CONSEQUENCES OF CLIMATE CHANGE
ALONG THE DANISH COASTS
SAFECOAST ACTION 5A
December 2008
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Disclaimer
The use of texts from the report with acknowledgement to the source is encouraged.
Although all care is taken to ensure the integrity and quality of this publication and the information herein, no
responsibility is assumed by the publishers or the author for any damage to property or persons as a result of
operation or use of this publication and/or the information contained herein.
A great deal of additional information on the European Union is available on http://europa.eu.int
For additional information on the SAFECOAST project please visit: www.safecoast.org
Keywords: Coastal erosion, flood hazard, impact assessment, sea level rise, climate change scenarios, coastal
complexity, fetch calculations
Author(s): Thorsten Piontkowitz & Carlo Sørensen
Publisher:
Danish Coastal Authority
Højbovej 1
7600 Lemvig
Denmark
Phone: +45 99 63 63 63
FAX: +45 99 63 63 99
Email: [email protected]
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Executive summary
Denmark has a long (7,400 km) and diverse coastline regarding its geological and
geographical characteristics where impacts on the coastal landscape of the latest
glaciations differ on local and regional levels. Open coasts and tidal flat coasts facing the
North Sea and sheltered coasts in fjords and embayments with fetches that vary from a
few hundred meters to several hundred kilometres add to the coastal complexity. The
societal interest in realistic estimations of the consequences of climate change has grown
greatly in recent years. Various interest groups (e.g. property owners, municipalities,
NGOs, media) with relation to the coastal zone are requesting reliable statements and
scenarios concerning climate change impacts on the Danish coastline. The range of spatial
scale may go from statements concerning all 7,400 km coastline to an assessment for few
kilometres of coastline at certain locations. Hence, impact assessments of the
consequences of climate change on the Danish coastline can be complex and multisided
depending on the scale of regard, which implicates the following questions:
•
How may we proceed in assessing the potential consequences of climate change
at different scales?
•
At which quality and informative value can impact assessments be performed at
different scales?
•
What is needed to perform these impact assessments at different scales?
Objectives and methods
Hence, the focus of this study, as a part of the EU Safecoast Action 5a, is on how an
approach for impact assessment of the consequences of climate change can be realised
for complex coastlines such as the Danish: How do we proceed in assessing potential
consequences due to climate change? The following scales and investigation methods are
adopted as starting points: At a national level, an analysis of the additional coastal
erosion by the year 2050 due to sea level rise along the entire Danish coastline is
undertaken and implemented by the production of a GIS-based coastal erosion atlas. The
definition of the coastal classification scheme, the regional wind-climate, wave energy
levels, and the littoral transport directions are incorporated in the production of the
coastal erosion atlas. At a local level, a more detailed impact assessment of the
consequences of climate change at four different coastal locations is performed. The pilot
sites reflect different types of coast, different types of coastal defence measures and
different degrees of hinterland occupation.
For the projections of sea level rise the A2 scenario (IPCC, 2007) is used. Local level
impact assessments are further made for the national UK scenario of 6 mm/yr sea level
rise in agreement with the overall Safecoast project outline. Additionally, at the two pilot
sites situated on the North Sea coast a modified A2 scenario is used that incorporates a
local 4 mm/yr sea level rise experienced there since 1973 (Knudsen, Sørensen and
Sørensen, 2008) which may affect the assessments of the future needs for coastal
protection measures along the North Sea coast.
Results
The produced erosion atlas gives an indication of the potential additional coastal erosion
until year 2050 due to a climate change related sea level rise at a national level. The
additional erosion amounts to between 0 m (rocky coasts) and 5-7 m on the exposed
sandy/littoral dune coasts facing the North Sea. The less exposed coastlines face an
additional erosion of 0.5-3 m. In relation to the coastal erosion atlas thematic maps of the
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coastal classification, of wind energies, wave energies and of littoral transport directions
are produced. The erosion atlas does not tell, however, if the additional erosion actually
will occur, as this locally will depend on the current and future states of the coast. Future
additional erosion may be negligible or turn out to become considerably larger than
assessed at the national level and has, therefore, to be investigated in more detail. It
further has become increasingly apparent that in many cases the sea level rise itself is
inferior to changes in the wind climate, and thus to the wave energies reaching the coast,
in determining the future rate of change. Finally does the transitional zone between the
North Sea and the Baltic Sea show a complex signature of water level variations with nonperiodic extremes that are not always directly related to storm events in the Danish
waters, as will the manifestation of a future global sea-level rise differ regionally due to
variations in glacial isostatic adjustment (GIA).
