per km„) develop
preferentially on the periphery of the cities (this map shows cities with populations of more than 50,000 in
1990). The maps of market pressure from the WALTPS study also show the importance of urban networks
and infrastructure in the integration of rural areas and the primary sector.
13
In the United States, the population and activities respond immediately to opportunities and market forces and move
freely within the Union, with net migration rates between States from -5% to +5% per year over a long period, with no
crisis. These movements are evidently facilitated by the infrastructure and land services policy followed diligently by the
Federal Government, or more than two centuries. China and India offer two other interesting examples of settlement
management on the scale of a sub-continent. In both cases, government unity, which sub-Saharan Africa evidently does
not have, plays an important role, but this factor should not hide the importance of infrastructure and land services
policies.
48
Figure 21. Rural densities and conurbations in 2000 (according to the AFRICAPOLIS programme).
3.1.2. ANALYSIS OF URBAN GROWTH
Historically, the major capital cities on the coast of West Africa have been driven by the tertiary sector since
the large factories of the colonial era. They developed as transit areas for exchanges between the continental
interior and the rest of the World.
Today, urbanisation is one of the most visible aspects of population redistribution. Between 1950 and
2006, the urban population of sub-Saharan Africa grew by a multiplication factor of 14, compared to 6 in
North Africa and 4 in the rest of the world14. Over and above the urban bias argument, according to which
this urbanisation process is the result of debatable policies, and is both dangerous and unsustainable, the
results of the WALTPS study should also be taken into consideration, for example, when they show that the
division of labour between consumers and producers of food, led by urbanisation, is an important driving
force for the transformation of the primary sector and the rural economy (multiplication of the density of
local activity by a factor in the order of 100 to 1000 and radical changes in the types of activities and
behaviour patterns of all the players involved).
What would be an acceptable level of urbanisation in West and sub-Saharan Africa? The two graphs below, based on
15
the official data (WDI and FAO) show that the correlation between PPP (purchasing power parity) GDP per capita and
the urbanisation indicator – represented by the ratio of the number of PNP (non primary population) consumers to the
number of PP food producers – is the same in sub-Saharan Africa as in the rest of the world: sub-Saharan Africa is
therefore not over-urbanised for its level of development. In Africa as elsewhere, urbanisation is both a consequence
and an engine of economic growth. This observation should not obscure the highly particular modalities of urbanisation
14
The reasons for this urban growth that is faster than everywhere else and their multiple implications have been clearly
explained in the ILTA (1984) and WALTPS (1995) studies, then in Ecoloc programme, to which one may refer.
15
World Development Indicator (World Bank)
49
in sub-Saharan Africa or the major deficiencies noted in terms of the management of urban growth, habitat and related
services, and the optimisation of the territorial encroachment of cities.
PIB par habitant en parité de pouvoir d'achat
et niveau d'urbanisation
: 46 pays africains
y = P IB p a r h a b it a n t e n P P A
y = P IB p a r h a b it a n t e n P P A
PIB par habitant en parité de pouvoir d'achat
et niveau d'urbanisation
: 151 pays du monde
y = 2300 *[(PNP/PP)^0.59]
y = 2100 *[(PNP/PP)^0.61]
ratio PNP/PP
ratio PNP/PP
Urban growth: how should it be assessed?
According to the statistics assembled in the United Nations directories (on the basis of national censuses16)
the ratio of urban to rural population was 44% in 2006.
Population urbaine et rurale (millions d'habitants), ratio U/R et niveau d'urbanisation U/P
Region
Year
1950
1960
1970
1980
1990
2000
2006
WA Littoral
Population, total P
19
26
33
44
60
79
94
WA Littoral
Urban population U
2
5
8
13
20
31
41
WA Littoral
Rural population R
17
21
26
31
39
48
53
WA Littoral
Ratio U/R
0,13
0,22
0,31
0,41
0,52
0,63
0,78
WA Littoral
% urban U/P
11%
18%
23%
29%
34%
39%
44%
16
The reliability of these national sources is often poor, and interpreting this data is complicated by the fact that the
definition of urban population varies from one country to another and even from one decade to another in certain
countries.
50
Taux de croissance de la population urbaine et rurale et du ratio U/R
Région
Période
50-60
60-70
70-80
WA Littoral
80-90 90-2006
Population, total P
2,9%
2,6%
2,7%
3,2%
2,9%
WA Littoral
Urban population U
7,7%
5,4%
5,0%
4,8%
4,5%
WA Littoral
Rural population R
2,1%
1,8%
1,9%
2,5%
1,9%
WA Littoral
Ratio U/R
5,5%
3,6%
3,1%
2,3%
2,6%
The WALTPS study - urban population, a dynamic aggregate: the re-evaluation of urban population on the scale of the
19 countries of West Africa over the 1950-1990 period was one of the most difficult tasks and one of the most substantial
findings of the WALTPS study. The U aggregate (urban population) is defined in this study as the sum of the population
of all localities with agglomerated populations of more than 5,000, where the density (number of persons per km‰) is
much higher than in the surrounding environment, or from some 10 persons per km‰, in rural areas or spread out, to
several thousand persons per km‰ in rurban areas or close peri-urban areas. The number of localities included in the U
aggregate and their geographic limits therefore vary over time, contrary to administrative entities (districts, communes,)
to which the States refer to calculate urban population: the division of the territory into “urban” or “rural” communes never
follows the reality of the situation... As all the localities included in the U aggregate on a given date are in order of size,
the WALTPS study databases can be used to calculate not only the “U5” aggregate ( “towns” with populations of more
than 5,000) but also “U10”, “U20” or “U 'n'”, corresponding to towns with populations in excess of 10,000, 20,000 or 'n'
thousand.
The AFRICAPOLIS study: exploitation and reinterpretation of the data. The pluridisciplinary approach adopted in the
AFRICAPOLIS study, which combines data from censuses at the smallest possible level of breakdown of the region into
administrative entities and the limits of the contour of agglomerations by interpretation with the aid of satellite
photography, provides the best possible measure of the agglomerated population of each locality on the day the census
data and satellite coverage is available.
For the period prior to 1980, when no comparable aerial photographs were available, the methodology applied by
AFRICAPOLIS can not really be used. For this period, the variations between the urban distributions by town size and
the levels of urbanisation given by AFRICAPOLIS are, with a few exceptions, quite consistent with the contents of the
WALTPS study databases, with slightly higher total urban population evaluations. Due to a lack of data on the actual
extent of urbanised areas in previous years, it is not possible to determine which of the two sources - WALTPS or
AFRICAPOLIS – is the most reliable for these distant periods.
For the 1980-1990 period, the AFRICAPOLIS study leads to slightly higher estimations of urban population (in the sense
of the U5 aggregate defined above) and urban growth over this period than the WALTPS study, the difference no doubt
being due to a better appreciation of the effect of the spatial growth of large towns and in particular of capital cities.
For the 1990-2000 period, the raw data from the AFRICAPOLIS study is the only data available, since the WALTPS
study could not take into account censuses after 1994. For certain countries, the rank-size distributions of localities
identified by the AFRICAPOLIS study should nonetheless be completed for towns with populations of under 10,000 (and
even with under 20,000 in the case of Nigeria), which can be done using the WALTPS methodology.
(millions
d'hab.)
12 pays côtiers
Population
urbaine
dérivée
d'AFRICAPOLIS
P o p u la t io n u r b a in e W A L T P S
P o p u la t io n u r b a in e s o u r c e U N
Année
1950
1,7
2 ,2
1960
3,7
4 ,3
4 ,6
1970
7,9
7 ,7
7 ,9
1980
13,8
1 2 ,9
1 2 ,7
1990
21,5
2 0 ,4
2 0 ,4
2000
33,0
2010
46,2
2020
66,1
7 7 ,2
3 0 ,5
In what follows, we will therefore use the data provided by the AFRICAPOLIS study, which lends
itself well to the analysis of the urbanisation process in the coastal zone. As a reminder, the urban population
on a given date is that of the localities with agglomerated populations of more than 5,000 identified in this
study, and the rank-size distribution queues for the 1990-2000 period for certain countries have been
completed. In the tables presented here, the coastal zone is defined as the territory situated less than 25
km from the coast. This coastal zone covers a surface area of 115,000 km², which is approximately 5% of
the total surface area of the coastal countries. The study zone, as mentioned before, only extends to Benin,
and therefore excludes Nigeria and Cameroon.
51
12 pays côtiers
Populaton totale
Population urbaine
Population rurale
Ratio U/R
Taux de croissance
Taux de croissance
Taux de croissance
Taux de croissance
(millions d'hab.)
de
de
de
de
12 pays côtiers
Année
1950
1960
1970
1980
1990
2000
2010
19,5
1,7
17,8
0,10
26,0
3,7
22,3
0,17
5,7%
2,9%
8,1%
2,3%
33,5
7,9
25,6
0,31
6,3%
2,6%
7,8%
1,4%
43,5
13,8
29,7
0,47
4,3%
2,7%
5,8%
1,5%
59,7
21,5
38,1
0,56
1,9%
3,2%
4,5%
2,5%
78,9
33,0
45,9
0,72
2,4%
2,8%
4,4%
1,9%
101,6
46,2
55,4
0,83
1,5%
2,6%
3,4%
1,9%
U/R
P
U
R
Superficie
de la zone littorale
(millions d'hab.)
Année
Zone littorale
Population des villes littorales
d o n t le s m é tr o p o le s d e s 1 2 p a y s c ô tie r s
Villes littorales en % de la pop urbaine totale
Taux de croissance de la pop urbaine littorale
Population rurale littorale
Population totale littorale
Niveau d'urbanisation du littoral
Pop. littorale en % de la pop. des 12 pays côtiers
Densité de population du littoral (hab/km²)
:115000
1950
1,1
0 ,8
62%
5,3
6,3
17%
33%
55
Km², soit : 5%
1960
2,1
1 ,5
57%
7,1%
6,3
8,4
25%
32%
73
de la superficie
1970
4,3
3 ,4
55%
7,5%
7,2
11,6
38%
35%
101
1980
7,7
6 ,2
56%
5,9%
8,4
16,1
48%
37%
140
totale des 12 pays côtiers
1990
11,8
9 ,6
55%
4,3%
10,5
22,3
53%
37%
194
2000
17,8
1 4 ,0
54%
4,2%
12,5
30,3
59%
38%
264
2010
24,5
1 8 ,4
53%
3,2%
14,8
39,2
62%
39%
341
The above tableau shows that the slightly more than half of the total urban population of the coastal
countries is concentrated in the coastal zone defined in around one twentieth of the total surface area
of these countries. This proportion seems to be decreasing slightly, from 57% in the 1960s to 53% in 2010.
This evolution is the result of two opposing phenomena:
 Opening up to external exchanges, with its consequences on the coastal tropism.
 The advancement of urban systems in the national territories (multiplication of “pr†fectures”, subprefectures and/or other secondary towns).
To these structural factors can be added, since the 1990s, the crisis of States and structural adjustment which
particularly affected the political capitals of the coastal countries.
Outside the capitals, population growth rates in coastal towns are of the same order as those of continental
towns and slightly higher than in the towns in the non-coastal countries of West Africa. These urban growth
rates are nonetheless steadily declining, from 7.8% in 1960-1970 to 4.4% in 1990-200017.
The relative weight of the coastal zone in the total urban population of the countries and the trend in these
ratios vary, of course, depending on the country, the surface area and morphology, and the degree to which
the national urban networks are embryonic (as in Mauritania) or mature (Ghana and C‚te d’Ivoire). In most
of the coastal countries with small surface areas, urban growth rates have reached or exceeded 10%
per year at certain times.
3.1.3. URBAN FOOTPRINT AND SPACE CONSUMPTION
According to a recent World Bank study based on the interpretation of Landsat images, the surface area of
towns increased on average by 3.2 % between 1990 and 2000, while the population of the towns in the
17
This downward trend in urban growth rates has been analysed in the WALTPS study, and is both structural (related to
the decline in relative importance of the reservoir of rural population) and linked to the economic situation (modern
economic crisis, structural adjustment, slowing down of migratory flows, etc.). The apparent one point decline in urban
growth rates over the past decade 2000-2010 could be partially explained by a statistical and methodological bias (urban
extensions subsequent to the last censuses and available pictures not taken into account).
52
sample increased at a rate of 1.5%. The size of towns is not a major factor for differences in growth rates:
from 1 to 2% for the population and 2.5 to 3.6% for the surface areas.
For the African cities included in this study, the growth differential between urbanised surface area and
urban population is greater than in the rest of the world, in the order of 3% or more. The urbanised
surface area per urban capita is in the order of 150 m² in Africa, compared to an average for the
developing countries of 125 m² (cities in Asia are, in general, built more densely than in the rest of the
developing world).
Surface construite et urbanisée par habitant urbain (m²/hab.)
Surface construite (1)
Tx de crois. 19902000
1990
2000
ASS
105
150
3,6%
PVD
105
125
1,8%
Moyenne mondiale
155
185
1,8%
Surface totale urbanisée (2)
210
Nota 1 Source : The Dynamics of Global Urban Expansion World Bank 2005
Nota 2 : d'après données AFRICAPOLIS
It emerges from the study that the urbanised area per capita depends mainly on income level. One additional
point of per capita GDP is reflected in an additional point in per capita space consumption by urban
population18. Although we have no data on this subject, it may be pointed out that in a context marked
by low investment capacity and lack of land ownership control, urban growth in Africa is often
horizontal and not very dense, with the exception of precarious housing settlements where the
densities are high.
As an order of magnitude for West Africa, the average standard is 150 m² urbanised space per urban
dweller (excluding parks, water features, land where building is not authorised, or which is not yet
developed or inhabited).
However, the footprint of the agglomerations is greater than developed land alone. According to the
AFRICAPOLIS study, the total surface area occupied by agglomerations in 2000 was in the order of 200 m²
to 300 m² per capita, and an average 210 m² per capita for the coastal agglomerations identified in this
study, if Abidjan, which is said to have an abnormally low rate of space consumption, is not included.
The AFRICAPOLIS report states: “The average density of agglomerations did not increase as much from 1950 to 2000
as we could have estimated in the current state of our work based on a sample of 97 towns for which we have the
th
surface area in 1960. This sample accounts for 1/9 of the total agglomerations with populations of more than 10,000 in
West Africa but 44% of the urbanised land. This primarily concerns the largest agglomerations… Between 1950 and
2000, the urbanised area of our sample increased from 766 to 6,381 km2, the average annual extension of urbanised
land was therefore 5.1% compared to 4.3 for the population."
For the coastal towns, the average standard was 210 m² per person in 2000, with an average growth
rate of 1% per annum.
18
This relation globally reflects the incidence of several factors of land use that change with income: increase in the size
of accommodation but especially of the space associated with accommodation (fall in net density); increased use of
(particularly) personal vehicles and correlative increase in space for driving and parking; increase in public facilities,
office areas, etc.
53
Surface agglomérée et consommation d'espace par
habitant des villes côtières en 2000 (source Africapolis)
Nombre de Pop
centres
totale
côtiers
(millions)
11
1,31
20
3,65
9
0,21
42
3,47
4
1,30
6
0,53
7
0,35
8
0,89
2
0,63
19
2,82
7
0,82
11
1,15
Pays
Bénin
Côte d'Ivoire
Cap Vert
Ghana
Guinée
Gambie
Guinée Bissau
Liberia
Mauritanie
Sénégal
Sierra Leone
Togo
Surface
Surface
par hab
totale( km²)
(m²/h.)
258
198
331
91
39
184
890
257
298
229
238
451
83
238
269
304
113
179
304
108
113
137
206
180
12 pays côtiers
146
17,12
3142
184
12 pays côtiers hors
Abidjan
145
13,97
2890
207
Emprise urbaine de toutes les villes de Côte d'Ivoire et du Ghana selon
l'étude AFRICAPOLIS en 2000
Surface
Population 2000 occupée par
(1000 hab)
habitant
(m²/hab)
994
7624
130
253
3148
80
741
4475
166
2128
7852
271
574
2516
228
1554
5336
291
Surface totale
occupée (km²)
Côte d'Ivoire
Ghana
196 centres
Abidjan
Autres centres
218 centres
Accra
Autres centres
3.1.4. MEDIUM AND LONGTERM DEMOGRAPHIC FORECASTS
The official projected population figures established by the United Nations do not take into account future
migrations between countries. Due to the differences in natural growth rates mentioned previously, without
these migratory flows, in 2050 the West African coastal region would only hold 29% of the population of
the WALTPS study region. This hypothesis is difficult to accept, because it is difficult to ignore the
differences in agroclimatic potential and market access between landlocked Sahelian countries and coastal
countries19. The stoppage or even reversal of net migratory flows between landlocked countries and coastal
countries could make West Africa one of the most unstable regions in the world and one of the most prone
to internal and intra-regional conflicts. Managing settlement means anticipating and facilitating future
population movements, or at least the movements which are structural and not linked to the economic
situation.
The population projections by region should therefore incorporate a hypothesis of net migrations for West
African landlocked countries towards coastal countries, of the same kind and of a comparable amplitude to
19
Even if all the ore deposits, oil products and other natural resources situated in land-locked countries were valorised,
the major part indirect activities induced by the exploitation of these natural resources would be found in the coastal
countries. Despite mineral wealth (including uranium and no doubt oil), Niger will never have the population of 58 million
predicted by the United Nations for 2050!
54
those suggested in the WALTPS study. With this hypothesis, coastal countries would concentrate 32% of
the population of West Africa in 2020 and 33% in 2050. This hypothesis concerning migration has clear
repercussions on the growth of the rural populations of landlocked countries and on the growth of the urban
and rural populations in coastal countries.
Population totale en millions d'habitants. Source : UN Pop. Division
corrigées pour les migrations entre pays enclavés et pays côtiers
Région
Composition
WA Littoral
12 pays
WA pays enclavés
1990
2000
2006
2020
2025
2050
60
79
94
131
148
231
7 pays
142
186
208
281
313
476
WALTPS
19 pays
201
265
302
412
461
707
ECOWAS
15 pays
179
236
267
366
409
629
Sub-Saharan Africa
49 pays
510
661
766
L it to r a l e n %
d e la r é g io n W A L T P S
3 0 %
3 0 %
3 1 %
1032 1151 1748
3 2 %
3 2 %
3 3 %
Taux de croissance de la population totale par région selon les données
UN corrigées pour migrations
Région
Période 90-2006 2006-2020 2020-2025 2025-2050
WA Littoral
12 pays
2,9%
2,4%
2,4%
1,8%
WA pays enclavés 7 pays
2,4%
2,2%
2,2%
1,7%
WALTPS
19 pays
2,6%
2,2%
2,3%
1,7%
ECOWAS
15 pays
2,5%
2,3%
2,3%
1,7%
Sub-Saharan Africa49 pays
2,6%
2,2%
2,2%
1,7%
3.1.5. WHAT ARE THE MEDIUM AND LONGTERM URBANISATION SCENARIOS?
Positing a medium and long-term vision of urbanisation trends that are inevitably based on supposition and somewhat
artificial hypotheses is clearly fraught with difficulties. However, these long-term views are derived from:
 the conceptual framework of the WALTPS study, which has been largely substantiated by the changes
observed in the course of the last fifteen years after the study appeared.
 the town by town projections established by the AFRICAPOLIS study for the 2010-2020 period, for only those
agglomerations identified in this study, using relatively conservative hypotheses, not taking into account the
redistribution of the population within the region of West Africa, nor the multiplication of small, "new' urban
centres with populations of 5,000 to 20,000, which were not yet visible on the aerial photographs but the
emergence of which can be expected, at the crossroads of the transport networks, on the periphery of large
cities and in the dense rural areas.
It should be noted that these are only averages which should not obscure the heterogeneous nature of land use and the
fact that certain more or less landlocked regions will not undergo such developments.
Future urban population growth rates are expected to fall by approximately 4% per year between 2000 and
2010 (when the probable underestimation of the AFRICAPOLIS study at 3.6% by 2020 then 2.5% on
average from 2020 to 2050 is corrected)
.
55
12 pays côtiers
Populaton totale
Population urbaine
Population rurale
Ratio U/R
Taux de croissance
Taux de croissance
Taux de croissance
Taux de croissance
(millions d'hab.)
de
de
de
de
U/R
P
U
R
1950
19
2
18
0,10
1980
44
14
30
0,47
4,3%
2,7%
5,8%
1,5%
2000
79
33
46
0,72
2,4%
2,8%
4,4%
1,9%
2010
102
46
55
0,83
1,5%
2,6%
3,4%
1,9%
2020
131
66
65,0
1,0
1,9%
2,6%
3,6%
1,7%
2050
231
140
92,0
1,5
1,4%
1,9%
2,5%
1,1%
 The rural population will continue to grow at over 1% per annum until after 2050, which
reminds us that, except for special cases (such as Mauritania or the Cape Verde islands) there is no
rural exodus in Africa: with the total rural population doubling between 2000 and 2050, rural
settlement will continue, with restructuring.
 The ratio of Urban to Rural population, the growth of which is roughly proportional to the
growth in number of urban consumers of foodstuff per farmer, should grow by almost 2% per
year from now till 2020, then at 1.4% beyond. Farmers' market productivity and cash incomes are
also expected to grow at these rates.
This future urbanisation scenario may no doubt appear excessive, particularly if we consider, based on the
econometric studies, that sub-Saharan Africa could reach the urban saturation threshold with levels of
urbanisation in the order of 40%. It is acceptable however, for compatible with the future economic growth
scenario at rates of around 6% per year in the long period presented further on, which is completely
plausible.
Unless the coastal countries in West Africa and more generally in sub-Saharan Africa are the seat of
generalised, repeated political and economic crises, by 2020 the governments and local authorities of
these countries could well have to manage an increase of over 40% in urban population, which would
then more than double between 2020 and 2050. It should also be remembered that certain urban centres
(in Liberia, for instance) regressed partly under the effect of certain crises.
3.1.5.1. URBAN GROWTH IN THE COASTAL ZONE: TWO POSSIBLE SCENARIOS
The tables below document two scenarios for urban growth in the coastal zone, both based on the same
snapshots of settlement and urbanisation in the 12 coastal countries in 2020 and 2050, as shown previously.
 Dominant scenario: this scenario accepts the fact that coastal tropism would continue to
constitute the most important integration factor in the future, which is consistent with the
hypothesis of economic growth, presented below, which would be reflected in particular by
growth in the “modern” economy and in exchanges between West African countries and the
rest of the world higher than proportional to the Gross Regional Product.
 “Controlling disparities” scenario: this scenario is based on the hypothesis of deliberate
policies to develop the land in the coastal countries, aimed at accelerating the development of
what the WALTPS study called “Zone 2”, distant from the coasts, corresponding, for example,
for C‚te d’Ivoire and Ghana, to the latitude of Yamoussoukro and Koumassi. Account taken
of the response times of the local dynamics to such deliberate development policies, the
“controlling disparities” scenario does not differ greatly from the dominant scenario except in
relation to the long term, in this study, that is the horizon of 2050.
56
Maîtrise des
déséquilibres
12 pays côtiers
tendanciel
Superficie de la zone littorale :115000 de la superficie totaleScenario
des 12 pays
côtiers
Zone littorale
(millions d'hab.)
Année
Population des villes littorales
d o n t le s m é tro p o le s d e s 1 2 p a y s c ô tie r s
Villes littorales en % de la pop urbaine totale
Taux de croissance de la pop urbaine littorale
Population rurale littorale
Population totale littorale
Niveau d'urbanisation du littoral
Pop. littorale en % de la pop. des 12 pays côtiers
Densité de population du littoral (hab/km²)
1950
1980
2000
2020
2050
2050
1,1
0 ,8
62%
7,7
6 ,2
56%
5,9%
8,4
16,1
48%
37%
140
17,8
1 4 ,0
54%
4,2%
12,5
30,3
59%
38%
264
3 6 ,0
2 6 ,0
54%
3,9%
18,0
54,0
67%
41%
500
8 3 ,0
6 1 ,0
59%
2,8%
27,0
110,0
75%
48%
1000
7 4 ,0
5 4 ,0
53%
2,4%
29,0
103,0
72%
44%
900
5,3
6,3
17%
33%
55
The total urban population of the coastal zone is therefore expected to double, from 18 to 36 million
between 2000 and 2020, while the rural population is expected to grow by half. The average population
density in this zone would therefore rise from 260 to 500 persons per square kilometre, with considerable
differences in this average depending on the country: the two extremes are Benin with 1,800 persons per
square kilometre and Guinea Bissau with 300 persons per square kilometre, if we exclude the two special
cases of Mauritania and Cape Verde.
From 2020 to 2050, the coastal urban population would increase from 36 to 83 million under the
dominant scenario and 74 million under the “controlling disparities”. The incidence on the average
rates of urban coastal population growth would therefore be 0.4% (2.4% compared to 2.8% in the dominant
scenario).
Even at the relatively distant date of 2050, a priori there is therefore little room to manoeuvre
between the two scenarios.
Bénin
Côte d'Ivoire
Cap Vert
Ghana
Guinée
Gambie
Guinée Bissau
Liberia
Mauritanie
Sénégal
Sierra Leone
Togo
12 pays côtiers
1950
60
100
20
310
40
30
50
20
0
290
80
60
1060
1980
610
1460
70
1550
690
170
150
410
210
1510
420
470
7730
2000
1480
3700
210
3580
1390
560
350
960
650
2910
870
1180
17840
2020
3600
7300
400
7000
2600
1200
700
2100
1400
5400
1700
2400
36000
2050
8900
16100
600
16700
6400
2800
2100
5300
3000
11100
4500
5600
83000
2050
7600
14000
600
14300
5600
2700
2000
4900
2800
10200
4000
4900
74000
57
Population des métropoles
Pays
Capitale
Bénin
Côte d'Ivoire
Cap Vert
Ghana
Guinée
Gambie
Guinée Bissau
Liberia
Mauritanie
Sénégal
Sierra Leone
Togo
Cotonou
Abidjan
Praia
Accra
Conakry
Banjul
Bissau
Monrovia
Nouakchott
Dakar
Freetown
Lomé
Scenario tendanciel
12 pays côtiers
Maîtrise des
déséquilibres
1950
1980
2000
2020
2050
2050
20
90
10
160
40
10
50
20
0
250
70
40
400
1290
40
1040
670
100
120
320
180
1270
380
420
910
3150
90
2520
1250
410
300
760
560
2260
740
1030
1900
5600
200
4800
2300
900
600
1600
1100
3900
1300
2000
4700
12200
200
11500
5600
2100
1800
4200
2400
8000
3600
4600
4000
10700
200
9800
5000
2100
1700
3800
2300
7400
3200
4000
800
6200
14000
26000
61000
54000
Distribution rang taille des villes côtières
en 2020
>5 millions
2 à 5 millions0
1 à 2 millions0
0,5 à 1 million
0,2 à 0,5 millions
0,1 à 0,2 millions
50 à 100 000
20 à 50 000
10 à 20 000
5 à 10 000
Nombre de villes
non
identifiées
identifiées
1
3
5
2
6
4
19
34
51
56
17
408
Total
142
464
total
Population
(1000 hab.)
1
3
5
2
6
4
19
34
107
425
5600
11000
8000
1500
2000
500
1400
1000
1600
3200
606
35800
Courbe rang taille des villes côtières en 1960, 1990 et 2020
avant ajout des villes non identifiées
Pop
1,E+07
1,E+06
Pop 1960
Pop 1990
1,E+05
Pop 2020
1,E+04
1,E+03
Rang
1
10
100
1000
58
Densité de population du littoral (hab/km²)Scenario tendanciel
Maîtrise des
déséquilibres
1950
1980
2000
2020
2050
2050
Bénin
Côte d'Ivoire
Cap Vert
Ghana
Guinée
Gambie
Guinée Bissau
Liberia
Mauritanie
Sénégal
Sierra Leone
Togo
233
59
52
99
43
86
21
28
0
75
61
193
428
190
72
263
164
229
32
69
29
172
109
499
846
398
108
489
290
512
56
127
70
290
182
1054
1800
700
100
800
500
900
100
200
100
500
300
2000
4000
1400
200
1800
1000
2000
300
500
200
900
700
4000
3600
1300
200
1600
1000
1900
300
500
200
900
600
3700
12 pays côtiers
55
140
264
500
1000
900
3.1.6. CONSUMPTION OF SPACE BY THE TOWNS IN THE COASTAL ZONE IN THE MEDIUM AND LONG
TERM
For the future, it is acceptable, as indicated previously, that per capita urban land use will increase at half the
speed of modern urban productivity (per urban capita GDP of the modern sector), and therefore at an
average rate of 1% per year over the long period, in conformity with the past trend noted by the
AFRICAPOLIS study. According to this conservative hypothesis, the urban footprint should reach an
average 260 mƒ per urban dweller in 2020 and 350mƒ per urban dweller in 2050, for effectively
developed surface areas of 180 and 250 mƒ20.
Surface construite et urbanisée par habitant urbain (m²/hab.)
Surface construite (1)
ASS
PVD
Moyenne mondiale
Surface totale urbanisée (2)
1990
2000
105
105
155
150
125
185
210
Tx de crois.
1990-2000
3,6%
1,8%
1,8%
Projections
Taux de
crois. futur
2020
2050
1%
180
250
1%
260
350
The results by country, given in the following table, are rather worrying. Even in the “controlling
disparities” scenario, the area occupied by towns (built on or not) would cover practically the whole of
the coastal zone in the case of Benin and Togo, and between 40 and 60% in Gambia, Ghana, and C„te
d’Ivoire. These indications are obviously very rough. They may act as an incentive to review the concept
and boundaries of the "coastal zone": different criteria should no doubt be adopted depending on the country
and the physical and human conditions. The “coastal zones” of Benin and Togo should no doubt be more
widely reviewed, along with Ghana and C‚te d’Ivoire perhaps. However, natural spaces not authorised for
construction should also be integrated, which, on the other hand, reduce the “useful area” of coastal land for
urbanisation. Added to this are the perspectives of shoreline mobility, which will clearly not help to increase
the amount of land available.
20
Note that this hypothesis remains very conservative, for the expected growth rate of per capita Gross Regional
Product is perceptibly higher than the rates observed in the past.
59
This very global, provisional analysis tends to prove that the coastal zone could, in the long term, become
a practically uninterrupted conurbation from Ibo country in Nigeria to Abidjan in C‚te d’Ivoire, with
metropolises with populations of several million every 100 km or so, and hundreds of satellites and farming
towns serving the agricultural conversion areas, intensive farming areas and areas of industrial-type animal
production, leaving little place for the development of tourism and the conservation of nature.
A few hot spots of coastal development are also identified, and the countries where it is most urgent to
implement specific urban policies aimed at opening up to urbanisation places that are not equipped today or
are not very suitable for urban development, due to lack of access and infrastructure or due to breaks such as
those resulting from lagoons and coastal water features, or preferably towards the inland countryside,
thereby arranging the sensitive areas on the fringe immediately adjacent to the shoreline.
This analysis could also act as an incentive to examining the possibility of setting up instruments on a subregional scale, with the principal institutions in the region and their partners (such as the CILSS-SWAC
pair). These instruments would be capable of closely monitoring the settlement of the coastal zone and
driving suitable land development policies, but these different points will be broached at an earlier point.
Consommation d'espace des villes littorales
(en KM² et en % de l'espace
littoral)
en % du
littoral
Surface urbainsée (km²)
Etat actuel
Pays
Bénin
Côte d'Ivoire
Cap Vert
Ghana
Guinée
Gambie
Guinée Bissau
Liberia
Mauritanie
Sénégal
Sierra Leone
Togo
Surface de la
zone littorale
2664
14260
4030
12370
10760
1725
12590
14090
14550
15330
10990
1854
12 pays côtiers
115000
c o n s o m m a t i o n d 'e s p a c e u r b a i n e n
m ² /h a b
Surface
en % du
urbainsée
littoral
(km²)
Maîtrise des
déséquilibres
Scenario tendanciel
2000
2020
2050
2050
2050
2050
311
777
44
751
292
117
73
201
137
611
184
247
947
1908
96
1825
680
305
195
549
354
1396
431
625
3106
5621
196
5828
2228
982
732
1863
1057
3870
1560
1951
117%
39%
5%
47%
21%
57%
6%
13%
7%
25%
14%
105%
2671
4905
196
5012
1968
951
692
1710
995
3573
1392
1723
100%
34%
5%
41%
18%
55%
5%
12%
7%
23%
13%
93%
3746
9312
28994
25%
25788
22%
2 1 0
2 6 0
3 5 0
3 5 0
3.2. ECONOMIC OUTLOOK
The development, equipment and occupation of the coastal zones will also largely depend on how the
economic situation evolves, which will determine investment capacity, in particular. A tentative long-term
picture of the coastal economy and its implications is provided in a separate document with the
corresponding economic models in a spreadsheet. This is an attempt to produce a long-term picture
showing the necessity of being able to provide environmental support to changes that may be considerable
under the pressure of the markets.
60
The departure point for an analysis like this resides in the data and national accounts logs which mainly
measure cyclical economic fluctuations. The structural changes over a long period are more difficult to
apprehend. The WDI21 (for 2010) published by the World Bank show that average per capita GDP was
expected to return in 2008 to the level it was at in 1975 before the crisis… Nonetheless, the structure of
population settlement has changed considerably in the meantime, while these indicators suggest that the
informal per capita GDP would have declined by almost 50% in that period. Observation of the living
conditions in rural localities or in popular residential areas makes this assertion debatable. Similarly, the
apparent near stagnation of primary value added by agriculture – which would be reflected in the decrease
of more than a third in per capita agricultural production – should be questioned, while the average food
ration – which is still inadequate – improved in terms of both quantity and quality.
ASS : PIB primaire par habitant
USD constants base 2000 source WDI
USD/hab.
130
120
A/P
110
100
90
80
1960
1970
1980
1990
2000
2010
Année
The actual performances of African economies therefore seem to have been underestimated by these
indicators, which should be cross-referenced with population growth perspectives.
A second point concerns the possibility of an economic recovery, preceded by a take-off phase, a situation
experienced by many regions in the world (Japan, URSS, China, India, etc.). 4% growth in per capita GDP
in the economic take-off phase, then 6 to 8% in the recovery phase are the actual values observed in these
different cases. The scenario proposed here supposes that after a long period of institutional maturation,
assimilation of post independence conflicts and opening towards the world, the time would be right for subSaharan Africa to begin the economic take-off phase. There is nothing extravagant about GDP growth of the
order of 6% per annum until 2020 (or 4% for per capita GDP) then 5.7% from 2020 to 2050, summarised in
the table in annex 2, and this is compatible with the picture of population redistribution and urbanisation
mentioned above. This scenario would be reflected in a growth trend of the order of 4% per annum for
primary productivity (that is to say, per capita value added in the primary sector), 2 to 3% per annum for
average productivity and income in the informal sector (or, rather "popular" sector) and 0.5 to 1% per
annum of productivity in the “modern” sector.
One of the particularities of this reading concerns change in the population redistribution between the
primary and non primary strata of the population, or between the rural milieu and city, reflected in the ratio
between primary and informal productivities. Currently this ratio is above 2. The stabilisation of population
settlement should see it decline in the long term (to around 1.5 in 2050), as growth in the popular economy
is the result of several factors:
 A take-off in the agricultural economy enabled by the expansion of the domestic market and the
intensification of city/hinterland exchanges. The known limitations of natural capital (soil types,
21
World Development Indicators
61