The impact assessment at the four coastal pilot sites Ballum-Koldy, Houvig, Løgstør and
Aabenraa showed that the consequences of climate change differ between the pilot sites.
For Ballum-Koldby inundation and more frequent flooding will be the main future
problem; at Houvig accelerated erosion will affect coastal protection efforts to keep the
coastline in place, and the pilot sites of both Løgstør and Aabenraa will experience more
frequent flooding of the hinterland and coastal erosion of the adjacent coastline during
storm surges. Central for all four pilot sites is the fact that the availability of different
data types and the data volume (e.g. the coastal topography and bathymetry, the
hydrodynamic regime, the existent coastal protection schemes) is scarce and varies
considerably between the pilot sites. Systematic data collection, e.g. within a program for
data collection, could not be detected at any pilot site. Available data and data quality
seem somehow coincidental in the four pilot sites.
The impact assessments at the four pilot sites also showed that there is not one single
model that can accommodate parameterizations for all different types of coast. Further,
it is unknown in detail how to prioritize the different parameters (e.g. hydrodynamic
forcing, seafloor gradient, morphological and geological conditions) controlling coastal
erosion, making the assessments difficult. Different methods are thus applied at each of
the four pilot sites. As coastal erosion rates are not linear in time in an ever evolving
coastal landscape with a large natural climatic variability, the assessment of it is not an
easy task. Where possible, the coastal erosion is looked into in morphological units that
take into account the coastal areas of erosion, transitional areas and areas of deposition.
One example is the pilot site at Løgstør where the breach towards the North Sea of the
Lim Fjord barriers in 1825 had a large impact of storm surge water levels and coastal
erosion rates, and another is Ballum-Koldby where it is still unclear if sedimentation
processes in the tidal basins can keep up with sea level rise or shifting of sand further out
in the Wadden Sea may lead to a different hydrodynamic forcing in the future. Valid
models for assessing coastal erosion at different types of coasts are still missing. In the
vast literature on coastal profile development and coastal erosion in relation to sea level
rise, no one model seems to satisfactorily describe and predict coastal development.
Conclusions and recommendations
In general, from the pilot sites and elsewhere along the Danish coastline, it may be
concluded that assessments of future coastal erosion and coastal flooding even at local
levels may be a difficult task especially in countries with a diverse coastal landscape such
as Denmark. Rates of impact of climate change on coastal erosion, inundation and
flooding were found to vary greatly between the pilot sites relating to the varying
coastline classifications, profile steepness, exposure and orientation of the coastline etc.
as well as within different morphological units at the individual pilot sites.
The production of the national erosion atlas shows that impact assessments at a national
level aim at simplistic approaches due to the tremendous lack of data and appropriate
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models to calculated future coastal developments. Simple approaches often go along with
a number of assumptions which have to be decided to implement national impact
assessments, that again include considerable uncertainties of the results. The quality of
these results will often be insufficient for planning and design due to the imperfect
information. Impact assessments at a national level may therefore be seen as suitable for
raising national initiatives which contribute to improve impact assessments at smaller
spatial scales.
Concerning the impact assessments at a local level, the quality of the results vary greatly
between the pilot sites which originates, amongst others, in the different availability of
data types and data volumes. The lack of models to assess coastal development at
different coasts complicates the performance of impact assessments at any spatial scale.
The expansion and improvement of these models still represents a huge challenge in the
coming decades, since the improvements of the models go along with better information,
including less uncertainty about further coastal impacts. However, due to the ability to
account for local variations in the coastal zone, decisions regarding long-term adaptation
measures should be made on the regional or local level. Moreover, in the process of
communicating adaptation measures, public perception on climate change and on its
consequences relate to local and thus well-known conditions and measures. The
availability and quality of data is crucial in the impact assessment. On a local level data
are easy to acquire or generate. The intention of a detailed impact assessment on a
national level to allow for sustainable decisions considering local variations is rather
unpromising due to the tremendous workload. On a local level the performance of impact
assessments including the generation of necessary data is manageable.
Due to the fact that adequate data are needed to perform coastal impact assessments to
help making sustainable decisions, it is recommended to initiate a Danish national data
collection program to allow for local impact assessments. Within this program the type
and quality of the data should be defined, as well as the general framework for impact
assessments to allow for the integration of neighbouring local impact assessments into one
regional assessment should be set. Further work is also needed on coastal change models
that accommodate the coastal variability and incorporates climate change. Tools for the
assessment of impacts of climate change are very much in demand by local authorities
and the work carried out on the four pilot sites are currently in the progress of being
transformed into guidelines and tasks for utilisation at a broader scale.
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Foreword
In the kitchen of rumours and horror scenarios, things are boiling in recent years. Global
climate change issues are discussed with large passion in the media, in expert groups and
in private. Especially in countries with many coastal kilometres, discussions are coined by
the threat of sea level rise, more frequent coastal flooding of low-lying coastal areas and
the intense loss of land due to coastal erosion.
However, often two circumstances follow with these discussions:
•
The call for immediate action, such as reinforcing flood defences and enhancing
protection measures against coastal erosion;
•
The assessment of climate change consequences is operated by narrow focus on a
small number of selected key parameters and basic information.
Since climate change will commence gradually over a longer time frame and as its degree
of impact is depending on how society adapts legislation, infrastructure and constructions
continuously to the different climate change impacts, the question arises: Is immediate
action required and adequate? Would a stepwise adaptation to climate change
consequences based on flexible approaches not be more suitable and sustainable,
especially when considering the large uncertainties we still face when assessing the
consequences of climate change?
Immediate action may raise the question of administrative level on which we should
react: local, regional or national – or at all levels? And what would be the particular
actions at each level? Are we actually in the position to react with sustainable solutions to
climate change at present when considering the available data for assessing climate
change impact? Do we already understand all hydrodynamic and geo-morphodynamic
processes in the coastal zone in such a way that allows us to decide on future measures to
meet consequences of climate change?
Together with the understanding of present hydrodynamic and geo-morphodynamic
processes in the coastal zone, assessing these coastal processes in future influenced by
changing climatic conditions is often formed by considering only a small number of key
parameters that are predicted to change in future, such as sea level. However, closer
inspections of e.g. a distinctive change of the predominant wind directions on local
coastal erosion processes or the impact of increased precipitation on flood defence
structures are rare.
This report intends to deal with some of the aforementioned questions and statements. By
conducting different actions, such as a thorough review on sea level variations in Denmark
and impact assessments at different scales, answers will be compiled to improve the
understanding and knowledge of climate change impact along the Danish coasts.
The report is the result of the Danish action within the project Safecoast being a 3 years
co-operation between a number of coastal management organisations in five European
countries bordering the North Sea: the Netherlands, Belgium, the United Kingdom,
Germany and Denmark.
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Contents
1.
Introduction ......................................................................................8
1.1 The project Safecoast ................................................................................. 9
1.2 Objectives and methodology of Action 5A....................................................... 10
2.
Climatic conditions affecting the Danish coasts......................................... 14
2.1 Sea level variations in Denmark ................................................................... 15
2.2 Storm surges and extreme water levels ......................................................... 19
2.3 Current rates and projections of sea level rise in Denmark ................................. 23
3.
Erosion atlas at a national level ............................................................ 29
3.1 Methodology ........................................................................................... 30
3.2 Wave energy at the coast ........................................................................... 34
3.3 Coastal classification schemes ..................................................................... 41
3.4 The Danish erosion atlas ............................................................................ 44
4.