agroclimatic constraints, saturation of land use) could, however, strongly hamper this
perspective22.
A slowing down of migratory flows towards the towns.
Growth in the size of towns and its effects on urban productivity.
An accumulation of capital and the valorisation of know-how developed in the course of the past
decades in a multitude of micro-companies and informal production units.
Increasing interaction between popular economy and “modern” economy.
A gradual restoring of the capital for local public investments (“IFL”) and improvement in local
governance as a result of decentralisation.
In this scenario, the growth of the non-primary “modern” economy draws on the change in urban population
involved in the modern sector. The sharing of territories (settlement, equipment), accompanied by the
corresponding public and private investment should enable the emergence of enterprises primarily centred
on the African market (a population of almost 2 billion), but also in turn taking advantage of the reduction in
comparative advantages in terms of labour costs in the countries with intermediate income levels, which will
also be confronted over this long term period with no less important environmental constraints. This
hypothesis of delocalisation of labour in low technology industries towards Africa should however be
weighted, taking account of the growing environmental pressure (carbon, hydrocarbons) of globalised
trading practices. In any case, such delocalisations would only concern the best equipped centres in terms of
services and infrastructure.
The persistent duality of the urban economy, both popular (informal) and “modern” would obviously be
reflected in the urban structures: “business districts” and residential areas with the standards of the
developed countries and vast peripheral areas with the minimum level of equipment compatible with the
local authorities' investment and management capacities and with the living conditions of the population. In
this intermediate phase of demographic transition, the disparities in productivity and income between the
strata (modern, informal and primary) of the metropolises, average towns, small towns and the rural milieu
could decline, but would remain high.
Between 2008 and 2050, the density of economic activity as measured by the Gross Local Product per km„
of coastal zone will be multiplied by a factor of 11 to 13. This is one of the parameters to be taken into
account when drawing up certain components of the coastal development schemes: definition of vocations
and rules for land use, zones devoted to heavy industry, practically non-existent today, urban environment
(industrial estates, waste management, water supply, treatment of waste water, lagooning, etc.) and
periurban environment (market garden belts, agri-towns, tourism areas, protected areas, etc..) development
and service policies, land ownership policies. Countries in which the industrial pressure on the coastal area
will be highest will be, like population, Benin (whose immediate proximity to Lagos makes the harbour area
of Cotonou practically an annex of Nigeria), Togo, C‚te d’Ivoire, Ghana and, more locally, Senegal with the
Cape Verde peninsula and the Grand Dakar district.
The need to accumulate private and public residential urban capital and the recurring cost to the local
authorities of maintaining public local investment capital evaluated previously in the case of Cotonou draw
attention to two critical issues for the sustainable development of the urban or economic zones:
 The balancing of development solutions that reconcile coastal development ambitions, goals in
terms of equipment standards and quality of service on the one hand, and the preoccupations and
priorities of governments and local authorities, and the capacity of local operators (enterprises and
households) to change behaviour patterns and bear any excess costs.
 The implementation of devices to mobilise local resources to face recurring expenditure and the
methods for accessing funding through borrowing on a par with needs, taking account of the
22
Giraud. P.N. & D. Loyer. 2006. – Capital naturel et développement durable en Afrique. In "A quoi Ša sert d’aider le
Suid ?"published by Serge Michailof.
62
contributive capacities of households and operators. We have seen that the cost of maintaining
public assets by the local authorities can and should therefore be wholly financed by a bearable levy
on the value of private assets, therefore with no external financing. It is clear that the main success
factor for the sustainable development of the coastal area would be to have all the local institutions
effectively capable of maintaining their capital, which is not the case anywhere in West Africa
today.
The possibilities of economic recovery could today act as an incentive to the countries in the region to
secure existing land reserves in the hinterland behind the coastal area that are strategic and equippable with
a view to setting up activity zones. This retreat to the hinterland would also be encouraged by the increasing
value of land, which would become prohibitive near the coast. This contrasted scenario also allows us to
highlight the critical issues for the management of an ultra-dense coastal zone towards 2020 and 2050, and
the necessity of preserving natural infrastructures and the corresponding ecological services, which will play
a fundamental role in supporting such developments.
3.3. TRANSPORT INFRASTRUCTURE
Colonisation in West Africa and the emergence of Nation States through independence led to the creation of
borders, the fragmentation and partitioning of a region that was originally roughly continuous, even though
the natural barriers, religious or linguistic contrasts and ecoclimatic gradients conferred great diversity on it.
Mobility has always been an essential and adaptive response for African societies to the extent that it is a
part of the subsistence and production strategy of many populations. The matrix of the territory of West
Africa, once structured by the rivers, landform, the edges of the major ecoclimatic zones and the distribution
of societies, from empires to forest and coastal micro societies, was brutally transformed with the
development of towns and the communication routes that are closely associated with them. The straight,
axial nature of urban development and the resulting connectedness of the urban areas (conurbations)
are today a major fact in most of the countries in West Africa, and this is particularly true in the coastal area.
3.3.1. ROAD AND RAIL NETWORKS
The road network, which is of varying serviceability, and for which we do not always have a reliable picture
on a regional scale (updated database23) is organised roughly into two types of routes:

Routes from the coast to the interior historically connecting West African port capitals with the
interior of the countries. Some of these old routes are paired with a more or less functional railway
line (Dakar-Tambacounda-Bamako; Conakry-Kankan; Abidjan – Bouak† –Ouagadougou) some of
which are essentially for the transport of ore (Conakry, Nouadhibou). In 1960, these “penetration
routes” were not connected to each other, except by the coastal route between Accra and Lagos,
linking Ghana, Togo, Benin and Nigeria. Another coastal road served a part of C‚te d'Ivoire from
Abidjan to San Pedro. In Senegal, the internal road system was already relatively structured.