Local assessments of four pilot sites ...................................................... 46
4.1 Pilot site of Ballum-Koldby.......................................................................... 47
4.2 Pilot site of Houvig ................................................................................... 58
4.3 Pilot site of Løgstør .................................................................................. 74
4.4 Pilot site of Aabenraa ................................................................................ 87
4.5 Findings of all four pilot sites ...................................................................... 94
5.
Conclusions and recommendations ........................................................ 99
5.1 Conclusions ............................................................................................ 99
5.2 Recommendations ................................................................................... 101
References ........................................................................................... 102
List of figures ........................................................................................ 110
List of tables ......................................................................................... 114
Annex A............................................................................................... 116
Annex B............................................................................................... 118
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1. Introduction
In recent years the issues of global climate change and sea level rise have caused societal
concern in many countries. Especially in coastal countries, such as Denmark,
consequences of climate change are elaborated in the media, in expert groups and in
private where future sea level rise often stand in the centre of discussion. At this, figures
for future sea level rise scattering within a large interval are subject to long-range
interpretations.
In the Intergovernmental Panel on Climate Change report (IPCC, 2007) this divergence of
sea level rise figures is due to the use of different climate change models and the
uncertainties in the applied emission scenarios. Low emission scenarios assume a decrease
of global atmospheric concentrations of carbon dioxide, methane and nitrous oxide,
whereas high emission scenarios expect an increase of global atmospheric concentrations
of greenhouse gases.
However, irrespective of the chosen emission scenario, fact is that climate will change
which raises two main questions:
•
What will be the consequences of a changing climate?
•
How do we adapt to these changes?
Climate change will, without doubt, cause changes in the coastal zone. Already today we
know that climate change will have potential impacts at different levels in the coastal
zone (Figure 1.1). First, direct impact will affect the hydrodynamic and hydrological
system in terms of (i) sea level rise, (ii) changes in wind climate and storminess and (iii)
increased precipitation.
Climate change
Impact on the hydrodynamic/hydrological system
Change in wind
climate and
storminess
Sea level rise
Change in
precipitation
Impact on coastal processes
Change in coastal
erosion
Increased hazard of
flooding
Increased river
discharges
(coast/shore protection)
(flood defence)
(sluice, outlet)
Impact on the socio-economic system
Increased probability of
economic damage
Increased probability of
loss of life
Figure 1.1 Different levels of climate change impact in the coastal zone.
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At the next level, these changes in the hydrodynamic and hydrological system will affect
the prevailing coastal processes at any type of coast. Sea level rise will cause permanent
flooding of coastal land. Changes in wind climate by e.g. the prevailing wind direction will
also affect the geomorphologic processes in the coastal zone and may turn out to be a
major force for considerable changes in coastal erosion and accretion processes (Figure
1.2).
(a)
(b)
(c)
Figure 1.2 Changes in coastal processes due to sea level rise: (a) permanent flooding and loss of
land, (b) coastal erosion, (c) increased hazard of flooding.
Besides the changes in the coastal dynamics, coastal defence measures will also be
affected by changing hydrodynamic and hydrological conditions. With increased
storminess, sea level rise increases the hazard of flooding of low-lying coastal hinterlands
during storms, incorporating a larger failure probability of the coastal flood defences.
Increased precipitation in the coastal zone will affect the regime in estuaries by increased
river discharges, which may implicate disturbances in the operation practices of sluices
and outlets. Furthermore, increased precipitation may have an impact on the stability of
certain coastal defence structures such as sea dikes.
The loss of land and more frequent storm surges, including an increased hazard of
flooding, will have an impact on the socio-economic system. This may be seen as next and
final level in the contents of climate change impact in the coastal zone. The probability of
economic damage and loss of life will increase if no adaptation to these consequences is
undertaken. But how to adapt to the consequences of climate change, where many facets
are still only coarsely quantified?
1.1 The project Safecoast
In 2005, a team of coastal managers and researchers from the Netherlands, Germany,
Belgium, Denmark and the UK decided to cooperate in a joint EU project named
Safecoast. The objective was to learn from each other about coastal risk management and
to plan for future challenges concerning the coastal zone. A main question was posed:
‘How to manage our North Sea coasts in 2050?’