East-West interstate connections of regional importance: Between 1960 and 1975 the East-West
interstate connections were developed with the connection of the whole area from C‚te d’Ivoire to
Nigeria. The network in Senegal was also improved, as was the interconnection of the network of
major cities in the non-coastal Sudanian-Sahelian countries.
The present day situation shows 3 major regional routes:
 The coastal route from Port Harcourt to Nouadhibou, with two functional interconnection
segments: Port Harcourt-Liberian border (Gulf of Guinea) and Kaolak-Nouadhibou in Mauritania.
23
Personal com. AFRICAPOLIS.
63
Between Liberia and Guinea Bissau, in many segments of the route serviceability is uncertain, and
the route is not coastal, since it bypasses Liberia and Sierra Leone and does not connect with the
coast. The same is true for the Boké (Guinea) to Bissau route. NEPAD is planning to build a transcoastal motorway link from Nouadhibou to Nigeria (6 lanes over a distance of 4,560 km at an
estimated cost of US $10 billion).
 The Sudanian-Sahelian route is more or less practicable from Bamako to Kano in Nigeria. The
link from Nouakchott or Dakar to Bamako is also more or less practicable, although conditions are
sometimes difficult. A project for a Trans-Sahelian route is under study by NEPAD, the DakarBamako segment is already scheduled.
 The Sudanian route, which links Kankan-Korhogo-Tamale-Kara-Parakou-Abuja. This route
should be consolidated and reinforced in the light of the force of attraction these zones represent for
agricultural and agro-food production.
A last route, still relatively undeveloped, could be described as post coastal. This route would
connect Nz†r†kor†, Yamussukro – Kumasi – Bohicon and could play an important role in the
integration of the area inland of the coast. It would constitute a pertinent, driving measure in
the reconfiguration of a future coastal area by relieving population pressure in the immediate
coastal fringe.
Figure 22. Position of the future three main regional road routes in West Africa.
(according to CEDAO/SWAC. Regional Atlas of transport and telecommunications in ECOWAS). For the moment, with
the exception of the C…te d'Ivoire - Nigeria coastal route, it is mainly North – South routes leading from the major sea
capitals to the capitals in the landlocked Sudanian-Sahelian countries which play a major role in regional exchanges.
The coastal region is reached in a variety of ways: either from a "comb tooth" system linking the coastal
access points to a road route parallel to the coast and situated further inland, or in some cases on the coast of
Senegal and countries in the Gulf of Guinea, directly by the coastal route.
On the straight stretches of sandy coast, traffic often uses the beach at low tide, with considerable impacts at
entry and exit points when the coastal rim dune system is crossed.
Coastal shipping is not widespread, except, subject to verification, in countries where the coast is largely
hemmed in (Liberia and Guinea Bissau).
64
Figure 23. Transport infrastructures in West Africa (according to SWAC).
65
Figure 24. Practicability of road routes in West Africa in 2005 (according to SWAC)
66
3.3.2. HARBOUR INFRASTRUCTURES
The countries in the region count 36 main identified ports (World Port Index), around fifteen of which
have volumes of traffic in excess of 500,000 tonnes. It is nonetheless difficult to envisage the West
African harbour network without including Nigeria, which is by far the largest port centre.
Most of these ports handle international or even intercontinental traffic, as the regional traffic is not
highly developed. These ports have a variety of different origins: built on the historical site of a
colonial harbour (Cotonou), established in a peri-urban zone (Lomé), genesis of a satellite harbour
town (Tema in the case of Ghana). In all cases, whether the ports were developed or extended in a
particular context (at least in the Gulf of Guinea), marked by a narrow shelf, low sediment stocks and
a strong longshore drift, these infrastructures have strongly disrupted coastal dynamics downstream of
and in the immediate proximity of the developments.
The picture of the ports of West Africa produced in figure 25 only partially illustrates this coastal
system, which should be completed with the multiple secondary and/or specialised harbour
infrastructures: ore wharfs, fishing ports that are roughly equipped and quays for landing catches,
coastal shipping points in particular in mangrove areas or border areas typified by informal trading
such as between Benin and Nigeria. Information on these secondary, but no less integral centres is not
directly available, and should be obtained from the national diagnostic studies that are in progress.
List of the main harbours on the coast of West Africa (source: World Port Index – WPI)
WPI
code
Name
Mauritania
45812
NOUADHIBOU
North
Latitude
Lat
Type
Lon
Type
Longitude
20,916
17,05
45814
Senegal
45818
45819
NOUAKCHOTT
18,033
16,0333333 North West
ST LOUIS
M BAO OIL
TERMINAL
DAKAR
16,0166
14,7166
16,5166667 North West
17,4
North West
Average
tide
(amplitude)
Size
Comments
Average
0.66
Restricted
Natural coastal
harbour
Open port
Restricted
Very restricted
Natural river port
Open port
0.33
0.33
14,6833333 17,4333333 North West
Average
0.66
45821
RUFISQUE
45822
KARABANE
45823
LYNDIANE
Gambia
45825
BANJUL
Guinea Bissau
45835
RIO CACHEU
45838
Bissau
Guinea
45850
KAMSAR
45855
CONAKRY
14,7
17,2833333 North West
12,5666667 16,6666667 North West
14,1666667 16,1666667 North West
Very restricted
Very restricted
Very restricted
Sheltered by jetty/
breakwater
Open port
Natural river port
Natural river port
16,5666667 North West
Very restricted
Natural river port
0.66
12,2833333 16,2333333 North West
11,8666667 15,6333333 North West
Very restricted
Very restricted
Natural river port
Natural river port
0.66
1.98
10,6333333 14,6166667 North West
9,51666667 13,7166667 North West
Very restricted
Average
0.99
1.32
45857
BENTI
Sierra Leone
45862
FREETOWN
9,16666667
Very restricted
Natural river port
Sheltered by jetty/
breakwater
Natural river port
0.99
45865
45867
Liberia
45940
8,58333333
7,53333333
Natural coastal
harbour
Natural river port
Natural river port
MONROVIA
6,31666667 10,8166667 North West
Restricted
0.33
45950
BUCHANAN
5,86666667 10,0666667 North West
Very restricted
Sheltered by jetty/
breakwater
Sheltered by jetty/
breakwater
45820
PEPEL
BONTHE
13,45
8,5
13,2
North West
North West
13,2333333 North West
13,05
12,5
North West
North West
Restricted
Very restricted
Very restricted
0.33
NA
0.33
0.66
0.99
0.99
0.99
0.33
67
WPI
code
45952
Name
GREENVILLE
45955
CAPE PALMAS
Cote d’ivoire
45963
SAN PEDRO
45965
SASSANDRA
45970 BAOBAB MARINE
TERMINAL
45990
PORT BOUET
46000
ABIDJAN
46007
LION TERMINAL
46008
ESPOIR MARINE
TERMINAL
Ghana
46040
TAKORADI
46045
SEKONDI
46063
46070
SALTPOND
TEMA
Togo
46090
PORT OF LOME
North
Latitude
4,98333333
Average
tide
(amplitude)
0.33
Lat
Lon
Type Type
North West
Size
Very restricted
4,36666667 7,71666667 North West
Very restricted
Comments
Natural coastal
harbour
Open port
4,66666667 6,61666667 North West
4,95
6,08333333 North West
4,95
4,53333333 North West
Very restricted
Very restricted
Very restricted
Open port
Open port
Open port
0.66
1.32
NA
5,23333333 3,96666667 North West
5,25
4,01666667 North West
5,03333333
4,8
North West
5,03333333
4,45
North West
Very restricted
Average
Very restricted
Very restricted
Open port
Port in lagoon
Open port
Open port
NA
NA
0.66
NA
4,88333333 1,73333333 North West
Average
Sheltered by jetty/
breakwater
Sheltered by jetty/
breakwater
Open port
Sheltered by jetty/
breakwater
0.33
Sheltered by jetty/
breakwater
Open port
0.33
Sheltered by jetty/
breakwater
Open port
0.66
4,95
Longitude
9,05
1,7
North West
Very restricted
5,2
1,05
North West
5,61666667 0,01666667 North East
Very restricted
Restricted
6,13333333 1,28333333 North
East
Very restricted
46095
Benin
46110
KPEME
6,2
1,51666667 North
East
Very restricted
COTONOU
6,35
2,43333333 North
East
Restricted
46115
SEME TERMINAL
6,3
East
Very restricted
2,65
North
0.66
0.33
NA
0.99
0.66
0.66
68
Figure 25 – Freight handling in the main ports of West Africa (2003). according to ECOWAS Transport Atlas (ECOWAS/SWAC-OECD)
69
These ports perform a variety of functions, some being almost exclusively for the handling of ore
(Nouadhibou and Kamsar), while other, hub ports are largely connected to the inland areas (Dakar,
Abidjan, Cotonou and Lagos). In the light of the recurring costs of these infrastructures and their
impact on the dynamics of the shoreline, we may wonder about the pertinence, from a regional
perspective, of such a legacy system in which some of the component elements even compete with
each other. The implementation of regional road (and/or railway) infrastructure planned by NEPAD
should help to rationalise the regional system with an increase in the hub role of certain major ports to
the detriment of ports whose existence is currently justified by the relative lack of a functional regional
road system.
It should also be noted that the growth in demand for raw materials (particularly the products of
mining) today leads to the possibility of the development of several more ore ports (Saint Louis for
phosphates, for instance, with the improved navigability of the Senegal river) or in Guinea Conakry
for the export of iron and bauxite. The essential role played by the private sector in implementing and
developing infrastructure should have as a corollary a building up of technical services in terms of the
strategic evaluation and tracking of impacts.
70
4. HAZARDS
4.1. ELEMENTS OF CLIMATE FORECAST
What kind of climate changes would have a direct effect on the state of the coastal sea and significant
impacts on erosion phenomena? Here is a summary of the different change scenarios to 2050 for the
different West African coastal climate profiles described previously.
4.1.1. UNCERTAINTY OF CLIMATE MODELS FOR WEST AFRICA
Global climate models (general circulation models) are solidly based on physical principles and are
capable of reproducing certain characteristics of past and present climates. Increasing confidence is
being accorded to Atmosphere-Ocean general circulation models (AOGCM) for their capacity to
estimate future large scale climate changes (Randall et al., 2007)24. The confidence in the forecasts of
certain variables such as temperature is better than for precipitations, for example. Thus the models
reproduce unambiguously the significant warming of the earth over the past few decades due to the
increased concentration of greenhouse gases (Randall et al., 2007). To assess the relative credibility of
the climate forecasts produced by the different models, it is assumed that the models capable of
reproducing past observations will also give the most plausible projections.
However, there are systematic biases in the simulation of the African climate by most of the climate
models that contributed to the 4th report of the IPCC (Intergovernmental Panel on Climate Change).
90% of these models overestimate the precipitations on a large part of the continent (Christensen et al.,
2007)25. The temperatures simulated also show bias, but this is not significant enough to call into
question the credibility of the projections.
The intertropical convergence zone simulated is moved towards the equator in most of these models.
The surface sea temperatures are overestimated by 1 to 2 degrees on the Gulf of Guinea. A large part
of these models have no monsoon, as they cannot properly reproduce the Northward movement of
precipitations on the continent. Only 4 of the 18 global ocean-atmosphere models in the 4th IPCC
report examined by Cook and Vizy (2006)26 are able to produce quite realistically the interannual
variability of surfacewater temperatures in the Gulf of Guinea and the dipolar structure of
precipitations between the Sahel and the Guinean coast.
4.1.2. CLIMATE CHANGE SCENARIOS
Future changes in climate depend on a number of natural and anthropic parameters. The most
important anthropic factor is the increase in greenhouse gases related to industrial activity. These
emissions depend in turn on a number of socio-economic factors, for which it is impossible to predict
future changes. This is why the IPCC produced a special report on emissions scenarios (SRES) to
enable the evaluation of climate change. Four families of scenarios were drawn up (figure 26). These
scenarios take into account various factors that could influence climate, such as population increase,
socioeconomic development or technological choices.
24
Randall, D.A. et al., 2007. Climate Models and Their Evaluation, Cambridge, United Kingdom and New York,
NY, USA.
25
Christensen, J.H. et al., 2007. Regional Climate Projections.
26
Cook, K.H. and Vizy, E.K., 2006. Coupled Model Simulations of the West African Monsoon System: Twentiethand Twenty-First-Century Simulations. Journal of Climate, 19: 3681-3703.
71
Figure 26. Schematic of the SRES scenarios.
4.1.3. PROJECTIONS TO HORIZON 2050
4.1.3.1. TEMPERATURES
The Intergovernmental Panel on Climate Change (IPCC) submitted its fourth report in 2007 and the
conclusions leave no room for doubt that the global climate is warming up, sea levels are rising and
the ice cap is melting (27).
Figure 27: Anomaly of temperatures observed and projected till 2100
compared to the 1900-1950 period for West Africa (IPCC, 2007).
According to this report, the decades to come will see warming of approximately 0.2ˆC per decade and
even if all concentrations of greenhouse gases were stabilised at their 2000 values, warming of around
0.1ˆC per decade would still be expected.
Maintaining the current pace of emissions would cause additional warming and would lead to
“numerous alterations to the global climate system in the course of the 21st century which would very
likely be greater than those observed in the course of the 20th century".
72
The projections for Africa show temperature rises which will very likely be higher than the global
average rise (Erreur ! Source du renvoi introuvable.8), with an accentuation on arid zones (figure
28). The average rise in temperature for West Africa on the 2050 horizon would be between 1.5 and
3°C.
4.1.3.2. PRECIPITATIONS
The global ocean-atmosphere model has more difficulty simulating precipitations than temperatures.
In several regions of the world, these models agree on the rise or fall in precipitations, but they diverge
greatly in their projection of precipitations in West Africa and the signal for variations in
precipitations on the Sahel and Guinean coast remain uncertain (figure 28).
The models therefore generate clear but contradictory answers. The extreme cases are the
GFDL/CM2.1 models, which predict a considerable reduction in the precipitations over the Sahel and
the Sahara, and MIROC3.2 which predicts strong increases. The overall average presents a downward
trend in JJA (June July August) precipitations on the West African coast to the North of the 10th
degree of latitude (figure 28), which is approximately the domain of the maritime trade wind and the
North of the Liberian-Guinean domain.
This decrease will be accompanied by an increase in the intensity of precipitations (figure 29) and a
reduction in the number of rain events (Tebaldi et al., 2006)27. In the South, on the domain of the
permanent Atlantic monsoon, the models do not agree on the change signal, even though the average
presents a slight upward trend.
The increase in the intensity of precipitations and the reduction in the return periods of certain extreme
events could cause the flooding of coastal zones and aggravate erosion phenomena (which could in
certain specific cases lead to increased siltation).
The expected global decline in rainfall in the course of the 21st century would cause a reduction in the
flow rates of the major rivers such as the Senegal and the Volta, which would be accompanied by a
sediment deficit and an aggravation of coastal erosion. To this should be added the influence of works
such as dams on these watercourses, which only aggravate the phenomenon.
27
Tebaldi, C., Hayhoe, K., Arblaster, J. and Meehl, G., 2006. Going to the extremes. Climatic Change, 79(3): 185211.
73
Figure 28: Projected changes in temperature and precipitations in Africa from multi-model simulations
of A1B scenarios. First line: change in annual average temperatures, DJF and JJA. Middle line: ditto
for fractional changes in precipitations. Third line: number of models projecting increases in
precipitations of the 21 (Christensen et al., 2007).
Figure 29: Average of all nine models (Tebaldi et al., 2006) of change in intensity of precipitations for
the 2080-2099 period compared to 1980-1999 (A1B scenario). The black dots show places where
more than half the models project a significant change.
74
4.1.4. THE SOUTHERN OSCILLATION (ENSO)
The El Niño phenomenon effects climate in places all over the world including Africa, where it is
accompanied by extreme events such as drought in West Africa. Van Oldenborgh et al. (2005)28
estimated the changes in the variability of ENSO in a future climate. The projected changes differ
from one model to another, however. Based on the 6 models out of 19 the most apt to reproduce the
current variability of El Nino, they found no statistically significant change. The uncertainties are too
high to permit an estimation of the future intensity of the El Nino/La Nina phenomena. The model
however projects a future weakening in the coupling of El-Nino and the monsoon (Philip and Van
Oldenborgh, 2006)29.
4.1.5. FREQUENCIES OF EXTREME EVENTS
Among the most important extreme events affecting the coasts of West Africa, the episodes of intense
precipitations, depressions and tropical storms can cause considerable damage. There is disagreement
between the different studies on the projected frequency of extreme events (including cyclones) as a
result of global warming. There seems to be more of an agreement on their increase in intensity
because of a perceptible increase in the temperature of marine surfacewaters.
The theory indicates, and the observations and results of models confirm, that a consequence of global
warming will be more violent cyclones. A quite recent study (Webster et al., 2005)30, based on satellite
data, has shown that the number of category 4 cyclones has almost doubled since the 1970s, although
there is no significant trend for the total number of these events over the same period.
In any case, it is difficult to establish series of data that lend themselves to statistical analysis for
events whose extreme nature has for corollary a low frequency.
Storm surges depend greatly on local conditions, in particular bathymetry and related tidal regimes.
This means studies of storm surge statistics are specific to each region and can not be generalised.
4.1.6. SIGNIFICANT WAVE HEIGHT
There are a limited number of studies on wave climatology projections (Weisse and von Storch,
2010)31. Most of the studies use statistical disaggregation to project the significant height of waves in a
future climate (Caires et al., 200632; Wang et al., 200433).
These studies allow for a considerable increase in the significant height of waves in the North Atlantic,
consistent with the deviation of storm paths towards the poles. These studies do not predict an upward
trend in low latitudes. For West Africa, the change will therefore come especially from the increase in
the frequency and duration of tidal wave events, in particular related to extreme events.
28
Van Oldenborgh, G., Philip, S. and Collins, M., 2005. El Nino in a changing climate: a multi-model study. Ocean
Science Discussions, 2(3): 267-298.
29
Philip, S. and Van Oldenborgh, G., 2006. Shifts in ENSO coupling processes under global warming. Geophys.
Res. Lett, 33.
30
Webster, P., Holland, G., Curry, J. and Chang, H., 2005. Changes in tropical cyclone number, duration, and
intensity in a warming environment. Science, 309(5742): 1844.
31
Weisse, R. and von Storch, H., 2010. Marine Climate Change : Ocean Waves, Storms and Surges in the
Perspective of Climate Change. Springer Verlag.
32
Caires, S., Swail, V. and Wang, X., 2006. Projection and analysis of extreme wave climate. Journal of Climate,
19(21): 5581-5605.
33
Wang, X., Zwiers, F. and Swail, V., 2004. North Atlantic ocean wave climate change scenarios for the twentyfirst century. Journal of Climate, 17: 12.
75
4.1.7. TREND: RISING SEA LEVEL AND STORM SURGES
The fact that sea level is rising seems to have been largely confirmed. The historical tide gauges show
a rise in the average level of the sea of 20 cm over the past 100 years. Current estimates for the end of
this century are between 20 and 50 cm. Much more dramatic estimates evoke (as a conservative
estimate) a rise of 3.3 metres should the West Antarctic ice sheet disintegrate completely34.
4.1.7.1. NON UNIFORM SPATIAL DISTRIBUTION OF SIGNAL
The spatial distribution of the sea rise signal is far from uniform. First of all, the surface of the oceans
is not regular and for example in the subtropical Atlantic we note a convex area of approximately 1
metre in elevation. This spatial distribution also depends on climate variability and the hazards of
marine circulation. These spatial disparities were already observed in the data for the decade 19932003.
At regional level, this rise can significantly deviate from the global average due to little known local
factors such as land subsidence, the change in atmospheric circulation and wind regime, the
redistribution of atmospheric pressure or the unequal distribution of thermal expansion. Our current
state of knowledge does not permit more accurate estimations.
Figure 30: Map of the geographic distribution of the speed of variation of sea level 1993-2007) according to
Topex/Poseidon and Jason-1. We observe that the speed of elevation can vary by a factor of 5 from one region to
another. This distribution appears to be quite stable between 1993 and 2007. LEGOS/CNES/IRD/CNRS http://www.legos.obs-mip.fr/fr/
34
Bamber. J.L. 2009.- Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice
Sheet. Science. 324: 901-903.
76
4.1.7.2. THE CAUSES
The causes of the rise in sea level are multiple and can be summarised as follows:
 Thermal expansion of ocean waters (variations in volume)
 Exchange of masses of water with the continental zones (variations in mass).
A few facts: the tidal gauges provide relative data but the Topex Poséidon and Jason-1 satellite programmes
provide absolute data. Between 1993 and 2006, Topex evaluated the rise in sea level at 3.3 mm +- 0.4 mm per
annum. The contribution of thermal expansion is estimated at 50% between 1993 and 2003 compared to 25%
between 1950 and 2000. The contribution of water exchanges is estimated at 40%. The literature also mentions
the moderating impact of major dam projects that have been built in the course of the 20th century.
Figure 31 - Different components of the rise in sea level between 1960 and 2000: thermal expansion in the first
700 m (red), thermal expansion in deep waters (orange), Antarctic and Greenland ice sheets (light blue), glaciers
and ice caps (dark blue), land retention (green). according to Domingues. C.A. 2008.- Improved estimates of
upper-ocean warming and multi-decadal sea-level rise. Nature. 453: 1090-1094p.
The rise in sea level is therefore the consequence of several concurrent phenomena. The greatest
contribution comes from thermal expansion due to global warming, then, in second position comes ice
melt. Some causes, like thermal expansion due to inertia, will be active for several centuries whatever
the perspectives for the reduction of greenhouse gas emissions.
According to the IPCC's 2007 report, in 2009-2010 average sea level will have risen by around 18 to
59 cm compared to 1890-1999. By 2050 the rise will be of the order of 10 to 20 cm (Erreur ! Source
du renvoi introuvable.32). This rise does not take into account the probable acceleration in ice melt,
which could add a further 10 to 20 cm. There is a lot of uncertainty surrounding these values, which
could be exceeded (Meehl et al., 2007)35.
35
Meehl, G.A. et al., 2007. Global Climate Projections.
77
Figure 32: Past, present and future changes in average sea level
compared to 2000 (IPCC, 2007
4.1.7.3. ALARMING CONCLUSIONS
The erosion and flooding (submersion) of coastal areas which largely contributes to the receding
shoreline will be aggravated in the course of the 21st century following an increase in average sea
level.
Africa is one of the regions in the world whose coastal zones and deltas are the most exposed to risks
of flooding due to the rise in mean sea level (Nicholls and Tol, 200636). This rise in sea level,
combined with increased intensity or frequency of extreme events, will have serious consequences for
the development of the coastal zone. Many coastal or island areas will be submerged or subject to
increasingly frequent flooding causing considerable damage.
In West Africa, although this rise cannot be estimated accurately, a rise greater than the global average
is expected. There could be dramatic consequences for several regions, such as around Nouakchott
which is already below sea level. Major conurbations are at risk. The destructive effect of this rise in
water level will lead to an increase in the frequency of storm surges and their destructive
potential, in particular in river deltas.
There will be more frequent intrusions of saline waters which will gradually make aquifers unfit
for consumption and agriculture (the advancement of the salt-water wedge and alteration of
freshwater lenses).
36
Nicholls, R. and Tol, R., 2006. Impacts and responses to sea-level rise: a global analysis of the SRES scenarios
over the twenty-first century. Philosophical Transactions A, 364(1841): 1073.
78
Figure 33. This graph shows the changes in mean sea level since 1993 according to Topex/Poseidon and Jason1. The mean increase observed between January 2003 and end 2007 was 3.06 +/- 0.4 mm/yr. From this value
should be subtracted post glacial rebound, estimated at –0.3 mm/yr), which leads to a rise in seal level of ~3.3
mm/yr for the last 15 years (Cazenave and Nerem, 2004, Ablain et al. in progress, 2008). Source:
LEGOS/CNES/IRD/CNRS - http://www.legos.obs-mip.fr/fr/
4.1.7.4. FORESEEABLE CONSEQUENCES OF THE RISE IN SEA LEVEL COMBINED WITH STORM
SURGES
The consequences are extremely difficult to evaluate and should only be envisaged through a detailed
study of local situations. There remains the hypothesis of a 1 metre rise in sea level would
significantly aggravate coastal hazards. The most sensitive coasts are clearly low sandy coasts and
mangroves, as well as coastal zones composed of sandstone or marno-limestone cliffs. The major
lagoon systems will also be affected. The lowest sectors will be subject to increased erosion or
temporary or permanent submersion.
Coastal systems are not in fact passive with regard to the rise in sea level, and there are numerous
threshold effects (“the coast has a rebound capacity"). For example in the case of the submersion