The societal concern on the issue of global climate change and associated sea level rise
was given reason to develop and communicate new messages, conclusions and
recommendations regarding further steps in the North Sea region. However, the
translation of the consequences of climate change to national, regional or local actions is
difficult and often obstructed by issues of downscaling.
Hence, the primary focus of Safecoast is to deepen knowledge and understanding of the
future challenges of climate change in the coastal zone by a range of stakeholders and
coastal defence experts from both private and public sectors. The participants performed
scenario building exercises, conducted risk assessments on the potential outcomes of
climate change detailing the social, economic and environmental repercussions, and
examined possible solutions such as improved technical flood defence systems.
Additionally, Safecoast emphasises the need for enhancing public awareness to the topic
of climate change in relation to coastal risk management. The objectives and
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implementation processes of the EU flood directive in the involved project countries are
considered in outcomes of project Safecoast.
Project Safecoast is based on a number of studies named ‘Actions’ and does not aim to
engage in any political process. Each action has a general objective and is related to
either comparative, strategic or more technical activities. Overall, project Safecoast
includes six actions which are conducted by the eight project partners from Germany, the
Netherlands, Great Britain, Belgium and Denmark. The following topics are covered by the
actions:
•
An inventory of climate change and spatial development scenarios;
•
Risk communication methods;
•
Comparison of different flood risk assessment methods;
•
An integrated master plan for Flanders coastal safety;
•
Flood risk assessments for now and in 2050 at various pilot sites;
•
Consolidation of findings into adaptive strategies in an integrated coastal
management framework.
The project runs from July 2005 to June 2008 and is co-financed by the European Union in
the framework of the INTERREG 3B North Sea programme for transnational projects. The
project budget is around €2.3 million. The overall project management is placed at the
National Institute for Coastal and Marine Management/RIKZ in the Netherlands which has
commissioned Mr. Niels Roode as project manager. The homepage of project Safecoast is
www.safecoast.org.
1.2 Objectives and methodology of Action 5A
Denmark has a long (7,400 km) and varying coastline regarding its geological and
geographical characteristics, and impacts on the coastal landscape of the latest
glaciations differ at local and regional levels (Figure 1.3). Open coasts and tidal flat coasts
facing the North Sea and sheltered coasts in fjords and embayments with fetches (length
of water over which a given wind can blow) that vary from a few hundred metres to
several hundred kilometres add to the coastal complexity.
Figure 1.3 Denmark and the Danish coastline.
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At the same time the societal interest in realistic estimations of actual or potential
impacts on the Danish coastal system due to climate variability and/or climate change has
grown greatly. Various interest groups such as property owners, municipalities, NGOs, the
press and organisations with relation to the coastal zone are requesting reliable
statements and prognoses concerning the impacts of climate change in the coastal zone at
different scales. The range of scale may consider statements from the entire coastline to
an assessment for a particular groin field at a certain coastal location. Hence, impact
assessments of the consequences of climate change on the Danish coastline can be
complex and multisided depending on the scale of regard. This implicates the following
questions:
•
How can we proceed in assessing potential consequences of climate change at
different scales?
•
At which quality and informative value can impact assessments be performed at
different scales?
•
What is needed to perform these impact assessments at different scales?
In the Danish national strategy for adaptation to climate change (Danish Government,
2008) the coastal zone is one of a total of 12 sectors where climate change may have a
large impact and where ad hoc adaptation strategies are advocated. Tools for decisionmaking need to be further developed and answering the above questions may be one step
in this direction; pointing to solutions as well as to areas where our current knowledge is
still insufficient. Further, a strategy for communicating impact assessments may lead to
more reliable and cost-efficient adaptation strategies being applied at all levels.
Klein et al. (1999) conclude that adaptation to climate change in the coastal zone has to
be considered a multi-stage and iterative process. They list four basic and iterative steps
in a coastal adaptation process, Figure 1.4: (i) information collection and awareness
raising, (ii) planning and design, (iii) implementation, and (iv) monitoring and evaluation.
The conceptual process by Klein et al. (1999) includes further the two basic efforts of
mitigation to remove the cause of impacts (climate change) and/or adaptation to the
impacts, which is conditioned by policy criteria and coastal development objectives and
interacts with existing management practices.
Climate
change
Climate
variability
Impacts
Mitigation
Information,
Awareness
Planning,
Design
Implementation
Monitoring,
Evaluation
Existing
management
practices
Other
stresses
Policy
criteria
Coastal development
objectives
Adaptation
Figure 1.4 Coastal adaptation process to climate variability and change including four basic and
iterative steps (Klein et al., 1999).
With respect to this study, the first basic step of information collection and awareness
raising is of main interest. Klein et al. (1999) conclude further that this first step is
indispensable for adaptation in the coastal zone, both to show the potential need for
adaptation and to inform the adaptation process. The more relevant, detailed and
accurate the information that is available to planners and decision-makers, the more
targeted and effective adaptation strategies can be disposed.
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Klein et al. (1999) list four categories of climate-related information being important for
coastal adaptation:
•
Potential impacts in the coastal zone due to climate change;
•
Anticipated trends of other potential stresses (e.g. population increase, land-use
change);
•
Potential interactions between climate-related impacts and non-climatic
developments; and
•
Anticipated autonomous adjustments to combined climatic and non-climatic
impacts.
The first two types of information seem to be easily developed. Nevertheless, the
practical implementation may give rise to a number of challenges. The collection of
information concerning potential impacts in the coastal zone raises uncertainty about e.g.
a complete list of all impacts and how to assess the impacts in time and space. Because of
these practical problems uncertainty of the available information can remain considerable
and will affect the coastal adaptation.
Objectives
The main objectives of Safecoast Action 5A are thus to analyse how approaches for impact
assessments of the consequences of climate change can be realised at different scales for
the complex Danish coastline. The study further intends to investigate the informative
value and quality of impact assessments at different scales. Besides the main objectives
this study aims at:
•
Highlighting basic principles needed to perform impact assessments at different
scales, such as input data
•
Improving the knowledge of the effects of climate change on the coastal systems,
•
Enhancing the understanding of the interplay between the coastal defence
system and the related socio-economic system under changing boundary
conditions,
•
Improving the process of informing relevant parties (policy makers, public,
media, etc.) about the consequences of climate change in the coastal zone in an
appropriate way.
Methodology
To achieve the aforementioned objectives, the following scales and investigations have
been adopted:
•
At a national level, an analysis of the additional coastal erosion by the year 2050
due to sea level rise along the entire Danish coastline is undertaken.
•
At a local level, a more detailed impact assessment of the consequences of
climate change at different coastal locations is performed.
Analysis of additional coastal erosion due to sea level rise at a national level is
implemented by the production of a GIS-based coastal erosion atlas that shows additional
coastal erosion up to the year 2050. The definition of the coastal classification scheme,
the regional wind-climate, wave energy levels, and the littoral transport directions are
incorporated in the production of the coastal erosion atlas.
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At a local level, four pilot sites reflecting the coastal variability are selected. The pilot
sites are characterised by different types of coast, different types of coastal defence
measures and different degrees of hinterland occupation.
This report is divided into five chapters. In Chapter 2 the state of knowledge and
theoretical background about sea level rise scenarios for Denmark are provided. The
chapter intends to review sea level variations due to eustatic and isostatic movements
over the millennia to yield a better understanding of the coastal variability experienced
today. Since extreme events will have a large impact on the Danish coasts and with
specific water levels being exceeded more often in the future with an increase in mean
sea level, water level extremes and the applied methodology are described and discussed.
Finally, a look ahead by projecting future climate change related global sea level rise is
dealt with in this chapter.
Chapter 3 deals with the production of a macro-scale coastal erosion atlas at a national
level. Before introducing the adopted methodology to generate the erosion atlas, a short
discussion on available models and methods is given. To differentiate between the
different types of coasts in Denmark, a classification scheme is put forward. In the
classification scheme simplifications have been made to account for the large diversity of
coasts with distinctive differences in the size of the bordering water body, coastal
orientation and hydrodynamic conditions.