hazard, coastal plant formations can trap sediment, river flow rates can be modified by the variability
of continental precipitations, lagoon or estuary outlet streams can be partially closed by the advance of
sand spits, etc.
Any evaluation of the impacts of the rise in sea level should therefore remain cautious and avoid
swinging into simplistic, reductionist or "mechanical" calculations or representations, in particular in
the field of economics. The submersion hazard, when the stakes justify this, can only be properly
considered through a local hydraulic approach.
Sandy coastline: Increasing erosion of sand systems is expected, aggravating the risks of submersion,
in particular in type 3 units of the typology given in annex 1, causing the shoreline to recede as has
already been observed. It would appear that the very special conditions for the application of Bruun's
rule make it impossible to quantify the receding in a realistic way37. A possible increase in storms and
37
The Bruun rule does not take into account longshore transfers. Slott. J. 2003.- Shoreline Response to SeaLevel Rise: Examining the Bruun Rule. Nicholas School of the Environment and Earth Sciences
79
exceptional weather events should also have an impact on the plant systems of dunes, reducing their
fixing action. This vegetation will also be disturbed by increased salination of the freshwater lenses
and aquifers behind the dunes. Halophytic plants will be favoured. Over the long term, for sandy
beaches, the erosion rate is considered to be double that of the rise in sea level38; 70% of sandy
beaches in the world are being eroded.
Dune ridges and lidos: lidos and dune ridges will migrate inland, at least for the narrowest lidos.
Certain lidos will be fragmented. A tracking programme should enable local identification of the lidos
able to migrate and those likely to fragment.
Figure 34. Manner of lido migration subsequent to a rise in sea level
(according to Titus. J.G. 1990.- Greenhouse Effect, Sea Level Rise,
and Barrier Islands: Case Study of Long Beach Island, New Jersey.
Coastal Management. 18: 65-90.
.
Lagoon systems: the hydrology of lagoons comprises exchanges with continental waters, but also
with the sea. In addition, these are located at a height close to sea level. The ecology of lagoons is
based on two main parameters: depth and salinity.
If the former is slightly modified (the migration of the lagoons, if this is possible, should conserve the
initial depth gradient in most cases even if the depth of the lagoons increases slightly), salinity would
be altered (i) following the rise in sea level; (ii) by the salinisation of aquifers; (iii) by a possible
decrease in the freshwater supply consecutive to a reduction in rainfall and therefore in flood peaks.
On the other hand, the tendencies for lagoons to fill in by supply from the land could be partially
counteracted.
Closing of lagoon outlets: the closure or strangling of lagoon outlets due to the development of spits
and local accretions lead to the eutrophication of the aquatic milieus concerned. The filling in of these
outlets also implies flooding in periods of spate.
Other wetlands: different behaviour patterns are possible depending on each local situation:
 The spontaneous receding of wetlands inland, made possible in certain cases by the
topography and lack of obstacles. Tracking is possible, in particular on the basis of the
analysis of the distribution of halophile plant communities.
Department of Earth and Ocean Sciences.
38
Zangh K. 2004.- Global warming and coastal erosion. Climatic Change. 64: 41–58, 2004.
80
 The elevation due to high sediment supply (vertical accretion), in particular in delta systems,
which can largely compensate for the rise in sea level.
 Wetlands can gradually disappear by submersion.
In every case, considerable changes should be anticipated in the composition of the flora and fauna of
these ecosystems, which will depend on each local situation.
Conservation: the destruction of wetlands by submersion during exceptional events can cause
irreparable damage to natural infrastructure systems39 as was shown by the consequences of cyclone
Katrina.
5. ISSUES AT STAKE
The analysis of the issues at stake and their spatial distribution shall be conducted in phase III of this
study, in particular depending on the results of the national diagnostic studies, but also depending on a
systematic interpretation of the way the coastal area is used on the basis of satellite pictures.
39
Marris. E. 2005. The vanishing coast. Nature. 438. 908- 909p.
81
6. RESPONSES
The social, political and technical responses to coastal erosion are multiple, ranging from coastal
development to independent adaptation strategies implemented by the local populations under threat.
These responses can also be regulatory or legal. In the future, they will above all be based on the
precautionary principle and the awareness of risky areas when locating human settlements. We know
in fact that rights do not always create facts, and that regulations are no substitute for a change in the
behaviour of individuals, local authorities, institutions and politicians. Note that this study is part of
the response on a regional scale.
The provisions related to the applicable law in coastal spaces can only be broached once the results of
the national diagnostic studies have been received. It is therefore in phase III, with the reception of the
national diagnostic studies and the results of specific studies, case studies and socio-economic studies,
that the different manners of response will be tackled.
However, it may well be useful at this stage of regional pre-diagnostic study to identify three regional
bodies likely to play key roles in response to the impacts of coastal dynamics.
6.1. SUBREGIONAL ORGANISATIONS
6.1.1. UEMOA
Article 4 of the UEMOA Treaty of 10 January 1994, at the initiative of which the present study is
conducted by IUCN, mentioned the necessity of taking the environment into account as part of the
coordination of national sector policies, without considering it a priority. Additional Protocol no. II
approaches the environment on the basis of "sector", as mentioned din chapter IV. Combating coastal
erosion is listed as one of the Union's environmental improvement goals.
Recommendation no.02/97/CM of 21 June 1997 adopted in Lom† is relative to the implementation of
a first generation environment programme. It determines different components and the measures to be
developed to undertake the harmonisation of national policies in these domains. There are eight
identified sub-programmes, including combating coastal erosion. The 1999 study relative to erosion
problems, validated in 2005, led the Council of Ministers of UEMOA to adopt rule
02/2007/CM/UEMOA aimed at implementing a coastal erosion programme.
The implementation of a community policy for environmental management (UEMOA - PCAE
politique commune d’am•lioration de l’environnement – common policy for improving the
environment), rendered by legal instruments (directives, rules, decisions) remains a necessity, given
the observed divergences between the environmental policies of the member countries.
The framework offered by UEMOA is certainly adequate for rationalising the development of coastal
areas and encouraging in the long term the sharing of certain large infrastructure systems, and
encouraging solidarity and reciprocity between coastal territories. This framework remains limited to
Member countries and should be reinforced by the effective action of other regional organisations such
as the Abidjan convention.
6.1.2. GUINEA CURRENT LARGE MARINE ECOSYSTEM (GCLME):
There have been two phases of the GCLME project since 1995, broaching multiple aspects of the
integrated management of coastal zones, including coastal erosion One of the important contributions
82
of the project is the existence of a shared diagnostic conducted on all the countries concerned, the
number of which has increased from 6 to 16 (Guinea Bissau to ANGOLA). The project took place
within the framework of the application of the provisions of the Abidjan convention and the
implementation of its Strategic Action plan. The GCLME also gave rise to the setting up of an interim
commission (Interim Guinea Current Commission), whose first ministerial meeting took place in
Abuja in 2006.
The valorisation of the results from the numerous workshops GCLME has conducted on a range of
topics (fishing and fish stocks, prevention of the impacts of pollution, particularly hydrocarbons;
coastal erosion; urban and domestic effluent management, etc.) should be envisaged as part of the
present study, but will require access to detailed items over and above the regional summary reports.
6.1.3. CANARY CURRENT LARGE MARINE ECOSYSTEM (CCLME):
The programme of the Canary Current Large Marine Ecosystem (CCLME): this programme,
also financed by the GEF, is in the start-up phase and will include Mauritania, Senegal, Gambia and
Guinea Bissau in particular.
6.1.4. THE ABIDJAN CONVENTION
The Abidjan Convention on cooperation for the protection and development of marine milieus and coastal areas of
West and Central Africa, (here known as the Abidjan Convention) and the Protocol relative to cooperation in terms
of combating pollution in the event of a critical situation (here designated as the Abidjan Convention Protocol)
were adopted on 23 March 1981 in Abidjan and entered into force on 5 August 1984. The Convention was signed
as part of the UNEP regional seas programme.
The Abidjan convention is the benchmark legal instrument for the governance of coastal and marine
milieus in West and Central Africa (from MAURITANIA to NAMIBIA). This geography covers three
major marine ecosystems: the Canary Current, the Gulf of Guinea Current and the Benguela Current.
Only fourteen countries out of 22 potentially concerned satisfied the conditions that place them
officially in a position as Parties to the convention (in the table below countries noted in grey are not
Parties to the convention).
Country
Mauritania
Senegal
Gambia
Guinea Bissau
Guinea
Sierra Leone
Liberia
Cote d’ivoire
Ghana
Togo
Benin
Date of signature of the convention
Date of ratification
23 March 1981
13 June 1981
10 May 1983
December 6 1984
23 March 1981
4 March 1982
7 June 2005
22 March 2005
15 January 1982
20 July 1989
16 November 1983
17 October 1997
23 March 1981
23 March 1981
23 March 1981
23 March 1981
23 March 1981
The initial foundation of the convention mentions two objectives: (i) the fight against forms of
pollution; (ii) the building of regional cooperation to protect the marine and coastal environment.
However, unlike many regional conventions, the Abidjan convention does not place constraints and in
this sense is closer to the spirit of the major Multilateral Environment Agreements.
There is an interesting item in the fight against pollution, consisting in inviting the countries to use
rational management for environmental questions, by not taking any measures that would result in the
“direct or indirect transfer of prejudice or risks from one zone to another ...”. In the case of coastal
83
erosion, the application of such a disposition would certainly have made it possible to balance the
transboundary effects of certain developments (ports, dams, etc.). Additional protocols expressing
concrete measures (such as the Protocol relative to “existing or potential situations critical for the
marine environment which constitute a significant threat of pollution”) are examples of provisions that
could be envisaged in relation to coastal erosion.
Harmonisation of environmental policies concerning the marine environment is also the concern of
other regional institutions such as UEMOA. The value added of regional coordination of
environmental policies is also derived from scientific and technical cooperation. The possibility of
exchanging data on the monitoring and evaluation of pollution could also be applied to the
problematic of coastal erosion and coast line development. Useful regional studies were conducted
within the framework of the UNEP regional seas programme - WACAF - (1982-1988), although today
the effective use made of their results should be assessed.
The internalisation of the provisions originating from the present study could be conferred on an ad
hoc task force reporting to the Secretariat and the Conference of the Contracting Parties. The recent
transfer of the Abidjan convention Secretariat is another disposition, which will contribute to the
efforts to revitalise the convention and deploy its system in the countries of the region.
This revitalisation programme, voted at the 8th conference of the contracting parties in Johannesburg in
November 2007, comprises different sections, including the improvement of the financial viability of
the convention.
In conclusion, the Abidjan convention could well be an adequate framework for improving
regional governance in terms of combating coastal erosion, for example through the adoption of a
specific protocol. In the light of the integral nature of many of the provisions to be adopted, which are
aimed in particular at land-use planning, in this additional protocol the convention should effectively
target the domains of interest that are actually within its jurisdiction, such as the regional coastline
monitoring programme, for example.
6.1.5. REGIONAL PROJECTS
The regional programme Adaptation to Climate and Coastal Change in West Africa (ACCC):
the adaptation to climate change programme is financed by the GEF and covers Cape Verde, Gambia,
Guinea Bissau, Mauritania and Senegal. It allows for the implementation of pilot protection and
adaptation operations in the five countries concerned. The implementation of the current phase will
end in 2010.
The West African Regional Coastal Zone and Marine Conservation Programme (PRCM). The
second phase of the PRCM began in 2008. On the basis of basket fund financing, the programme
includes more than twenty projects concerning biodiversity, the networking of marine protected areas
in West Africa, and the sustainable development of fisheries in West Africa.
84
7. TOWARDS A REGIONAL COASTAL EROSION
PROGRAMME
The results of this study should lead to a set of recommendations able to frame and guide the
implementation of UEMOA’s regional programme to combat coastal erosion. In this current stage of
regional pre-diagnostic it is obviously premature to identify the measures this plan will comprise.
Nonetheless, a global architecture could be envisaged that would comply with best practices in terms
of risk prevention.
Associating a shoreline monitoring programme with a coastal development scheme focuses on coastal
erosion, this coastal risk prevention programme would incorporate three major compartments deigned
to mutually reinforce each other:
 Protection – attenuation of the impacts related to shoreline dynamics
 Watch – Vigilance – Knowledge of the hazards and experience feedback.
 Preparation of populations and decision-makers.
These three main lines would be underpinned and stimulated by regional provisions regarding coastal
risk management, in application of a regional strategy of which the present study would contribute to
identifying the main elements. This regional plan should contribute to a regional policy of global
security for the population and coastal areas. The provisions and recommendations that will
originate from this should also be made compatible with the provisions that have already emerged
from other regional initiatives (Abidjan Convention, GCLME). The points on which the harmonisation
of policies could be given priority should be identified.
The structure of regional proposals emerging from the study would make it possible to centre and
develop synergies with the other components of the UEMOA PRLEC.
85
Figure 35. Organisation of the regional coastal erosion programme
7.1. SECTION 1: PROTECTION AND ATTENUATION OF IMPACTS
The protection of coastal areas, support to development, and the attenuation of the social and
economic impacts of the shoreline dynamics should be envisaged on two separate spatio-temporal
scales:
7.1.1. STRATEGIC MEASURES (DEVELOPMENT SCHEME)
These measures will originate from a cross between the acceptance of (i) the specific features of a
future coastline the study of the sensitivity of which will help to draw the contours and (ii) the social
and economic issues associated with the development of coastal countries, apprehended prospectively.
These measures will be essentially rendered in strategic terms and in terms of land-use planning.
The major development solutions that may be proposed will also allow a valorisation of natural coastal
areas for the ecological services they offer in terms of risk prevention and sustainable valorisation of
natural resources.
86
7.1.1.1. COASTAL DEVELOPMENT SOLUTIONS: TOWARDS A FUTURE COAST OR ANTICIPATING
RETREAT
Developing a strategic retreat: the expected advance of the shoreline, which will vary according to
the site, should lead us to envisage, wherever possible, redrawing the contours of a future coastline,
arranging a natural "buffer" space between shore and risk. The width of this zone should be calculated
depending on different elements:
 The extent should not only depend on the concern for the safety of people and goods, but also
on the dynamics specific to coastal ecosystems. The areas necessary for pushing back
wetlands or lagoons will often be at least as important as in the case of dune formations. It is
therefore a question of arranging the space necessary for natural coastal systems to adapt
spontaneously preserving the way they operate and the ecological services they offer.
 One question that is essential concerns the preservation of wetlands, both for the ecological
services they offer, but also for the resources, in particular the fishing resources they provide.
In this respect, conservation actions are justified anew, by helping to maintain these complex
systems in an operational state and therefore preserve their capacity to adapt to change, which is also
related to the diversity and complexity of the mosaic of wetlands. In accordance with this objective,
the prevention of the fragmentation of wetland systems is also important and should be taken into
account not only in coastal defence actions, but also when developing the use of land.
The justification of arranged retreat should be supported by an analysis of the costs-benefits
incorporating the ecological services rendered by wetland areas.
7.1.2. PRIORITY MEASURES (LOCAL EROSION MANAGEMENT)
Operational measures: In the light of the national diagnostic studies and case studies, this study will
help to identify certain priority zones where development actions could be launched. These measures
to protect against-attenuate coastal risks will be accompanied by a review of the existing provisions
and developments and their assessed medium term impacts (i) as regards solving the problems that
motivated them; (ii) as regards the unplanned consequences recorded on the sites following these
developments. This review may or may not be exhaustive, but will draw on the data collected in the
coastal countries, but also on the valorisation of experience acquired through other programmes such
as the European EUROSION programme.
7.2. SECTION 2: WATCH – VIGILANCE – KNOWLEDGE OF THE HAZARDS AND
EXPERIENCE FEEDBACK
7.2.1. TOWARDS A STRATEGIC REGIONAL OBSERVATORY OF GROWTH AND SETTLEMENT IN
THE COASTAL ZONES
87
The observatory function should also take into account changes in the population settlements in the
coastal zones and their growth and development in the widest sense. The paradigm of economic
recovery can be debated, but it remains one of the credible hypotheses and the monitoring of the way
land is used in the coastal zones constitutes a necessity for anticipating unresolvable situations
accompanied by late decision-making. It would be useful to envisage this role, which could be filled
by institutions such as the Sahel and West Africa Club (OECD) situated at the interface between the
countries on the one hand and their technical and financial partners on the other.
7.2.2. THE COASTLINE MONITORING PROGRAMME
This programme should be organised on the basis of the networking of researchers and institutes
working on the subject in each country. It could include (i) fundamental research or completed
research carried out by doctoral students on the different topics related to coastal dynamics, but also
(ii) systematic, regular tracking of changes in the coastline in the different sediment cells.
This coastline monitoring is only of interest if it leads regularly updated information that is made
available to the regional decision-making bodies and the different countries. The Regional coastal
observatory which UEMOA is planning to fund could legitimately run this information
collection and presentation process.
The networking of African researchers on coastline dynamics would also make it possible to have
technical and scientific resources that could be mobilised at regional level, able to intervene in various
frameworks (assistance with decision-making, assessments of the developments budgeted by PRLEC,
or simply to procure greater depth of knowledge on coastline dynamics).
An essential point in the setting up of a programme for monitoring the coastline would be the
identification of the scales of times and space targeted. On scales inferior to a decade, the observation
of local sedimentation dynamics may be acceptable, but on longer scales (and larger areas), the whole
of the coastal system should be taken into account (coastal tract), including the area behind the coastal
zone and the foreshore.
Figure 36. The whole morphological sequence to be considered
for the long term prevision of coastal dynamics (coastal tract).
according to Stive. M. 2004. How important is climatic change for coastal
erosion? Editorial comment. Climatic change. 64: 27–39.
The tracking of the positive and negative impacts of existing developments could also be included in
the remit of the coastline monitoring programme.
88
7.2.3. NECESSARY MONITORING AND REINFORCEMENT OF MARINE PROTECTED AREAS
(COASTAL SENTRIES)
The third component of this tracking system concerns the marine protected areas of West Africa
through the RAMPAO network. The monitoring of these natural areas and of the pressures that bear
on them also constitutes an important indicator of the changes in the coastal zones.
7.2.4. EXCEPTIONAL EVENTS AND STORM SURGES: DEVELOP MONITORING FUNCTIONS AND
WEATHER ALERTS
The last component concerns the setting up of an alert system to signal exceptional events and storm
surges (perhaps accompanied by a regional marine weather mechanism) in order to deliver timely,
quality information, for all of the coastal zones in West Africa, regarding the occurrence of these
events, which can take on dramatic proportions as witnessed by the information produced by the first
national diagnostic studies which are currently being processed.
The ViGiRisC40 programme, which is in the start-up phase, is an opportunity for perfecting such a
system, all the more so since the funding of the storm surge pilot component of this programme, which
was to be carried out by a Mauritanian project, has been postponed for administrative reasons.
7.3. PREPARATION OF POPULATIONS AND DECISION-MAKERS TO COMBAT
COASTAL EROSION
Preparation of populations and decision-makers: the fight against coastal erosion should be
organised at all levels and will require considerable efforts in terms of informing and raising the
awareness of all the categories of people confronted with it on the one hand; in terms of organising the
governance of this specific risk on the other hand at national level, for example through legislative and
regulatory measures and impact studies. This section of the programme should be considered crossover in relation to the others.
Different levels, approaches and products will be considered in terms of capacity building depending
on the different categories of players. The partnership with existing programmes will be turned to
advantage (ANCORIM) for designing the regional plan.
7.4. IMPLEMENTATION OF REGIONAL COASTAL RISK GOVERNANCE BODIES
It is imperative that the response to and prevention of risks related to coastal dynamics should include
a regional dimension, if only for the coordination of major developments with transboundary impacts
and for the analysis of the risks and impacts related to these developments.
In the light of their interdependence, the coastal areas in the sub region should be considered as a
single, consistent, functional entity, whether for the management of shared sediment reserves or
renewable natural resources (populations of migratory fish for example, spawning and reproduction
grounds of regional importance). These regional discussions should also permit economies of scale at
40
"Vigilance et Gestion Intégrée du Risque Climatique", programme funded by the French Fund for the Global
Environment and run by ACMAD. This programme is also conducted in liaison with the continental CLIMDEV
programme (BAD, OUA, United Nations Economic Commission for Africa).
89
the level of each country and the possibility of solutions for progress such as the sharing of certain
major harbour infrastructure, for example.
The proposed plan could be the subject of preparation of project sheets in a stage subsequent to the
completion of the study. As this study was carried out under the aegis of UEMOA, it so happens
that some of the countries concerned are not in fact members of this Union. If some of the study
recommendations should lead directly to the mobilising of means allocated to UEMOA PRLEC to
combat erosion, it would appear to be a possibility that the regional dynamics generated by the
study will evolve and continue to grow, including with non-member countries.
90
8. OUTLOOK AND CONTINUATION OF THE WORK
The publication of this pre-diagnostic report brings to a close phase II of the study, which was initially
scheduled to end in mid February 2009 (outlook validated at the regional workshop in September
2010). Phase III of the programme is centred around the same components, whose activities will be
continued, with an objective of finalising the work of diagnosis, and drawing up a provisional
development scheme. In parallel, networking activities are continuing and given concrete form
through the implementation of several tools designed to be updated and used in the future.
Some delays may have been experienced, in particular in the response times to the call for proposals
concerning the case studies, and in the conducting of the national diagnostic studies, for which it took
longer than expected to identify the national teams (see below the outline of conditions at end
February 2010). These delays make the decision to adjust the initial completion deadline for the study
to the end of 2010 appropriate, to overcome the difficulties involved in coordinating teams from
eleven different countries.
8.1. PROGRESS REPORT AND STRUCTURE OF PHASE III
Phase III will comprise different sections:






Finalisation of the national diagnostic studies and of the regional diagnostic study
The conducting of case studies and the layout of the maps for their presentation.
Validation and finalisation of the 1:250,000 scale map.
Cartographic interpretation, drawing up and rafting of the provisional development scheme.
Production of the cartography of the development scheme on a scale of 1:500,000.
Various communication and technical and scientific networking operations on a regional
scale.
This phase will extend from 10 March 2010 to 1st September 2010. The deliverables expected are the
following:





Final regional diagnostic study.
Final cartography of sensitivity at 1:250,000.
Final case studies
Provisional development scheme and recommendation for shoreline monitoring.
Cartography of the development scheme at 1:500,000, provisional version.
This third phase will be completed at the beginning of September 2010 by a regional seminar to
present the results.
For each of these sections, information is provided below regarding (i) the state of progress of the
work; (ii) the actions planned for phase III.
91
8.1.1. FINALISATION OF THE NATIONAL DIAGNOSTIC STUDIES AND OF THE REGIONAL
DIAGNOSTIC STUDY
8.1.1.1. PROGRESS REPORT
This regional pre-diagnostic study fixes a general framework which should be enhanced and
completed (i) by the date from the national diagnostic studies; (ii) by the systematic analysis of the
issues at stake for the coast, which remains to be carried out for the 1:500,000 cartography for the
development scheme. The current situation with the national diagnostic studies is summarises by
country in the table below:
Country
Mauritania
Senegal
Gambia
Guinea Bissau
Guinea
Sierra Leone
Liberia
Cote d’ivoire
Ghana
Togo
Benin
Commencement
of work
Work
commenced
Work
commenced
Work
commenced
Work
commenced
Work
commenced
Work has not
commenced
Work
commenced
Work
commenced
Work
commenced
Work
commenced
Work
commenced
Stage
report
Not
received
Received
Received
Received
Received
Received
Received
Not
received
Not
received
Received
Provisional report
Comments
-
Increasing delay
Received and
satisfactory
Received, being
checked
-
Final report underway, digital
cartography delivered
Satisfactory progress in the light of
delayed start-up
Late
Received being
checked
provisional report
pending
Provisional report
to be completed
Not received
Received end
February 2010
Received and
satisfactory
No satisfactory quotes and proposals
from a consultant
Progress compliant
Completed provisional report pending
Increasing delay but started late
Provisional report being checked
Satisfactory progress pending final
report
8.1.1.2. WORK TO BE CONDUCTED
TASK
Finalisation of national diagnostic
reports
Support for finalisation of national
diagnostic reports (reviews)
Finalisation of the regional
diagnostic and integration of
contributions from national
diagnostics
Drafting of chapters on issues and
coastal land use
Comments
Diagnostic reports
underway in 10 countries
Spread out according to
reports from national
teams
Following exploration of
the coast by satellite
image (see 1:500,000
cartography)
41
OPERATOR
Teams of national
consultants
EOS.D2C
DEADLINE
15/04/2010
EOS.D2C
15/05/2010
EOS.D2C
30/05/2010
15/04/2010
92
Integration of corrections and
further detail obtained from
circulation of the regional prediagnostic
Conducting of the socio-economic
study on Togo
Conducting of the socio-economic
study on Senegal
Database of national consultants
Additional contacts to be made
(universities, operators in Europe
and West Africa)
National consultants and
EOS.D2C
30/03/2010
M. KOLOR
30/03/2010
M.L. de Noray Dardenne
30/03/2010
EOS.D2C
EOS.D2C
30/03/2010
15/04/2010
These contacts will
provide greater depth on
the key points of the
diagnostic.
8.1.2. LAUNCH OF THE CASE STUDIES
8.1.2.1. PROGRESS REPORT
The terms of reference of the call for proposals were published, and the call for proposals launched.
Several responses were recorded within the allotted time. For different reasons 4 responses arrived
late. These are being examined.
List of proposals for case studies received within the allotted time
Project owners/Parent structures
Design office IRC-CONSULTANT
Title of the study
Country
concerned
Evolution of the coastal are of Nouakchott: characterisations
and associated risks
Mauritania
Dynamics of the beach and coastal ridge at the level of coastal
Aboubracry THIAM, IUCN consultant
Mauritania
artisanal fishing development points: case of PK 100 and 144
south of Nouakchott
Mame Demba THIAM, Ma‹tre de
conf…rences, Tahirou DIAW, Professor,
Department of Geography, Cheikh Anta
DIOP University, Dakar
Coastal erosion and changes in the shoreline in the light of
natural and anthropic processes. (Example of Pointe SarŒne to
Joal-Fadiouth – Sangomar and Bargny Gueth, Senegal)
Senegal
Boubou Aldiouma SY et al., Laboratory
Leidi, Geography Section, Gaston
Berger University of Saint Louis
Study of the dynamics of the coastal ridge of the Saint Louis
Gandiolais, at the mouth (Tar…)
Abdoulaye NDOUR, Doctoral student,
Changes in the coastline from 1954 to 2009 and impacts of
Department of
protection works on the Rufisque coast
Geology, Cheikh Anta DIOP University,
Dakar
Sadou BARRY et al., researchers,
Centre for Scientific Research ConakryRogban… (CERESCOR)
Erosion of the coasts of the rice plains of Kaback, prefecture of
For…cariah (Republic of Guinea)
CONAKRY
Ibrahima DIANE et al., researchers,
Centre for Scientific Research Conakry-
Guinea
Morphodynamic study of the coast of Koba
Rogban… (CERESCOR)
Ernest KONAN, doctoral student,
Evolution of the shoreline and sediment dynamics of the coast
Abidjan Oceanology research Centre,
from Abidjan (Cˆte d'Ivoire) to Afor…nou (Ghana)
University of Cocody
C•TE D’IVOIRE
93
KOUADIO SALOMON doctoral student,
Abidjan Oceanology research Centre,
University of Cocody
Characterisation of the morpho-sedimentary dynamics of the
western coastal perimeter of Cˆte d’Ivoire (Case of the coastal
perimeter of San-Pedro)
Adot… BLIVI et al., Centre for Integrated
Coastal morphodynamic cells and referential profile of land use
Coastal and environmental
between the Volta and Mono rivers
Togo
Management, University of Lom…
Characterisation of the sediment dynamics of the sector of
OYEDE LUCIEN MARC et al., Earth
sciences Department, Abomey-Calavi
coast adjacent to the mouth of the Mono - Couffo rivers in
Benin
southwest Benin (Gulf of Benin, West Africa)
University
Hubert OSEI-WUSUANSA, Water
Directorate,
Impact of good management and construction practices on the
Ministry of Water Resources, Works &
implementation of shoreline stabilization works – A case study of
Housing, Accra
the Keta sea defence project
Ghana
Ernest KUSI-MINKAH, Hydrological
Services Department, Accra
These proposals were all assessed by the GEOMER laboratory in Brest. The budgets were rather high
with risks of duplicating actions, so some of the operators were asked to revise their proposals prior to
contractualisation.
A first mission of the GEOMER laboratory took place in Dakar at the CSE in January 2010. This
mission should give rise to two reports which are in progress and which concern:
 The cartography guidelines for case studies to be carried out with CSE in Dakar.
 The guidelines for the presentation and format of cartographic data to be produced by the
operator for cartographic production by CSE in Dakar.
8.1.2.2. WORK TO BE CONDUCTED
TASK
Production of case studies
Cartography of case studies
Comments
The case studies should
be launched in March
2010 if administrative
constraints are overcome
Spread out according to
reports from national
teams
Creation of the coastal erosion
portal and of the database and
meta-data accessible on the
web.
Networking of coastal erosion
researchers
CSE will produce a
methodology proposal for
this networking in March
2010.
OPERATOR
Case study teams
DEADLINE
15/05/2010
GEOMER – CSE Dakar
15/06/2010
GEOMER – CSE Dakar
15/06/2010
GEOMER – CSE Dakar and
case study teams
30/06/2010
94
8.1.3. 1:250,000 CARTOGRAPHIC ANALYSIS
8.1.3.1. PROGRESS REPORT
A first model of this cartography should be delivered by IUCN with this report. It comprises the whole
of the digitised interpretation of classes and segments according to the typology given in annex 1. This
model is provisional and should be checked (including by the national consultants). It constitutes and
intermediate product for the preparation of the 1:500,000 cartography for the development scheme.
8.1.3.2. WORK TO BE CONDUCTED
The 1:250,000 cartographic analysis should be fully checked and verified by the authors, and in
particular depending on the remarks of the national consultants. It should be completed in
accordance with the following elements:





Results of the national diagnostic studies (in particular developments and at risk zones).
Hierarchical hydrographic network, toponymy.
Results of the stage/training in progress on mathematics applied to remote sensing.
Geology depending on additional documentary work.
Results of the case studies
A finalised map surround comprising the insertion of illustrated inserts on the case studies, and a
breakdown into regular map sheets - the reproduction and digital conversion will be done once the
model has been finally validated.
The paper format for the map sheets will be selected at the regional seminar at the end of phase II,
as will the scale (a final hard copy at 1:500,000 could be easier to handle if the units remain
clearly visible – the 1:250,000 format would amount to around 15 metres of map sheets!).
TASK
Preparation of a base map for
verification
Review by national consultants
Review and verification of the
provisional model for phase II and
corrections.
Detail of urban spots
Toponymy of caps and localities
Preparation of the final rendering
Printing of final map
Drawing up of meta-data
Comments
To be produced using
AFRICAPOLIS urban spots
OPERATOR
EAM - GEOME
DEADLINE
15/03/2010
National consultants
EAM-GEOME
30/04/2010
30/03/2010
EAM-GEOME and
trainee from CNRS
Laboratory
EAM-GEOME
EAM-GEOME
EAM-GEOME
EAM-GEOME
15/04/2010
30/03/2010
15/05/2010
15/05/2010
15/05/2010
8.1.4. 1:500,000 CARTOGRAPHY OF THE DEVELOPMENT SCHEME
8.1.4.1. PROGRESS REPORT
95
A first report map base at a scale of 1:500,000 was prepared, combining the different layers considered
indispensables, but these should still be competed or corrected. The 1:250,000 sensitivity analysis will
enable the cross-referencing of issues with the vulnerability which is the basis of the rough draft of the
development scheme.
8.1.4.2. WORK TO BE CONDUCTED
Preparation of the base map
Many layers of the base map will be taken from the 1:250,000; certain layers are still to be assembled,
however, corrected if necessary and finally validated:
 Hierarchical road network: the mission has not yet been able to obtain a proper layer of the
road network (the AFRICAPOLIS study in fact points out the deficiencies in this domain42).
Nonetheless, it will be established, if necessary from scanned paper documents.
 Toponymy (in two stages: 1 – Placing of the toponymy; 2 – Selection of the pertinent
toponymy).
 Hydrographic network (same as for the 1:250,000, but to be hierarchised)
 Ports and major infrastructure (on the basis of the national diagnostic studies)
 Coastal developments (on the basis of the national diagnostic studies)
 Protected areas: from the database of national diagnostics, as the data from the World
Commission is not sufficiently reliable. The ones to be selected will be only those in contact
with the shoreline. For Senegal, the protected areas will be taken from the recent 1:200,000
map.
 Discrimination of urban spots from the results of the remote sensing stage which is in
progress.
Cartographic analysis of the issues with a view to producing the development scheme
The cartographic analysis of issues will be performed through:
 A full sweep of the study zone on Google Earth with transfer to the base map.
 Refining of the data depending on the results of the national diagnostics.
 A tentative classification of the density of settlement and of the methods of use of rural zones.
With a view to crossing the issues – vulnerability, the segment system of the 1:250,000 map will be
simplified, and featured on the 1:500,000 cartography of the development scheme.
The indicative typology for identifying critical coastal issues will be organised as follows:
CLASS
“OFFSHORE” FEATURES
Harbour
infrastructures
COMMENTS


International transit port
Ore port
42
AFRICAPOLIS, and Nelson. A. & al. 2006. - Towards Development of a High Quality Public Domain Global
Roads Database. CIESIN. Columbia University.
96
Tourist facilities
Biodiversity and
heritage












“OFFSHORE” FEATURES
Road coverage
Hydrographic network
Main catchment areas
Dams (if information available)
Habitat





Port with local traffic, coastal shipping
Equipped fishing port
Principal points for fishing catch landing on beach and commercialisation
(if possible) important sites of artisanal transformation
Professional tourist hotels (international class)
Local, especially urban
Residential, small hotels
Tourism heritage site under or undeveloped
Approximate boundaries of protected areas
Zone with conservation potential
Historical heritage sites
Other sites of particular interest for landscape
AFRICAPOLIS distinguished urban spots (type of habitat, un-built land)
AFRICAPOLIS Periurban extensions of agglomerations
Dense coastal habitat (type of plantations, palm tree groves)
Averagely dense coastal habitat
Sparse or uninhabited
The schedule of deadlines is as follows:
TASK
Preparation of the working base on a scale of
1:500,000
Digitisation/updating of road network
Identification and discrimination of methods of land use and
issues (interpretation of satellite pictures)
Toponymy
Hierarchising of hydrographic network
Digitisation of protected areas
Digitisation of land use and issues
Digitisation of the information from the national diagnostic
studies (at risk zones, extreme events, etc).
Digitisation of the development scheme
Formatting and printing of provisional document (cutting into
sheets, map surround)
OPERATOR
EAM - GEOME
DEADLINE
15/03/2010
EAM - GEOME
EAM-GEOME
30/03/2010
15/05/2010
EAM-GEOME and trainee from
CNRS Laboratory
EAM-GEOME
EAM-GEOME
EAM-GEOME
EAM-GEOME
15/04/2010
EAM-GEOME
EAM-GEOME
30/06/2010
15/07/2010
30/03/2010
30/04/2010
30/05/2010
30/05/2010
8.1.5. CONCEPTION OF THE DEVELOPMENT SCHEME
The development scheme will be drawn up on the basis of the cross-referencing of the issues observed
and the vulnerability of the coasts exploited through a 1:250,000 map.
The development scheme document will comprise (i) strategic measures; (ii) priority measures will be
illustrated by the 1:500,000 map. This document, which will be produced in phase III, will be a
provisional document intended to be presented at the regional seminar in September 2010. The
preparation process should extend from April 2010 to July 2010.
8.1.6. COMMUNICATION AND NETWORKING
97
A set of contents will be produced by the different people involved in the study, enabling distribution
by means of a newsletter ((4 editions – April – May – June – July). The distribution of this newsletter
will justify the creation of a regional contacts database (technical and scientific services, NGOs), but
also including contacts relative to coastal erosion in the Northern countries.
A web portal will be developed to be linked to the portal set up by CSE for the date of the case studies.
This portal will provide access to the final products of the study. Setting it up early will allow time for
adequate referencing.
8.1.7. PHASE IV AND FINALISATION OF THE STUDY
The following products will be presented at the regional meeting in September 2010:
 Final regional diagnostic study
 Final 1:250,000 cartography
 Recommendations for combating coastal erosion and the provisional version of the
development scheme accompanied by 1:500,000 maps
 Results of the case studies
 Detailed products of the national diagnostic studies
 Specific, studies, in particular socio-economic ones
The validation of these products will make it possible to begin the last phase of the study which
will aim to produce a final version of the recommendations for controlling coastal erosion and
the provisional version of the development scheme accompanied by 1:500,000 maps. These
documents will be presented in the form of an illustrated summary suitable for a wide distribution.
These final products will be presented at a regional workshop at the beginning of December 2010
and, depending on UEMOA's expectations, again for the departments of UEMOA and/or the
Union's Council of Ministers.
9. ANNEXES
9.1. CARTOGRAPHIC INVENTORY OF TYPES OF COAST AND APPROACH TO
THEIR GEODYNAMICS IN RELATION TO COASTAL EROSION
98
This cartographic inventory has four objectives:
 Define a uniform reference framework based on a typology applicable to the whole of the West
African coast, in order to place in context and compare the various local manifestations of
coastal erosion.
 In the selected typology, take into account criteria of exposure to potential natural risks of
human settlements on the edge of the coastal area or likely to be built there in the future.
 Diagnose the methods of sediment transport from the continent to the coastal area and their
contributions to the conditions of equilibrium or disequilibrium in interaction with the coastal
drift and sediment transport currents.
 Highlight elements of reflection on the possible impact on coastal geodynamics resulting from
hypotheses about rising sea levels.
The work of cartography has been presented on a scale of 1:250,000 (which means, for the study
zone, approximately 16 metres of map) and a summary work on a scale of 1:500,000 is planned.
Aware that a scale of 1:250,000 does not provide the accuracy of LANDSAT satellite image
interpretation required on a number of sites to present the context of erosion, the diagnostic study was
completed (as it was impossible to locate all the information on the map) by means of a systematic
“zoom” of the coast at a scale of +/- 1:25,000 (using tools such as Google Earth, among others).
The layers of vector information used for the transfer of this inventory are the following:
Coastline
Bathymetry
Hydrographic network
Protected areas
Roads and transport
Agglomerations with populations over 5,000
Hypsometry
NOAA (scale 1:75,000).
Curves generated from the 30 arc seconds
model of the General Bathymetric Chart of the
Oceans –– IOC/IHO UNESCO
HydroSHEDS USGS Data
IUCN World Protected Areas Commission
VMAP 1 and 0
GEOPOLIS
Curves generated from the digital terrain model
de SRTM (CGIAR-CSI)43 from 90 metres
The proposed typology of the coastal systems centres around two complementary readings expressed
on the 1:250,000 provisional geodynamic analysis map accompanying this report:
 The nature of the shoreline (typology of sea fronts): analysed through the evaluation of around
300 segments in 22 geomorphologic classes, including dynamics and sensitivity.
 The analysis of the 25 km coastal fringe with evaluation of sedimentological characteristics
and sensitivity to coastal erosion.
9.1.1. TYPOLOGY OF SEA FRONTS
Description
“Apparent”
coast regularly
covered by tides
Code
Summary
500 000
1A
Code
analysis
250 000
B2
1B
1C
B3
B4
Definition and geodynamic coastal context
Exposure to risks of human
settlements
Erosion and sand or mud siltation subject to the
interaction of coastal currents and tidal currents
serving the mangrove channels and any
continental tributaries they may have.
1A: Evolving but permanent and continuous
unsuitable for human
settlement except flood
protection and agriculture
43
The quality of this digital terrain model was confirmed in a study performed by CIAT: Comparison of SRTM derived
DEM vs. Topographic map derived DEM in the region of Dapa.
99
(intertidal zone,
typically
mangrove)
2A
B1
2B
A2
A3
A1
Naturally highly
unstable sand or
mud coast
Sandy coast with
straight
longitudinal
profile backed
onto sandy
formations
3A
3B
3C
Undulating coast
with rocky
headlands and
sandy coves
Rocky coast
4A
C1
D1
E1
C2
D2
E2
C3
D3
E3
F1
F2
4B
G1
G2 0
5
H1
H2
mangrove barrier.
1B: Undergoing great change, erosion or
accretion, mangroves discontinuous or absent.
1C: Narrow mangroves and coast type 4.
Islets, narrow coastal ridges in islands or
adjacent to mangroves.
Very active erosion and siltation.
Forest exposed to storm
hazards and erosion (fishing
villages and tourist areas)
Estuary sites with complex fluvio-marine
dynamics, spates, low water flows, tidal and
coastal currents leading to areas that are
evolving constantly
Coastal drift current and sediment transport
always present and in variable intensity, shaping
sandy rims and terraces with no rocky obstacles.
3A: Presence of back-lagoons and canals.
3B
3C: very narrow coastal rim with continuous
edge of lagoons and channels
High risk of fluvio-marines
flooding, very active
topographic change.
Dynamics of coastal currents regulated by the
headlands and rocky obstacles which act like
groynes.
4A: coves with wide radius curvature backed
onto sandy terraces and rims.
4B: Frequent rocky headlands with sandy coves,
in places less exposed to dominant ocean
waves.
Rocky coast, small sandy beaches in places.
Erosion variable depending on the degree of
resistance and alteration of rocks.
Risk of storm hazards and
erosion variable on the scale
of the sites. High for building
on the beach and altering
coastal current system
High generalised exposure
to storm hazards. Risks
related to erosion very
variable following small
regions and sediment
reserves. High risk of breach
of rim in 3C. High impact of
any development that alters
coastal transit.
Risk limited to loose or
weathered rock.
This table includes (column 2) the simplified typology of sea fronts which will be used for the
1:500,000 cartography (development scheme maps).
9.1.1.1. MUD OR SANDY COAST REGULARLY COVERED BY THE TIDE
This is an “apparent” coast materialised in a line of mangroves. The “apparent” shore lies under the
elevation of the highest seas.
B2
APPARENT COAST OF MANGROVES MATERIALISED BY CONTINUOUS BARRIER
OF TREES
Definition and geographic location: this is a “fictional” shore as it is subject to daily flooding and evacuation by
the tide (intertidal zone).
The border of mangrove trees materialises a fragile balance between:
o
o
a height subject to tidal flows that is acceptable for the maintaining of the aerial roots of the mangrove
trees anchored in unconsolidated sandy mud soil rich in organic matter.
The maintaining of the sediment deposit process favoured by the root network of the mangrove trees,
with the maintaining of water circulation to ensure oxygenation of the soil milieu.
100
The map boundaries of this category were obtained by generating a contour at 0 m from a digital terrain model
(SRTM30). However, at the scale of 1:250,000 the shores of many small interior channels of the mangroves have
not been drawn.
The confrontation between the contours generated from the DEM and the spectral response of the limits of the
mangrove areas match perfectly.
The mangroves in Senegal, Guinea Bissau, Guinea CONAKRY and Sierra Leone are largely concerned by this
category. In addition to the mangroves on the sea front, this also covers the apparent shores of the estuaries of
mangroves, channels and rias that penetrate deep into the continent, which means a considerable extension of
this category.
Coastal geodynamics: the genesis of the evolution of this type of coast is governed by a number of factors,
which must be grasped on a local scale. The main factors are summarised as follows:







44
The dominant influence of the CDC-ST , or their absence.
The height of the tidal range (typically very high in Guinea and Guinea Bissau).
The interaction at the level of the estuary between CDC-ST and the flow and flushing effect of the
mangrove channels.
The sediment supply transported towards the ocean from the continent, low towards the rias with small
catchment areas or towards low-lying sediment trapping catchment areas, important for certain river
outlets, in particular the Corrubal in Guinea Bissau or the Jong in Sierra Leone.
Change through meander formation in the mangrove channels with continuous destruction and
reconstruction of the mangrove shores.
The impact of recurring drought on the balance of mangrove shores (Sine Saloum, Casamance).
Lastly, in proximity to the major agglomerations or fish processing sites, the destruction of mangrove
trees for firewood.
Human activities on the edge of the coastal area and issues: continuous susceptibility to tidal flooding
excludes any type of habitat. To this constraint is added the non consolidation of the mud soil unsuitable for
installing breakwaters.
The shore is therefore only attainable by waterway as part of the activities fishing or gathering utility wood, sticks
and firewood from the mangrove trees (in difficult exploitation conditions).
A real boundary of practicability between the land and water activities, access to the shore is developed locally on
the sites with the most favourable topography for access to pirogues.
Rise in sea level hypothesis: possible retreat of the line of mangrove trees but adaptation possible depending
on the ratio between the speed of sea level rise and that of siltation.
B3
APPARENT COASTLINE WITH DISCONTINUOUS BORDER OF MANGROVES
AND/OR COASTAL MUDFLATS.
Definition and geographic location: Only the most extensive segments (more than 3 km) have been traced;
they appear to be associated with gently sloping mudflats continued on the edge of the foreshore by mud shoals.
This situation was observed in Guinea CONAKRY, Guinea Bissau and Sierra Leone, but is relatively rare
compared to the percentage occupied by continuous mangroves. This category also encompasses the special
case of the mud shores of the Banc d’Arguin in MAURITANIA.
Coastal geodynamics: it is difficult to assess here beyond some hypotheses that should be confirmed to explain
this type of landscape and the following two cases are possible:


Sites where the mangrove barrier is being destroyed by erosion and/or alteration of the milieu, or in
some cases even by anthropic destruction.
Sites which are being colonised by the mangroves, as the sediment balance conditions have changed
due to sediment trapping by shoals.
101
Human activities on the edge of the coastal area and issues: same considerations as for category B2, the
reconstitution of a shore of mangroves in periurban sectors where the mangrove has disappeared due to wood
cutting is not yet on the agenda (as has been done in South-East Asia).
B4
APPARENT COASTLINE TYPEFIED BY THIN FRINGE OF MANGROVES
BACKED AGAINST LOCALLY ROCKY HILL TOPOGRAPHY.
Definition and geographic location: On the ocean front, this type of coast is only rarely present, except in
Guinea Bissau in the Bijagos Archipelago. On the other hand, when the shores of the rias that penetrate inland
are taken into account, the frequency of this type of coast in combination with the type B2 is much more
important.
The mangrove population is an average 50 to 200 metres wide to the inland shore of the highest waters, located
at the foot of hills. This contact area may have, at a shallow depth, the presence of levels of hardpan at the
bottom of the slopes.
Coastal geodynamics: In the case of the Bijagos, it seems that this type of shore is prolonged on the foreshore
and sometimes further out to sea by rocky banks that facilitate siltation which is then colonised by the mangroves
as soon as the conditions allow it. These shallows play the role of "submerged groynes”.
Human activities on the edge of the coastal area and issues: Due to the narrowness of the mangrove barrier
to be crossed, these sites are often used as routes for taking pirogues between the sea and the habitat located on
the nearby hills (frequent case in Guinea Bissau).
Rise in sea level hypothesis: situations vary depending on the sites and the context of current and sediment
systems: mangroves regaining hold by a return of ocean wave heights, oxygenation of silt milieus or continued
regression of mangroves due to a tidal range that has become too extreme.
9.1.1.2. SANDY OR SILT COAST LOCATED IN NATURALLY HIGHLY UNSTABLE SITES
SANDY OR SILT COAST ASSOCIATED WITH MANGROVES AND NOT COVERED BY THE TIDE
B1
SPITS ADJACENT TO MANGROVES
Definition and geographic location: this type of coast is very close to the characteristics of those
mentioned for A2:
o
o
Narrow and very low sand rims behind the beach.
Presence of mangrove regularly reached by the tide that border towards the continent the
coastal rims and recent sand deposits.
The typically straight character of the longitudinal profile of the shore is the first criterion that
differentiates this category from A2 and the break between B1 and B2 is sometimes somewhat
approximate.
Guinea Bissau and Guinea possess the major part of this category.
Coastal geodynamics: the largely predominant influence of the CDC-ST without obstacles explains
the straight shoreline, even though this current redistributes sediment from the mangrove channels.
Furthermore, the angle of the shore to the direction of the ocean waves is constant in B1 whereas the
undulating profile of A2 type coasts is much more contrasted locally.
102
Human activities on the edge of the coastal area and issues: fishing conditions and practices are
very close to those of A2. The foreshores and shallows are less diversified in B1, so this context is
probably poorer in the biodiversity of fauna and les sheltered than certain sites in category A2.
Rise in sea level hypothesis: Same as for A2, retreat with disappearance of rims but long term
reconstitution at the expense of the mangroves.
A1
ESTUARY AND COASTAL RIVER COASTS
Definition and geographic location: with the exception of the vast estuaries of the Gambia, Senegal, Corrubal
and Konkour„ rivers, these estuary sites present similar schemas whatever the extent of the catchment area:
o
An estuary mouth that is opened by the waters from the continent and by the tidal flux for the largest
rivers.
o
Two spits on either side of the mouth, sand rims shaped by the drift current, the sediment transported by
which is enriched by river sediment supply. The sand rim and the beach have a straight longitudinal
profile facing the ocean and more or less steep curvature at the estuary mouth.
o
Behind the sand rims and in parallel to the estuary is a complex of wetlands, salt marshes or freshwater
marshes with the sequences of vegetation adapted to the local hydrological and topographic conditions.
In addition to the major rivers (Cavally, Sassandra, Badama and Volta), these estuary zones are particularly
present on the coasts of Liberia, Western C…te d’Ivoire and Ghana. They are associated with the geological and
topographic conditions of the Precambrian basement (craton) with a high density hydrographic network supplied
by abundant precipitations.
The estuaries of Senegal and Sine Saloum present a slightly special schema, marked by a long, narrow spit
parallel to the shore and estuary that is undergoing cyclical movement.
Coastal geodynamics: the estuary sites are constantly evolving in interaction with:
 The coastal drift current and sediment transport (CDC-ST) which tends to close the estuary outlets by
constitution (and often reconstitution) of the coastal rim.
 The hydrological regime of the coastal rivers related to the seasons with periods of spate maintaining or
reopening outlets and low water flow periods during which they tend to be filled up by longshore
sediment transport.
 The result of these two contradictory, perpendicular forces reflects the local variability (extent of
catchment areas of the coastal rivers, exceptional spate events and the corresponding flows). For many
small coastal rivers, the transit of flows to the sea is occasional, through the temporary opening of an
outlet or percolation of rainwater (stored in a perilittoral lagoon) through a rim (frequent case for the
coasts of Liberia and West C…te d'Ivoire).
 The role of tidal currents for the largest estuaries, which interact with the river flows, particularly during
high water periods.
 The occasional role of certain storm surges opens a breach in a fragile coastal rim, thereby favouring a
new outlet for the estuary lagoons, which receive the waters originating from the continent.
Note also the impact of changes in coastal geodynamics for the estuary sites of regulated sites (system of flood
peaks and sediment transfers) by large dams (Bandama and Volta for example).
NOTE THAT COASTAL AREAS CLOSE TO ESTUARIES HAVE A HIGH NATURAL INSTABILITY
THAT IS DIFFICULT TO CONTROL.
Human activities on the edge of the coastal area and issues: Estuary zones are precious sites for biodiversity
which is favoured by the salinity gradient and fluvial-marine ecotone:
103
o
o
a place of exchange, reproduction and migration of the aquatic fauna native to sea, brackish waters
and fresh water.
Highly diversified plant communities and habitats in lagoon areas and peripheries.
The exploitation of these biological resources has long been an incentive to the population to settle permanently
or temporarily on these sites which are also favourable for the sheltering of fishing vessels and for access to
navigable rivers. Places for exchange, trading, halts for changing transport vehicles and colonial trading posts; the
human settlement of estuaries has also been facilitated by the availability of fresh water resources.
The response to the constraints and risks associated with the dynamics of change of these fragile milieus has
traditionally been resolved by the movement and mobility of "light" human settlements. The same can not be said
for the "heavy" infrastructure (harbour, urban, hotel facilities), the sustainability of which is confronted with change
forecasts that are unreliable at the level of detail of the sites. (History of Grand Bassam or of the tourist village of
Assinie in C…te d’Ivoire). These questions are all the more important as these sites, due to their landscape
diversity, the proximity of still water lagoons and sheltered interior beaches, often have a greater tourism potential
than the surrounding coastal areas.
Rise in sea level hypothesis: Increased instability of spits, reconfiguration and migration towards interior of perilittoral wetlands
A2
COAST LINE OF SANDY POINTS AND WIDE ESTUARY ZONES OF MANGROVE
CHANNELS
Definition and geographic location: this type of coast is characterised by typically curved
longitudinal profile, with the curvature tracing an angle close to 90† in places, at the level of the
estuary mouths of large mangrove channels.
The coast is adjacent to very low altitude sand deposits, either very recent or current, that in places
trace topography of ridges and channels whose width varies from around a hundred metres to a
kilometre, in contact with older sandy terraces or mangrove that is regularly reached by the sea.
This situation is represented particularly in Guinea, Guinea Bissau and the Sherbro islands in Sierra
Leone.
Coastal geodynamics: the constitution and balance of these coastal profiles is the result of
interaction between the coastal drift current and the tidal currents (tidal ranges often exceed 5 metres
in these zones) of the large mangrove channels at the outlets of their wide sea estuaries, or even
beyond their direct influence. The resultant force of these two forces of opposite directions is by nature
unstable at the level of detail of the sites concerned.
Human activities on the edge of the coastal area and issues: this type of coast rarely possesses
soil suitable for agriculture, is hemmed in and in an almost insular situation, with very low drinking
water resources, and is not very favourable to permanent human settlement.
On the other hand, it offers preferential conditions for fishing activities with camps of fishermen who
are quite often migrant and from other countries.
Rise in sea level hypothesis: Disappearance of very low sand spits, reconfiguration of spits further
inland with the retreat of the mangroves.
A3
COASTLINES OF SANDY ISLETS AND INSULAR SPITS
Definition and geographic location: this category comprises coasts that are predominantly sandy,
but very complex individually, including, in places, mudflats and mangroves that have developed
typically in an insular situation. This is the case for the Turtle Islands in Sierra Leone, the small islets
104
in the Bijagos archipelago in Guinea Bissau, and the narrow insular sandy spits of Sin„ Saloum in
Senegal.
Coastal geodynamics: the emergence of these islets is typically favoured by the presence of rock
outcrops or hardpan which encourages the trapping and depositing of sediment. As a general rule,
these islets are the tips of surrounding shoals that are emerged at high tide. In places, they are the
relict of larger, insular spits. Their instability and that of their shores is certainly high and obeys a
current system that is probably complex and different from the two categories described previously.
Human activities on the edge of the coastal area and issues: the surrounding shoals are important
for the diversity of marine biological communities and are known to local fishermen. The emerged tips
are the site of fishermen's camps.
The Parc National Marin des Poil‡o (Bijagos Islands) encompasses this type of site, and the Turtle
Islands in Sierra Leone should benefit from a similar protection effort.
Rise in sea level hypothesis: Disappearance of certain sandy islets, reduction of surface area and
retreat of the coast, reconfiguration of detail.
9.1.1.3. SANDY COAST WITH STRAIGHT LONGITUDINAL PROFILE
In the absence of rocky obstacles, these coasts are continuously shaped by coastal drift currents and
sediment transport.
Any alteration of these dynamics by the building of breakwaters in the opposite direction to the current
rapidly leads to an accumulation of sediment upstream of the breakwater and downstream erosion,
which may affect several kilometres of coast (history of the ports of Nouakchott, Abidjan, Cotonou,
Lom„, etc.).
The apparently perfect straightness on a local scale should be tempered by the larger scale where
slight undulations on a period of the order of twenty kilometres, or even more, seems to be emerging
with sectors that “tend towards erosion” and others that “tend towards accretion” (the West coast of
C…te d'Ivoire, for example or Benin – see glossary). These undulatory phenomena with wide periods
(time and space) are reflected by a more or less regular displacement of the curvature of the shore in
time and space over a period of several decades.
Within the framework of this work, subdivisions were produced, taking account of the presence
(proximity to coast and density) of channels and lagoons parallel to the shore.
This criterion is important for identifying the vulnerability of human settlement on the deg of the coast,
in the current situation faced with marine erosion and under the assumption of a rise in sea level.
SANDY COASTS ADJACENT TO DUNE FORMATIONS
C
Definition and geographic location: this type of coast covers the whole of MAURITANIA and
Senegal down to Dakar, practically uninterrupted.
The beach is bordered by a current coastal rim or changes to roughly stable dune formations with no
transition. These dune formations are typified by a wide diversity of:
o
dune landforms and history of Aeolian deposits often in several phases in the Quaternary era.
o
stability and alteration under the action of Aeolian erosion and transport (aridity and density of
the corresponding vegetation, wind regime, land-sea and sea-land wind exchanges)
105
The following three categories have been distinguished:
C1
C2
C3
Absence of channels or depression zones less than 2 kilometres from the shore
Depression zones present at 300 metres and 2 km from the shore.
Low-lying areas less than 300 metres from the shore separated by a recent dune ridge of
variable altitude and in places on the edge of coast reached by the sea during certain
exceptional storms.
Category C2 and especially C3 are largely represented in the southern half of the Mauritanian coast,
with certain depressions (sabkhas) at an elevation below sea level.
Coastal geodynamics: the CDC-ST is powerful and supported by a prevailing wind regime
(Harmattan) North-South to North-East – South-West, directions that are practically parallel to the
coastal wind regime. These winds are capable of Aeolian sand transfer from the dunes to the coast.
However, the sea winds (West-East) can mobilise the sands on the foreshore (which dry rapidly in arid
conditions) and dunes behind the beach in almost the opposite direction from the prevailing winds.
The winds in the rainy season (even of short duration) also exert an influence in a North-West/ North–
Easterly direction, in particular on the dune coast of Senegal where coastal areas have been replanted with trees to limit the advance of the volatile sands from the coast inland to the continent.
Human activities on the edge of the coastal area and issues: human habitat very rare on the edge
of the coast, as populations of fishermen, and fish farmers traditionally settle inland. Note that there
are some exceptions that facilitate access to common activities of landing the catches from artisanal
fishing, or residential tourist facilities (example of the near surroundings of Nouakchott).
The growing use of four wheel drive vehicles has changed the way the coast is accessed, the beach
becoming a more practicable route at low tide than land route in an unstable dune milieu. The
“boulevard” of the beach is therefore connected to the interior road network by satellites that cross the
dune rim.
The potential appeal for coastal resorts is modest on a comparative scale of international tourism.
While sunshine is guaranteed, the wind and the relatively cool water temperatures associated with the
Canary current, along with dangerous currents are so many constraints. For the moment, tourist
frequentation is limited to the urban population of Nouakchott and to a lesser extent from Dakar,
driving to the beach for picnics.
The observation of a few constructions “on the water's edge” on the coastal area around Nouakchott
shows the risks run by such infrastructure: the fragility of the coastal rim, difficulty controlling Aeolian
transits, impacts of storm surges.
Rise in sea level hypothesis: reconfiguration of undulatory dynamics on a large scale, but especially
an increase in the risk of a breach of the rim and encroachment of the sea for type C3 coasts.
D
COASTLINES ADJACENT TO SANDY TERRACES LARGELY DEVELOPED
TOWARDS THE INTERIOR
Definition and geographic location: on a width of between 2 and 10 kilometres, these coastlines are
adjacent to recent sandy terraces, of an average elevation of between 5 and 15 metres, more
transverse topography marked by channels of varying depth, parallel to or curved in relation to the
present day shoreline.
In the absence of rocky elements, these materials are extremely sensitive to all forms of marine
erosion, are continuously remobilised under the action of ocean waves, CDC-ST currents and episodic
high swell.
106
Taking into account the risk exposure of human settlements, three categories have been distinguished
depending on the narrowness of the coastal dune rims and the proximity of peri-littoral flooded or
flood-prone channels and lagoons.
D1
D2
D3
Absence or rarity of flood-prone channels at less than approximately 2 km from the shore.
Presence of channels roughly continuously along a stretch of several kilometres of located
less then 500 metres from the shore.
Presence of channels often continuously along a stretch of several kilometres of located less
then 500 metres from the shore. In the event that marine erosion advances, all of the
infrastructure and built facilities would have to be "moved" inland beyond the channels.
This type of coast is particularly developed uninterrupted:
 On the East coast of Cote d'Ivoire from Fresco and continuing in Ghana round to Axim.
 In Ghana, Togo and Benin, from the Volta delta to the border with NIGERIA.
There are other, less extensive portions of coast that present a similar profile, in particular from the
Sherbro islands in Sierra Leone to Monrovia.
Coastal geodynamics: Due to their geographic situation, these coastal areas are generally oriented
East-West and exposed to ocean waves which almost all year round are South-West/North-Easterly.
The CDC-ST is regularly supported by these dynamics with a high potential for marine erosion when
these are aggravated by a deficiency in sediment reserves. The foreshore and the beach offer
numerous signs of a situation that is changing greatly and continuously: high berms, crescent shaped
beaches, foreshore channels, bars and breaking ocean waves “work” the sandy material constantly.
Human activities on the edge of the coastal area and issues: with the exception of the palm
groves and their related subsistence crops in Ghana and Togo/Benin, an ancient tradition, residences
at the edge of these coastlines is a recent practice. Note that the crossing of the bar and the dangers
of the sea are not an incentive for small fishing, reserving this activity for more specialised fishing.
Human settlement ahs been developed with the creation of vast palm groves in C…te d’Ivoire
accompanied by planned villages, while today vast sectors of coast (hemmed in, it is true, and with
soil that is mostly unsuitable for agriculture) are almost devoid of population (southern coast of Sierra
Leone, Liberia for example).
The development of major agglomerations close to this type of coast (Abidjan, Lom„ and Cotonou),
their axial extension along the road network that is constantly being improved and is easy to install on
sandy terrace, the increasing use of four wheel drive vehicles, have been the impetus in a few
decades for a major transformation of certain coastal areas (Abidjan, Lom„-Cotonou). In addition to
the residences of rural populations in demographic densification to which are added social categories
or age groups with urban-based incomes, resident for reasons of rural comfort and the cost of
accommodation which is lower than in urban zones.
However, the most visible transformations are due to the development of second homes for leisure, or
for prestige, for certain categories of urban dwellers on a model that tends to move as close as
possible to the sea front. Added to this, the installation of coastal road systems in urban zones, and
the construction of hotel complexes and tourist facilities that follow the same logic.
In financial terms, the coast concerned here has a concentration of the most costly stakes of the
impact of coastal erosion. This point of view does not exclude the socio-economic importance of the
disappearance of agricultural land or habitat of relatively modest value.
Rise in sea level hypothesis: the situation is preoccupying, the coastal trend here should be
considered general. The local installations are ineffective, some even counter-productive (walls on the
edge of the foreshore exposed to storms) accentuating the reflection of waves and cross-shore
transits from the land to the sea.
107
SANDY COASTS ASSOCIATED WITH NARROW TERRACES OF VARYING AGES
E
AND ELEVATIONS
Definition and geographic location: this type of coast presents characteristics very close to those
described for category D and the same distinctions have been made depending on the presence
and proximity to the coast of channels and lagoons.
E1
E2
E3
Absence or rarity of flood-prone channels at less than approximately 2 km from the shore.
Presence of channels roughly continuously along a stretch of several kilometres of located
less then 500 metres from the shore.
Presence of channels often continuously along a stretch of several kilometres of located less
then 500 metres from the shore. In the event that marine erosion advances, all of the
infrastructure and built facilities would have to be “moved” inland beyond the channels.
These terraces are narrow (0.5 to 1 km wide) by 1 to 5 km long, with a complex topography in
transition with the surrounding hills. They are associated with the Precambrian basement of the coasts
stretching from Monrovia to Fresco in Cote d'Ivoire and from Axim to Tema in Ghana.
Only the most developed have been mapped on the scale of 1:250,000. Coasts of type F and G, the
characteristics of which are detailed below, also present, in the sandy coves, frequent small terraces
and coastal rimes but more modest in size.
Coastal geodynamics: this is comparable to that described for category D, the main difference
residing in the local modulations of the CDC by the headlands that frequently limit the extension of
these coastal areas.
Human activities on the edge of the coastal area and issues: for historical, cultural and
demographic reasons, human settlement on potential natural sites in the close vicinity is contrasted:
o
Very high density of palm groves in Ghana with frequent residential facilities for leisure or
even tourism.
o
Very low human presence on the coasts of C…te d'Ivoire and Liberia.
Due to the narrowness of the sandy deposits of the rim, and the topographic complexity of the area
behind the beach, the stakes related to erosion are high, in particular on the sites with tourist facilities.
Note that this category, for reasons of landscape, often presents a considerable potential for tourism.
Rise in sea level hypothesis: increased risks of erosion in a context of coasts adjacent to geological
formations with low sandy, mobilisable sediment reserves. Risk of partial disappearance of coastal
rims and radical reconfiguration of certain sites with profound alteration of the coastal landscape.
108
F
SANDY COAST WITH SLIGHTLY UNDULATED LONGITUDINAL
PROFILE
Definition and geographic location: the longitudinal profile, and coastal geodynamics, are
determined by the presence of headlands separating sandy coves with lengths and curvature radiuses
with varying degrees of accentuation.
According to these criteria two categories have been distinguished:
F1
F2
corresponding to coves 2 to 10 kilometres long with a low curvature radius.
reserved for coastal areas with a higher density of headlands separating the coves and 1 to 3
kilometres, with a clearly marked curvature radius; this configuration is exposed to ocean
waves in the most irregular way, especially when the direction of the ocean waves change
according to the season (example: Senegalese 'petite côte').
This type of coast is associated with predominantly sandstone geological formations that are highly
weathered. The levels of sandstone that are still relatively unaltered or the inclusions of ferruginous
cuirass offers increased resistance to marine erosion on certain sites, allowing the headlands to
emerge.
The sandy coves are adjacent to recent deposits in narrow coastal rims, gradual onset terraces,
isolated lagoon channels in places, the whole occupying a variable width but most often very narrow
(50 to 200 metres), and rapidly relayed by footslopes of hills with typically modest elevation.
The "petite côte" in Senegal, Basse Casamance at the level of Cap Skiring, Caravela island in the
Bijagos Archipelago and the coast of Ghana from Sekondi to Tema offer numerous examples of this
type of coast.
Coastal geodynamics: the headlands and their rocky inclusions, even of short extension, play a role
of hydraulic groynes that alter the path of the CDC-ST currents. The downstream part of the
headlands is subjected to a slightly erosive return current, the upstream part is more favourable to
accretion, but in the long term with a compensatory sediment balance.
The headlands' exposure to the ocean waves in certain sections of the coast also plays a role in the
local alteration of the CDC-ST paths, depending on their relative orientation, in particular in coastal
zones where the direction of the ocean waves is subject to seasonal variations (zone situated to the
th
North of the 9 parallel).
This is also the case of certain metamorphic rock formations in the Precambrian shield that are
considerably weathered but with resistant seams of the quartzite type or basic rock (Ghana coast).
Human activities on the edge of the coastal area and issues: the proximity of headlands offering
better shelter against ocean waves has long been used for embarking and landing of artisanal fishing
and today is modernised in places (Joal, Fadiout). On the coast of Ghana, certain sites have been
used historically for landing small vessels. The coastal habitat remained for a long time limited to
isolated villages centred on fishing activities, and ahs been profoundly modified especially by the
recent development of seaside tourism, in particular on the 'petite côte' and in Basse Casamance.
In addition to the hotel facilities aimed at international tourism, the concessions and second homes of
the country's residents have multiplied, saturating the available land almost continuously along the
edge of the beach, on practically continuous stretches of coast.
109
The careless location of certain constructions given the inherent geodynamics of this type of coast
(and which are perhaps accelerating) has led to increasingly costly impacts.
The extreme fragility of many of the headlands should be stressed (for example Varela in Guinea
Bissau) which stand on layers of rock that are often thin and highly altered. In a few cases where the
alteration of the rock does not preserve the visible rocky materials, there is a strong temptation to
extraction at the level of the headlands, and even preventing this type of "artisanal” practice, the long
term consequences of such practices could have heavy impacts.
Rise in sea level hypothesis: the risks seem to be concentrated on the fragile headlands subjected
to an acceleration of their erosion by rock falls from the upper part of their slope which often poorly
consolidated.
G
SANDY COAST, ROCKY IN PLACES
WITH AN UNDULATING LONGITUDINAL PROFILE
Definition and geographic location: Type G coastlines are associated with granite or metamorphic
geological formations on the Precambrian shield and mostly profoundly altered into ferralitic soil and
alterites in several tens of meters of thickness.
This alteration affects in a much more irregular way, however:
o
seams of resistant rock, quartzite, pegmatite, gabbro
o
Some of the granite is crumbling into rocky "balls" that have been preserved from weathering.
The substance the most resistant to the contact of marine erosion determines numerous headways, a
few sections of rocky coast, blocks of rock on the foreshore or reefs situated out to sea. The total
length of this rocky presence on the coast is low (probably less than 10%); the remainder is present in
sandy creeks and coves. While the headlands are directly adjacent to hill topographies, the creeks
and sandy coves are bordered by a thin coastal rim relayed by a lagoon channel into which a number
of small coastal rivers flow. The transition between channels and hills in the hinterland often comprises
a fine layer of older sandy terrace of colluvium apron. The subdivision that was made between type
G1 and type G2 is based on the highest frequency of small segments of rocky coast and often of rocky
reefs that typify the type “G2”.
The major part of the coast of Liberia and the West coast of C…te d'Ivoire is concerned by these types
of coast, which are also frequent in Ghana around Cape Three Points.
Coastal geodynamics: due to their geographic situation with a perceptibly East-West orientation,
these coasts are exposed to South-South-West/North–North-Easterly ocean waves which keep the
same direction for the most part of the year at this latitude in a normal situation as in a storm.
This overview should however be modulated at the level of the actual site, depending on the
orientation of the coast whose undulating profile is not strictly East-West and that of the headlands
and rock seams which reinforce it in a North-South direction. The local impact of ocean waves, the
CDC-ST currents' trajectory in the same predominant West-East direction generate high local
variations in coastal geodynamics.
These considerations should be taken into account in assessing the risks relative to marine erosion,
and the impact of developments likely to modify coastal geodynamics, which should always be
approached site by site.
Lastly, and despite the contributions of a very dense hydrographic network, there are limited
continental sediment reserves and with the narrow continental shelf and the consequent low sediment
stock, there is a high risk of beach erosion.
110
Human activities on the edge of the coastal area and issues: the coasts of Liberia and Côte
d'Ivoire long remained forest and were practically uninhabited. Even the advancing agriculture front
which has been conquering the forest for several decades has only very partially affected the coast.
Outside the delta mouth sites, there are only a few small villages scattered along the coast, and their
inhabitants do not have a great fishing tradition. Note also that to the topography near to the coast, of
highly dissected hills to which should be added high rainfall, are not favourable for the building or
maintenance of roads and tracks, a considerable constraint against improving access. The creeks and
small sandy coves, their hilly surroundings of tropical landscape offer a "dormant" potential for suitable
tourism sites, which to date has begun to be developed o the coast of Ghana.
Rise in sea level hypothesis: resumption of degradation of headlands, rock falls.
H
PREDOMINANTLY ROCKY COAST
Definition and geographic location: the long history of the geological and pedological alteration of
West Africa has left little rock still resistant to marine erosion long the coastal fringe. The rocky
sections of the coast which is the subject of our study cover less than 1% of the coastline and within
this estimated figure, the portions of coast with rocky cliffs are the exception. Among the principal
ones are: the Cape Verde peninsula in Senegal, the rocky breakwater at Cap Verga and Conakry in
Guinea, Freetown in Sierra Leone, Roberts Port and Monrovia in Liberia.
Coastal geodynamics: differential coastal erosion has isolated these rock masses which constitute
obstacles that profoundly alter the trajectories of the CDC-ST, generating counter-currents and high
instability on the beaches located in the surrounding areas.
Human activities on the edge of the coastal area and issues: Historically, a good number of these
sites constituted landmarks for coastal shipping and the relatively sheltered natural harbour locations
in a geographic context in which protected sites with this aptitude were rare.
With the demands of modern maritime traffic, only the Dakar site has retained this role, and the
alternatives of port infrastructure had to be found for the historical sites (insufficient depth for the drafts
of modern vessels, in particular).
We should remember the complex current systems in proximity to these sites, and the fragility of the
urban beaches close to the major agglomerations (Dakar, Monrovia and Freetown).
Rise in sea level hypothesis: low local impact, except for facilities built without foresight at the foot of
cliffs facing the sea.
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9.1.2. SEDIMENTOLOGY MAPPING OF THE COASTAL FRINGE
The different units employed for this cartography are summarised below:
ZONES WITH ESSENTIALLY AEOLIAN SEDIMENT TRANSFERS
Ancient or recent
sandy dune
formations
Zones subjects to
tides (foreshore)
Continental wetlands
S1
SD
SD1
Coastal dune areas
smoothed dune formations SDe = active dunes.
High longitudinal dune ridges and depressions between the
dunes.
SPA
Alternance of bared surfaces (reg) of sandy clay, rocky in places
and very mobile sandy dune formations.}
SC
SM
LM
LS
“Dior” type sandy soils, ancient Aeolian coverage deposits
Sand-silt foreshore of the Bancs d’Arguin
Estuary alluvial deposits from Senegal river - argilloarenaceous
Temporary +/- saline lagoons, sabkhras and occasionally floodprone salt marshes, argilloarenaceous.
ZONES WITH ESSENTIALLY FLUVIAL SEDIMENT TRANSFERS
Coastal rim and
recent sandy terraces
Ancient terraces
Profoundly altered
sediment formations
with inclusions of
hardpan and resistant
rock
S1
S2
SH
SHS
S.CT
CT
ST
TB
GR
K
Intrusive or
metamorphic rock
P
PR
Flood zones
influenced by tides
(intertidal)
LN
LV
M
MV
L/Ls
V
Continental wetlands
AL
ALx
Sandy coastal ridge
Sandy terraces with ripples and channels, landform smoothed in
places
Smoothed terraces that are hydromorphic in the rainy season
SHS ditto SH but +/- saline
Sandy materials from the ancient terraces and undifferentiated
sediment formations.
Sandy or argilloarenaceous formations with inclusions of
hardpan originating from profound soil alteration
Formation of “tertiary sand beds” in C…te d’Ivoire
Sand or clay materials from the “terre de barres” formation.
Ancient alluvial deposits from the Volta.
Sandy materials of varying degrees of altered sandstones (rare)
Clay, sandy, pebbly materials from limestone and marnolimestone to ferruginous cuirasses
Profoundly altered granite and metamorphic rocks. Deposits of
harder rocks in places.
Solid rocks such as quartzite, gabbro and basalt, resistant to
erosion.
Lagoons of variable salinity that collect continental waters. LV:
more plant life.
Organic sandy-silts from mangroves and sandy +/- saline
materials from islets and "tannes".
Alluvial deposits of varying gradation from complex delta zones
of channels, lagoons and spits.
Lagoons not connected to tides (L), saltwater lagoons (Ls),
margins of saltwater lagoons (LsD)
same as for MV but outside of intertidal level, often located on
periphery of MV.
Alluvial plains, materials with variable gradation
ALx hydromorphic +/- swamp
112
X
Xs
LSD
XS
Low-lying, somewhat flood-prone ground, low alluvial pediment
with +/- saline tables (Xs) variable gradation.
Sandy margins of type LS depressions, sandy dunes and sandy
clay.
Nyayes-type sandy depressions, dune sands and sandy clay.
9.1.2.1. AEOLIAN SEDIMENT INPUT ZONES
The coastal zone stretching from Nouadhibou to Dakar, with the exception of the mouth of the
river Senegal, is typified by a large range of sandy, predominantly dune coverage. The continentcoastal interface is highly subject to erosion and Aeolian transport under the action of winds from
substantially different directions:
 The North-South to North/East– South/West Harmattan which blows continuously during more
than six months of the year.
 The West-East sea wind and occasional monsoon winds during the short rainy season which
are South/West - North/East.
The Harmattan is the driving force behind the coastal drift currents (CDC) and sediment transport from
continent to coast, but the role of winds in the opposite direction is far from negligible in reconfiguring
the dune formations along the edge of the littoral and sediment transfers towards the continent.
Starting from June in particular a South-North current cell forms opposite the Senegalese Grande
C…te.
This global schema should be nuanced however with the increasing importance sand transfer from the
coast to the continent gradually descending towards the South, in particular from the mouth of the river
Senegal.
With the exception of the river Senegal and the very occasional coastal wetland outlets, no streams,
even temporary, reach the coast.
Note also the existence of the Khayar canyon to the north of the Cape Verde peninsula, whose role in
trapping sediment should be taken into account.
S1
Definition and
geographic
location
Geodynamics
SANDY FORMATIONS ON COASTAL RIM
Formation typified by recent dune formations, very unstable irrespective of
potential stabilising vegetation in this climate, these systems form a continuous
border to the beaches along the coast. Clearly individualised when narrow,
separating the coast from intra continental depressions (cf. Mauritania), it switches
in transition to older continental dune deposits.
Changes in the coastal dune ridge can be taken into account to approach the
problem of stock-taking between Aeolian sediment transfers and those mobilised
by the coastal drift currents. The map interpretation suggests the following
approximate zoning:
A zone to the North of the latitude of Nouakchott where the net results of transfers
appears clearly in favour of the Aeolian contribution from the continent in almost
the totality of the duration mobilised by the CDC. This is attested by the thinness of
the dune ridge adjacent to the nearby major sabkhras in the central and southern
coastal areas of Mauritania, which have a lesser supply of nearby mobilisable
sand than the rest of the coast. Everything suggests that the continental part can
scarcely manage, except in places, to maintain the equilibrium of the current rim.
On the other hand, there is a perceptible tendency of coastal sand to advance
inland (this in fact justified the ribbon of stabilising replanted trees along the
113
Hypothesis of
rising sea level
and/or storm
surges
S2
Definition and
geographic location
"grande c…te" in Senegal.
As mentioned regarding coasts of the types D2 or D32, this hypothesis greatly
increases the risks of the dune rim being affected and ocean wave and storm
situation and marine intrusion or flooding from the topographically weak points of
the rim.
PRINCIPALLY DUNE TYPE SAND FORMATIONS
Without going into the complexity of forms and history of sandy deposits, the
following categories can be identified:
SD 1 Longitudinal, prominent dunes, mostly present in Mauritania
SD
SC
SH
Geodynamics
Hypothesis of rising
sea level and/or
storm surges
S4
Definition and
geographic location
Geodynamics
Hypothesis of rising
sea level and/or
storm surges
Dune formations, undulated to varying degrees, on the near
coastline in Senegal
Smoothed sandy formations on the Senegalese southern Sahel
sandy soils of fluviomarine terraces, hydromorphic to a certain
extent, in proximity to the mouth of the Senegal river and the Banc
d’Arguin
Corresponds to interrupted “barkhane or sif” dune formations
localised in flat sabkhras, rocky outcrops, ancient alluvial plains.
SPA While the volumes of sand are lesser than in the case of the
previous formations, Aeolian transits are very active (desert zone to
the North of Mauritania, mainly).
The sediment contributions to the coast of all these formations seems to be
significant, mainly to the North of the mouth of the river Senegal.
Low impact except for certain formations of the type SD1 directly in contact with
the shore and on which coastal erosion coastal could accelerate.
FORMATIONS ASSOCIATED WITH LOW-LYING AREAS
In Mauritania, these extend largely to the North of Nouakchott (“grande sabkhra”)
at an elevation inferior to the average sea level in places.
South of Nouakchott to the Senegalese border, an almost uninterrupted strip of
low-lying land borders the coast situate very close by.
South of the mouth of the Senegal, the relict of a similar network of depressions
that has disappeared subsists amidst a highly sandy milieu (Nyayes zone).
In addition to the real, almost permanent lagoons in Diawling Park (L), the
following can be distinguished:
low-lying decantation zones, argilloarenaceous to a varying
LS Sabkhras,
degree, occasionally flooded with high rainfall, salt water table near the
surface.
sandy margins of the sabkhras but at an elevation that remains
LSD The
low, exceptional flooding in places.
The functional gulleys allowing these low lying areas to be emptied are rare, two
or three in proximity to the Chott Boul. It is problematic to identify traces of other
fossils under the sand of the coastal rim and the Aeolian sediment contributions.
If the sea level rises this could facilitate the re-opening of fossil gulleys and the
corresponding marine intrusions. Similarly, a climate change hypothesis that
envisages years with exceptional precipitations could also lead to the reopening
of the gulleys which would empty he waters accumulated under these low-lying
areas.
114
9.1.2.2. FLUVIAL SEDIMENT INPUT ZONES
PRE-COASTAL EMERGED ZONES WITH MAINLY FLUVIAL SEDIMENT TRANSFERS
This zone stretches from Dakar to Cotonou, in addition to the remobilisation of fluviomarine sediments
by the current (CDC-ST), the sediment contributions from the continent are transmitted to the coast
through a fluvial or fluvio-marine hydrographic network in the case of the channels of mangrove
estuaries. These transfers in the nature and volume of sediment depend on a certain number of
factors:
 the extent of the river basin, the system of flood peaks, interception in wetlands before sea
outlet, the type of sediment coming from the river basin
 The presence of major dams playing a sediment trapping role upstream, but especially
regulating the flood peaks which are the mains sources of sediment supply.
 The density of the hydrographic network of small coastal rivers that flow into the sea and have
fast sediment transport (coast of Liberia, west coast of Côte d'Ivoire) or the presence of a precoastal hydrographic network with sediment trapping shallow, low-lying areas (Guinea Bissau,
Guinea Conakry, Casamance and Sierra Leone).
Whatever the final destinations of these sediments (coastal redistribution by CDC-ST) or deposits on
the continental shelf or to deep sea trenches, it seems that a large part of the current continental sand
reserves are localised in the vast belt of sandy terraces and/or mangroves.
S
Definition and
geographic location
RECENT SANDY FORMATIONS
Without going into the complexity of forms and history of sandy deposits, the
following categories can be identified:
Spits and coastal rims of recent fluvio-marine contribution
remobilised in places by Aeolian erosion. Their genesis depends on
SD 1 coastal drift currents, erosion or accretion depending on the sites.
S2
SH
S-CT
Geodynamics
Terraces attributed to the recent Quaternary era in several siltation
episodes, with transverse profile of ripples and channels at an
average elevation of 10 to 15 metres and close to sea level for the
channels (Sherbro islands, Abidjan terraces, Grand Bassam).
Low terraces, relatively flat and flood-prone in seasonal high
precipitations, often planted with rice (Guinea Conakry, Sierra
Leone), with habitat concentrated on slightly higher sand ridges.
Corresponding to ancient terraces in transition with peneplains with
roughly sandy soils associated with geological sediment formations
often with sandstone and profoundly altered (Casamance, Guinea
Bissau and Guinea and Sierra Leone).
Should also be mentioned, inside the mangrove areas are numerous
islets not reached by the regular tides, of low elevation, sandy to silty
and with varying degrees of salinity (tannes). Their contour is traced
by the high sea level mark and in light of their small dimensions;
many are not identified by a code as part of the 1:250,000 maps.
In the absence of any rocky obstacles, the most frequent situation, these
formations are highly remobilisable by the CDC. Their permeability excludes the
development of an active hydrographic network towards the coast; hydrological
erosion is restricted at local level to the filling in of the channels. Although they
border almost ¾ of the coast, their width is very irregular, varying from 100
metres to 5 km, and so the most easily mobilisable sediment reserves they
115
constitute are liable to large variations from one section of coast to another.
Hypothesis of rising
sea level and/or
storm surges
SCT
Definition and
geographic
location
PRE-QUATERNARY SEDIMENTARY FORMATIONS, SOFT OR
PARTIALLY CONSOLIDATED IN PLACES
All these formations dating from the post Cambrian to the end of the Tertiary have
in common:



Geodynamics
sedimentary sandstone origin (exceptionally marno-limestone in places in
Senegal).
Deep alteration supplies materials with a high sand load and a current or
previous hydropedological change implying the genesis of ferruginous
cuirass layers.
A peneplain topography with a dense network of low-lying ground with
plants the collect the waters and capture a large part sediment from slope
erosion.
These formations cover a vast pre-coastal belt stretching from the south of Dakar
to Monrovia, at times largely separated from the sea front by major sandy terrace
and mangrove areas, at times very close to direct contact with the sea in the form
of headlands.
The role of the headlands associated with these formations and mentioned in the
characteristics of type F coasts is the highest contribution to coastal geodynamics.
The amplitude of sandy sediment transfer to the coast is difficult to assess given
the importance of interception of the products of hydrological erosion by the
mangroves or low-lying areas and the level of remobilisation of current coastward
transport.
Hypothesis of
rising sea level
and/or storm
surges
P
Definition and
geographic
location
PRECAMBRIAN BASEMENT GRANITE AND/OR METAMORPHIC
FORMATIONS–INTRUSIVE BASALT FORMATIONS
There are two major systems in contact with the coast:

The intrusive granites and metamorphic dispersion trains in Liberia and
the west coast of Côte d'Ivoire.

The complex of granite and various metamorphic rocks, including
numerous deposits of greenstone on the coast of Ghana.
All these formations have been and are still subjected to deep weathering
leading to ferralitic soils in transition with the alterites in varying degrees of
116
thickness approaching the healthy rock. Relicts of former evolutions, ferruginous
cuirasses still occupy the summit parts of the hilly landform or dissected
peneplain, drained by a dense hydrographic network. The eroded materials
likely to be transported on the coast are of varying gradations, clay, loam from
ferralitic soils, the quartzy gravel of the stone-lines, ferruginous gravel, coarse
sandy alterites, quartzy, which are the continental “signature” of many small
coarse sandy beaches in the zone concerned.
Note that with the clearing of the forest, surface erosion tends to accelerate,
accompanying the normal evolution in the forest milieu with subsidence of
slopes.
Geodynamics
The hardest rocks (gabbro, basalt) enabling the genesis of real rocky coasts of
type H have been coded PR.
The differential weathering of certain solid rocks that are fractured in places, or
of deposits isolating numerous small rocky headlands, have been emptied of all
or part of their soil and alterite at the elevation subject to ocean waves. As
mentioned for type G coast, the angle of the headlands and deposits of hard
rock facing the direction of the dominant ocean waves plays a very important
role in modulating the coastal drift currents.
The importance of the continental sediment contribution is probably significant
for the major rivers with open estuaries, but the proximity of deep ocean floor
does not enable notable storage that is remobilisable on the coast.
Hypothesis of
rising sea level
and/or storm
surges
WETLANDS UNDER THE DIRECT INFLUENCE OF THE TIDE
M
Definition and
geographic
location
MANGROVES AND MUDFLATS REGULARLY SERVED BY TIDAL
FLOWS
These milieus extend across vast areas in Senegal (Sine Saloum and Casamance),
Guinea Bissau, Guinea and Sierra Leone, and to a lesser extent in Gambia or very
locally in the estuary and coastal channel area from Cote d'Ivoire to Benin (cfr
category MV). The mouth of the river Senegal and probably the Banc d’Arguin area
present conditions of soil comparables to those of the mangroves which grow in
less arid zones.
The substrate of the populations of mangrove trees corresponds to fine sediments
rich in organic matter with a sandy fraction, which is also fine. The somewhat dense
network of channels that guide the tidal flows is constantly evolving, with the
formation of meanders and the destruction and reconstruction of mangrove shores.
Geodynamics
The zone traced here as mangrove (code M) includes areas of varying size of free
water and mudflats devoid of vegetation, as well as numerous sandy islets with
saline halos tracing the contour of the near shore reached by the tide.
It would appear to be useful to distinguish here the interface with the "sea front"
coast exposed to ocean waves and the “interior” coast subject only to the calm
arrival of the tide in transition over a strip of a few metres with emerged islets of
mangroves or the continent.
117
The interface between mangroves and sea coast was broached regarding type B
coasts; it can be summarised, mentioning the contributions from the mangrove
channels (reconfiguration of meanders but especially estuary outlets of the main
rivers to the coast by a flushing effect and redistribution by the coastal drift
currents). But the tide flowing upriver can be responsible for certain interior
transfers, and in addition to the currents, the trapping role played by the network of
mangrove routes should not be neglected.
Hypothesis of
rising sea level
and/or storm
surges
As a general rule, the tide-continent interface at the level of the interior coast is
limited to the intervention of the highest tides liable to remove the saline surface
sediments. On the other hand, in the particular case of narrow mangroves framing
an interior coast of foothills, it may be subject to the effects of storm swells even
partially attenuated by the thin rim of mangroves. A situation of this type is frequent
in the Bijagos Archipelago, where the two types are sometimes only separated by a
hundred metres.
If the sea level were to rise, even by only 1 metre, the whole of the potential impact
in the zone defined here as mangrove could be considerable:

Acceleration of the natural evolution of th channels that guide the tide and
reconfiguration of the hydrological system.

Associated reconfiguration of the distribution of mangrove populations, the
different species of which have specific milieu demands.

Reduction in surface area of many small islets and tannes emerged today
and reconfiguration of the conditions of salinity.
On the level of certain traditional human activities in mangroves, the impact could
be considerable for rice growing with fields encased by breakwaters to control tides.
Suitable land is typically located in a situation of significant tidal height. The building
of breakwaters in soil and with materials that are not consolidated, and maintaining
them, demand work that cannot be mechanised and is very demanding in terms of
qualified labour. Already in difficulty today because of these constraints, this system
could be greatly compromised. The other rice growing systems hydrologically
connected with the tide could also be greatly disrupted.
For Guinea Bissau, and to a certain extent Guinea, Sierra Leone and Casamance,
these issues related to the rising sea level appear very preoccupying.
MV
TIDAL DELTA AND CHANNEL COMPLEXES
Definition and
geographic
location
This category encompasses highly complex systems:



Topographically: land regularly covered by the tide, land around the edges
of this land, spits and alluvial deposits beyond the reach of the tide.
Hydrologically: lagoons, surface waters and bodies of saline, brackish and
fresh water in interaction with the tide and seasonal rains and flood peaks.
Ecologically, with highly diverse sequences of plant communities adapted
to the milieu, biodiversity of land and aquatic flora and fauna.
The fluvialmarine deltas on the coasts of Liberia and the west coast of Côte d'Ivoire,
central Ghana and on a significant part of the coastal fringe of the left bank of the
118
Geodynamics
Hypothesis of
rising sea level
and/or storm
surges
LN
LV
Definition and
geographic
location
Volta round to Benin are characteristics of these milieus.
These zones are liable to play a role in sediment retention, but this role is probably
only temporary in delta complexes where the sediment is remobilisable during
certain flood peaks depending on the size and rainfall of the river basins.
A hypothetical rise in sea level would lead to a reconfiguration of these ecological
contexts both globally and in local detail.
LARGE DIRECTLY TIDAL LAGOONS
The major lagoons are in direct contact with the tide but its influence (tidal range,
input of freshwater, salt water,) is subject of high local variations:




Importance of volumes of freshwater input to the lagoon, contributions from
rivers and streams, emptying of phreatic fluvial sheets or slopes and, of
course, precipitations on the body of water.
Extent and volumes of water, bathymetric profile and depth of lagoons.
Seasonal contributions of fresh water and evaporation.
Effectiveness and functionality of gulleys enabling communication with the
sea.
The shores of the large lagoons are also subject to considerable variations from
sandy beach to belts of swampy fixed or floating vegetation.
They are present from the South of Sierra Leone to Benin. The largest lagoon system
is in Cote d'Ivoire, and is roughly parallel to the shore. It stretches for more than 250
km almost continuously with 4 gulleys or passes that communicate with the sea, one
of which is more or less operational (Grand Bassam). There is also the Cotonou
119
lagoon system in continuity with Nigeria and the Robertsport lagoon which is silting
up. In addition to the large lagoons, there are numerous prelittoral lagoons and
channels that are narrow but sometimes follow the coastal rim for several dozen
kilometres (Code LV) a large part of which has brackish and/or freshwater vegetation.
Lastly, there are large lagoons located inland because their communication with the
sea is broken today, like the large lagoon on the edge of the Keta in Ghana since the
road breakwater which blocked communication with the ocean or the isolated lagoon
that has become saline (code LS) on the left bank of the Volta delta.
Geodynamics
Hypothesis of
rising sea level
and/or storm
surges
In the absence of any rocky obstacles, the most frequent situation, these formations
are highly remobilisable by the CDC. Their permeability excludes the development of
an active hydrographic network towards the coast; hydrological erosion is restricted at
local level to the filling in of the channels. Although they border almost ˆ of the coast,
their width is very irregular, varying from 100 metres to 5 km, and so the most easily
mobilisable sediment reserves they constitute are liable to large variations from one
section of coast to another.
These systems work as sediment “traps” or at least “filters” for the sediment
contributions from continental river basins. Their tendency to silt up is attested by the
large number of hydromorphic alluvial plains at elevations close to that of the lagoons,
and which have been increasing recently and still are today. (typical example of the
Tanoe delta on the approach to the Eby lagoon in Ghana – Cote d'Ivoire).
The final evolution can lead to a major regression (Bandama delta), or even total
disappearance of lagoons in the last quaternary episodes (Sewe river plains and
connected rivers in Sierra Leone).
In these conditions, it seems obvious that in the West of Cote d'Ivoire and on the
coast of Togo and the NENION, the continental sediment contribution is low and the
reserve is limited to remobilisation by erosion of coastal sandy terraces.
WETLANDS VARIABLY INFLUENCED BY THE TIDE AND CONNECTED TO COASTAL GEODYNAMICS TO VARYING
DEGREES
AL - ALY
RIVER DELTA COMPLEXES
Definition and
geographic
location
These river complexes develop largely in low-lying subsidence areas and/or from the
filling up of lagoons. Conventionally their topography comprises bank ridges on the
main watercourse and distributaries and lateral zones that are lagoonal in places.
The main channel, and even certain distributaries, are subject to the tide for a varying
distance (can be locally important as in the case of the Volta). These complexes have
their main outlet on the maritime coast or major lagoons.
The principal delta systems are located in COTE d’ IVOIRE (Bandama, Comoe),
Ghana (Volta), Sierra Leone with the particularly complex topography of alluvial plains
or grooves along the Jong, Sewe and Moa rivers.
Geodynamics
Despite the size of their catchment areas, many rivers only have a very small delta
output, in the same order of surface magnitude as the many small coastal rivers that
flow into the coast from Monrovia to Sassandra. These microdeltas with very complex
topography have a frequent rate of small lagoons.
Delta complexes are platforms for sediment transport, direct by the principal channel,
indirect through the erosion of delta deposits. The greater part of this transport is by
120
flood peak and, in the case of rivers whose flow rates are regulated by large dams,
only the downstream part of the river basin plays the principal role (flood peaks and
sediment that is not trapped upstream by the dam).
The other level of interface to be taken into account concerns the alluvial phreatic
sheets which, in proximity to the coast, are in a freshwater system near the surface
but at a variable depth are “supported” on water that is saline to varying degrees. The
extent and depth of the salt wedge is influenced by multiple local factors
(pluviometrics, topography, nature of aquifers). It is acknowledged that global flooding
by flood peaks is the prime factor in recharging the water table and the absence of
such flooding in seasons of modest rainfall causes the water table to salinate (to be
confirmed for the left bank of the Volta).
WETLANDS INDIRECTLY INFLUENCED BY MARINE HYDROLOGY
X
Definition and
geographic
location
WETLANDS INDIRECTLY INFLUENCED BY MARINE HYDROLOGY
This category includes wetlands with variable local hydrological conditions which all
have in common:




A low elevation, under 5 metres and often under 3 metres above the
average tide level.
Flat geomorphology of low-lying land, pediment and foothills in smoothed
sandy terraces or depressions (alluvial grooves, silted channels).
Frequent flooding in the rainy season the duration of which varies.
A fluctuating water table close to the surface influenced by the depth of the
brackish waters which are also fluctuating following the conditions of the
sites and the season.
In the coastal fringe analysed in this study this category has mainly been defined on
the edge of the “interior” coast in contact with the areas of mangroves and low lying
ground of low terraces of continental transition (Senegal, Guinea Bissau, Guinea
and Sierra Leone).
In certain climate contexts, the zones affected with recurrent salinity have been
identified (code XS, for example Sine Saloum).
From Sierra Leone to Benin, the wetlands are most often associated with ancient
alluvial depressions, isolated and partially backfilled lagoon channels.
Geodynamics
This is especially sensitive at the level of sediment transport: a network of low-lying
ground, both dense and largely developed landward plays a very important role of
retaining the sandy sediment from continental erosion.
The low hydraulic slope approaching the internal coast leaves little transport energy
for a deficient hydrographic network. In Guinea Bissau, numerous sections of low-
121
lying ground situated between the mangrove and the upstream part (the “lalas”) are
covered with savannah vegetation that captures sand and the deposits retained
often lie on former mangrove soils, which gradually subsided under the weight of
the sand supply.
Hypothesis of
rising sea level
and/or storm
surges
If we remember that there is no ocean swell accompanying the upriver flow of the
tide on the “interior” coast and the modest remobilisation of sediments during the
descent, the conclusion to be drawn is a low sediment contribution from these
wetlands towards the marine coastline.
The movement of the interior coast towards the continent will be sensitive on certain
low-lying areas with very gentle slopes, but the most remarkable impact could
concern the phreatic rise of brackish waters, in particular the movement of the saline
fringe in contact with the “interior” coast and emerged continent. Sin„-Saloum would
be particularly concerned by this.
9.2. ANNEX 2: SOME TERMS AND CONCEPTS RELATED TO COASTAL
DYNAMICS
This glossary is in an initial phase in this intermediate report; it is intended to be enhanced and
completed over the subsequent phases of the study.
DEFENCE OF COASTS AND DEVELOPMENTS: if the issues at stake are important, or even critical
and cannot be moved, coastal defence solutions will be chosen, knowing that, as was largely shown
by the European EUROSION programme (2004), heavy defences (“hardening” of the coast) generally
lead to an aggravation of the initial situation in the long term. The costs must also be in phase with the
stakes to be protected and funding must be available. Reducing these impacts requires in-depth
acceptance of the articulation between the developed and the undeveloped areas. The
implementation of heavy developments is particularly problematic, when the direction of the coastal
drift current is not constant throughout the year.
Protection works: These are interventions based on the provision of materials from outside
(tetrapods, blocks of rock, masonry) aimed at fixing the shoreline. The idea is to physically oppose the
retreat of the shoreline.
Blocking exchanges with the sea: this blocking, which has sometimes become necessary
(breakwater), leads to a rapid acceleration of the filling in rates of the lagoons. It directly threatens the
ecosystems in the intertidal zone situated opposite the breakwater.
Works that alter longshore sediment transport: these are solutions perpendicular to the shore:
groynes, rockfill. These works logically lead to increased downstream erosion largely observed and
today well known among coastal planners and developers.
Solutions parallel to the shore: these are, in particular, breakwaters. These installations are placed on
the shoreface not far from the coastline. They have the effect of setting up a connecting bar between
the structure and the shore. The effects on coastal transport can be relatively significant, as sediment
deposited in the developed zones is no longer available in the undeveloped zones.
122
Development of breakwaters in the North of Brazil (in the Recife region). From left to right: 1 – former
developments, and placing of tombolos; 2 - more recent developments, the effects are not yet very visible; 3 – the
beach located downstream of the developments is being depleted.
Soft solutions: these solutions create a lesser impact, which does not always mean their cost is
lower.
Re-sanding of beaches: this is the soft solution most often adopted today when the issues at stake
justify it.
Drainage of beaches
PMD OR PUBLIC MARITIME DOMAIN: this typically corresponds to a distance (not geometric)
calculated from the high water mark, which is considered as the mean level reached by the sea during
tides with a coefficient of 120 (for France). Permits to use the public maritime domain can be issued in
the form of concessions (breakwaters, moorings, beaches, ports) or "AOT" – temporary use permit.
The PMD is a legally valid document.
COASTAL DYNAMICS: coastal dynamics are expressed and perceived at different scales of time and
space:
123
Coastal system observation scales.
(according to Desmazes. F. 2005.- Characterisation of sand bars
on a beach on the Aquitaine Coast. Thesis – University of Bordeaux).
On soft coasts, the coastal dynamics are expressed in particular by the forms of coastal
accumulation: coastal rims, spits, deltas or mudflats
Different forms of accumulation
(BRGM. 1984.- Elements of coastal sedimentary dynamics. UNESCO – WACAF III)
These forms of accumulation are the result of different phenomena induced by the forcing of the
coastal system. The energy responsible for the forcing is supplied by the wind, ocean waves, current
and water level, the continental waters – these are morphogenic agents. The dissipation of this
energy is reflected in the transport of particles, including sediment of varying gradation.
Forcing in the coastal milieu. The sediment is transported by the different currents: (a) induced by the wind; (b)
rotating currents generated by the tide. Some of these currents are induced by the transformation of wave energy:
(c) mass transport; (d) return current; (e) coastal drift. according to DEZMAZES. F. 2005.- Characterisation of
sand bars on a beach on the Aquitaine Coast. Thesis – University of Bordeaux).
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The direction of the ocean waves is evidently a determining factor which scientists have long
investigated44 differentiating the effect of waves perpendicular to the shore (oscillation waves) that
generate cross-shore transport, with high energy, and oblique waves (translation waves) that
generate longshore transport, essential to understanding coastal dynamics.
On sandy beaches, these dynamics are exacerbated during storm surges which may generate
spectacular and brutal reconfigurations of the beach profile.
Schematisation of the principal currents generated by the ocean waves (according to Cazes-Duvat. V., DelmasFerr…. M. and R. Troadec. 2002.- Manual for monitoring and treating coastal erosion – Indian Ocean Countries.
Regional Environment Programme – Indian Ocean Commission. 45p.).
44
Johnson. D.W. 1919. – Shoreprocess and shoreline development. New York. Chapman and Hall. 584p.
125
Schematization of the crosshore current – return current (A) and longshore current (B) (from CASTELLE. B. 2004.Modelling of sedimentary hydrodynamics above the sand bars subjected to the action of the ocean waves:
application to Aquitaine coast. French Thesis – University of Bordeaux).
MORPHOLOGICAL AND SEDIMENTARY DYNAMICS OF BEACHES: the visible (emerged) part of a
beach is only one element of a sediment system the immerged part of which is subject to the agitation
of the sea (see figure ….).
The behaviour of sandy beaches (formation of bars, migration of bars in the profile, shape and
typology of bars) is still poorly understood and is the subject of large numbers of doctoral theses. The
influence of the morphology of the seabed with the related refractive phenomena partially explain this
great complexity and the diversity of observable forms, which largely exceed the current possibilities of
modelling at a cost compatible with the investigation of considerable expanses.
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Changes in the transverse profile of a beach (from Cazes-Duvat. V., Delmas-Ferr‚. M. and R. Troadec. 2002.- Manual
for monitoring and treating coastal erosion – Indian Ocean Countries. Regional Environment Programme – Indian Ocean
Commission. 45p.).
The dynamics of sandy beaches is dominated by different systems of bars:
 The ridge and runnel system (1- formation of a bar roughly parallel to the beach; 2accretion behind this bar; 3- formation of a tidal pool downstream of the bar; 4- migration of
the system in the direction of coastal drift). The speed of migration (propagation) of these
undulations observed on a 35 km section of sandy coast showed an average displacement of
2.4 m/day, with 16% of the formations migrating in the opposite direction from the prevailing
coastal drift45.
 The crescent bars observed in the subtidal part of many sandy beaches. These are systems
with a marked rhythmicity with a wavelength varying from 1 to 1,000 metres, with an average
of 100 metres.
 The channel systems observed in particular during storm episodes associating longitudinal
bars interrupted by channels not parallel to the beach that originate in the upper beach.
Bars may constitute serious obstacles to boats, in particular when embarking from the beach, with
frequent accidents affecting artisanal fishermen.
45
Lafon, V., Dupuis, H., Howa, H., Froidefond, J.-M., 2002. Determining ridge and runnel longshore migration rate
using spot imagery. Oceanologica Acta 25, 149–158.
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Bar system (tide going out – West of the port of Abidjan, Cote d'Ivoire, source: Google Earth)
Changes in sandy coastlines should be observed on a variety of temporal and spatial scales. The
accretion and erosion episodes in a given point can be associated with the passing of “sediment
trains” which may correspond to a stabilisation of erosion. In Benin, the siltation wave advances at an
estimated speed of 2.8 km a year and its return period?? is approximately 25 years.
Siltation wave on the coast of Benin in time and space
(UNESCO. 1986.- Quaternary coastal geology of West Africa and South America. 179p.)
The capacity of sediment to be mobilised by morphogenic agents largely depends on its cohesion
(soft, cohesive or consolidated sediment), and gradation.
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OUTLET STREAMS: coastal release points (waste water, rainwater)
FORESHORE: also known as intertidal zone and seashore, the region of shoreline between the limits
of mean high and mean low tide levels.
COASTAL AREA: In practice, how coastal area is defined often depends on why, from the “state”
coastal area limited to the PMD46, to the developer's coastal area (population basin affected by the
marine economy), or again that of the environmentalists, which includes the different natural elements
contributing to the dynamics of the marine and coastal ecosystems. Often more than territorial, this
definition of coast originates from a reflection on function, and in fact leads to a collective
representation that is quite generally shared, but the territorial limits of which remain globally ill-defined
on the land side, including in the most sophisticated legislation.
The recognition of the role of the coastal area as a development area that contributes to the national
economy in multiple ways; like the recognition of the different resources linked to it, leads us to take
this coastal strip into account in an extended, more social than territorial way, which, on the level of
legal systems, remains shared between the public law that applies on the PMD47, and the private law
that applies outside the PMD. In West Africa, common land ownership law is still often recognised
locally.
The emergence of a specific notion, called “proximity to the sea” makes it possible to add rules and
procedures common to the two areas - land and sea. The purpose of these rules and procedures is to
guarantee the compatibility of usages in the whole of the coastal strip, and harmonise the modes of
development according to the imperatives of preserving natural milieus that are both fragile and vital
for national economies.
According to this functional representation of the coastal area, any zoning arrangements should
incorporate usages and players, with a view to proposing a system of spatial units compatible with the
recognition of management responsibilities, which may possibly be subject to contractual policies.
The notion of coastal area therefore spreads through the terrestrial milieus located outside the PMD
according to a principle of the general equilibrium of the territory, the public management of which
incorporates proximity to the sea, and in particular the constraints and natural risks susceptible to
affect the security and continuity of terrestrial activities.
The coastal area therefore remains above all a complex territory that groups together all the milieus
directly marked and influenced by proximity to c o a s t a l w a t e r s . It is easily conceivable that in such a
definition, based primarily on functional considerations, the territorial limit on the land side should be
defined in each local situation. The interlinking of the following components of the terrestrial part of
the coast should be successively distinguished:




The public maritime domain.
The territories of communities with a sea front.
The population basins where products from the sea are processed and commercialised.
Natural coastal areas and conservation sites.
Note that the geomorphologic definition consisting in considering the sediment formations originating
from deposits and fashioned by marine currents, and linked to the interaction of the two systems of
sedimentation, terrestrial and marine (intertidal zone, dune belts, lagoon systems, delta formations,
brackish waters and wetlands) whose regime is in particular linked to tides, is often applicable.
The same problem of boundary may also be posed on the maritime side of the coastal interface. An
extension of coastal maritime space to the entire EEZ seems exaggerated if it is a question of
qualifying coastal waters. However, the morphology of the continental shelf and the bathymetry of
nearshore coastal waters can play a significant role in the organisation and dynamics of coastal
currents, as in the storage and redistribution of sediment reserves.
The ambiguities evoked above concerning the very notion of coast are also perceptible in other
domains, where the diversity of the players and stakeholders has for corollary a diversity of points of
46
Public Maritime Domain
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view. Certain notions such as zoning or the vocation of areas can give rise to interpretations that are a
simplification with respect to the functional aim of the development. For, in fact, there are potentially
multiple ways of zoning the coast depending on the point of view we begin from. The identification of
the vocations of areas with a view to identifying the stakeholders in the development should above all
preserve their multifunctional nature. Even if the accent can be placed pertinently in a given sector
on such and such a type of activity, use or occupation, this should respect the principle of territorial
equilibrium, in particular by respecting the principles of compatibility between usages.
BRUUN'S RULE: BRUUN's rule postulates that maintaining the equilibrium profile of a beach, faced
with a rise in sea level, implies that sediment is eroded from the beachface and deposited on the
foreshore, to increase the height. This increase is directly proportional to the height of sea level.
Behaviour of the equilibrium profile of a sandy beach resulting from variations in sea level (Bruun's rule, 1962)
SIGNIFICANT WAVE: the surface of the sea appears very disordered, even chaotic. Statistical
methods are therefore required to express the height, period and energy of waves. Significant wave
height is defined, and measured during a given time (often 10 minutes) as the average heights of one
third of the highest waves in the series observed. In the same way, the significant wave period is the
average of one third of the highest periods in the series observed. The average energy of the wave
per unit of marine surface is proportional to the square of its height.
DEVELOPING A STRATEGIC RETREAT: given the mobility of the shoreline, against a background of
climate change likely to intensify it, there are few alternatives and the choice essentially depends on
48
each local situation . Among these alternatives, one solution that is viable in the long term is to
develop a strategic retreat.
The expected advance of the shoreline, which will vary according to the site, should lead us to
envisage, each time this is possible, redrawing the contours of a future coastline, arranging a
natural “buffer” space between shore and issues. The width of this zone should be calculated
depending on different elements:
48
Klein R.J.T., Nicholls R.J., Ragoonadeu S., Capobianco M., Aston J. Buckley E.N., 2001. Technological options for adaptation
to climate in coastal zones. Journal of Coastal Research, 17, 3, p. 531-543.
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 The thickness should not only depend on the concern for the safety of people and goods, but
also on the dynamics specific to coastal ecosystems. The areas necessary for pushing back
wetlands or lagoons will often be at least as important as in the case of dune formations. It is
therefore a question of arranging the space necessary for the spontaneous adapting of natural
coastal systems preserving the way they operate and the ecological services they offer.
 One essential question concerns the preservation of wetlands both for the ecological services
they offer, but also for the resources, fishery resources, in particular, that they procure. In this
respect, conservation actions are justified anew, by helping to maintain these complex
systems in an operational state and therefore preserve their capacity to adapt to change,
which is also related to the diversity and complexity of the mosaic of wetlands. In accordance
with this objective, the prevention of the fragmentation of wetland systems is also important
and should be taken into account not only in coastal defence actions, but also when
developing the use of land.
The justification of arranged retreat should be supported by an analysis of the costs-benefits
incorporating the ecological services rendered by wetland areas.
Determining a setback line: This is a question of anticipating the retreat of the shore to allow it to
conserve its natural and functional characteristics. The methods that can be used (see inset) are
interesting despite the following constraints:
 They apply especially when the retreat is more or less gradual and measurable (sandy coasts)
and are not suitable for taking into account other, special situations (the connecting of lagoons
or low areas with the sea, breach or fragmentation of lidos or dune ridges).
 They require historical data to assess the speed of shoreline retreat independently of any
structural works installed there.
 They require observation to be able to assess the retreat due to surges.
The setback line can be measured49 by extrapolating the historical shoreline displacement trends
measured towards a future date to be set, adding the accidental retreat due to storm surges:
Lr = (ΔrLT x Y) + ΔrCT where
Y = the number of years chosen
ΔrLT = the displacements of the shore in the Long Term expressed in m/yr
ΔrST = the retreat of the shore in the Short Term (exceptional storm) expressed in m/yr
49
Sabatier. F. & al. 2008. Dƒtermination d’une ligne de recul sur les littoraux en ƒrosion: exemple des plages du Golfe du
Lion (Determining a line of retreat on coasts surffering from erosion: example of beaches in the Golfe du Lion. Proceedings
of the international pluridisciplinary symposium on coastal area "le littoral : subir, dire, agir". Lille France – 16-18 January
2008.
Ferreira O., Garcia T., Matias A., Taborda R., Dias J.A., 2006. An integrated method for the determination of set-back lines
for coastal erosion hazards on sandy shores. Coastal Engineering, 26, p. 1 030-1 044.
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Figure: Determination of the setback line (according to Sabatier & al. 2008).
To this strip an additional width should be added to enable the sedimentary equilibrium of the dunebeach system to be maintained.
MARITIME SIGNALING: localised characteristics of lighthouses, buoys and lights.
STORM SURGE (or storm tide): exceptional rise in sea level due to the conjugation of different factors:
(i) high sea coefficient – spring tide; (ii) atmospheric depression; (iii) strong ocean swell. Certain storm
surges can reach a height of 3 metres. On sandy beaches, a storm surge can cause a brutal
reconfiguration of the shoreline
COASTLINE: the coastline is the limit of the highest level of the sea in spring tides and exceptional
tides. It separates land from sea. It is not clearly distinct from the natural upper PMD limit, which is
more easily characterised by the limit of the vegetation characteristic of salt water milieus.
Conventionally (in Europe) the coefficient 120 is used to define the highest spring tide without a surge
nor wind set down. Numerous factors (wind, swells, atmospheric pressure) can however, and with a
same tide coefficient, modify this line, which is therefore primarily a convention.
At first sight, the notion of coastline seems intuitively easy to understand: t h e l i n e t h a t s e p a r a t e s t h e
o c e a n f r o m t h e c o n t i n e n t . In reality, the boundary of the coastline implies the fixing of a “static” limit
within a milieu, the shore, that is primarily characterised by its dynamic nature and dual inclusion in the
land and the coastal waters. It should therefore be accepted a priori that the coastline is generally
50
mobile . The instability of the shoreline also hinges on different time scales: short (waves, tides),
longer (deposits or extraction of sediments (re)mobilised during exceptional or seasonal events);
geological time (eustatism, marine transgressions and regressions).
In every case, the line chosen is therefore only a compromise between different positions of the
shore. Except in the case of unaltered rock formations, the position of the coastline therefore remains
difficult to define and should theoretically be the result of the average of repeated measurements.
As part of the work conducted for the Aquitaine Coast Observatory, the French oceanographic
institution IFREMER and Geological and mining research Bureau (BRGM) define a n a v e r a g e d y n a m i c
c o a s t l i n e (“a line of equilibrium marked by the berm crest in good weather or springtime”) and a
m a x i m u m d y n a m i c c o a s t l i n e , equivalent to the “line of dynamic action marked by the peak of the
winter surges, the erosion beach scarps and the high watermarks of tides”. This definition is certainly
the most commonly encountered on an international level. Note that for France the difference between
these two measurements concerns an average height of approximately 1.20 m.
50
Especially on soft coastlines, but on other time scales rocky coasts are also subject to change.
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On another level, the length of the coastline itself depends on the scale of its cartographic expression
and generalisations (simplifications of the line) which are made when the scale is reduced, for in fact
the coastline is a fractal object.
Lastly, in the case of estuaries, the limit to be placed can only be arbitrary, given the highly seasonal
nature of the distribution of the salinity gradient of the waters.
While the definition of the coastline today appears somewhat obsolete for geomorphological studies
and tends to be replaced by morphodynamic monitoring of the shoreline making it possible to
apprehend and characterise the processes governing its evolution, it is still necessary for placing
the legal and fiscal ownership boundary of the public maritime domain, currently often based on
the analysis of the distribution limit of adjacent terrestrial plant formations characteristic of saltwater
milieus.
HYDROGRAPHIC ZERO: level of the lowest spring tides (chart datum), lower limit of the foreshore
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