Chapter 4 includes the results of local impact assessments at four pilot sites along the
Danish coast. The four pilot sites are selected to reflect the variability of the Danish
coastline and the different challenges which will be faced by the particular communities
in the year 2050. The chapter closes with a summary of the overall findings in assessing
the four coastal sites.
Finally, in Chapter 5 results and conclusions concerning the different methodological
approaches for assessing potential consequences of climate change at different scales
along the Danish coastline are presented. Recommendations for future work are listed.
Danish Vertical Reference (DVR90)
All national elevation data presented in this report refer to the Danish datum DVR90.
The “tilting” of Denmark and the rise in the relative sea level led the Danish National
Survey and Cadastre to shift from the old datum, DNN, based on high precision levelling
and mean water levels in the late 19th century to the new datum, DVR90, being the
outcome of the Danish third precise levelling in the 1980s and the sea level observations
performed by the Danish Meteorological Institute since the early 1890’s. Further
information about DVR90 can be found in Section 2.1.
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2. Climatic conditions affecting the Danish coasts
Denmark has 7,400 km of coastline (Schou, 1949) that is broadly divided into the west
coast of Jutland facing the North Sea, the “inner Danish coasts” covering the Lim Fjord
region, the Kattegat, the Belts, and, the shores towards the western Baltic Sea, Figure
2.1. Spanning the open coasts towards the North Sea and the Baltic Sea to very sheltered
coasts in the inner Danish waters, the coastal landscape offers a great variety of coastal
landforms and coastal types.
Figure 2.1 The Danish waters.
Geologically the majority of the coastal landscapes of Denmark has been built up from
Neogene and Holocene sediments and is primarily a result of the sedimentation and
sculpturing effects of the last two glaciations, the Saale and the Weichsel, in combination
with marine impacts during the Holocene period. The entire area of Denmark was last
covered during the Saale glaciations. During the Weichsel several ice advances covered
Denmark except for the south western parts of Jutland. The ice moulded the landscape
and glacio-tectonic structures can be found in a large variety in many coastal cliffs. Apart
from moraine tills, areas of calcareous Cretaceous marine layers have been folded up into
130 m high cliffs e.g. on the Baltic Sea coast, and Palaeocene marine layers of moler
(diatomite) are found in the Lim Fjord area. Only on the island of Bornholm in the Baltic
Sea hard rock coasts can be found. For a thorough description of the Danish sedimentary
history, refer to Sand-Jensen (2006).
Besides the geological and geographical great variety of coastal landforms, the allocation
and use of the coastal hinterlands play an important role in assessing the consequences of
climate change along the Danish coasts. As about 80 % of the population live in urban
areas connected to the coast, and half of these within 3 km of the coast, issues of an
assumed sea level rise in relation to climate change is of major interest in physical
planning for many coastal municipalities and at the governmental level. To people living
by the coast it may be irrelevant whether the sea rises or the land subsides. However, in
order to assess the impacts of a climate change related sea level along the Danish coasts
at different scales, differentiation between the relative and absolute rates of movement
is important.
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Hence, this chapter intends to review sea level variations due to eustatic and isostatic
movements over the millennia to yield a better understanding of the coastal variability
experienced today. Since extreme events will have a large impact on the Danish coasts
and with specific water levels being exceeded more often in the future with an increase in
mean sea level, water level extremes and the applied methodology are described and
discussed. Finally, a look ahead by projecting future climate change related global sea
level rise is dealt with in this chapter.
2.1 Sea level variations in Denmark
Sea level variations are found at a variety of time scales from seconds to millennia. The
object here is not a complete review on variations on different scales and their possible
causes, but merely to reflect upon variations of major relevance to the Danish coasts as
experienced today.
Long term phenomena
Sea levels have shown variations in the order of hundreds of meters on a geological
timescale. These may be due to plate tectonics or to periodic and non-periodic
astronomical causes like sun radiation or variations in the orbital parameters of the Earth.
Several periodicities of 21,000; 41,000 and 100,000 years have been found in relation to
the Neogene glaciations over the last couple of million years. The glacio-isostatic
adjustment (GIA) succeeding the Fennoscandic ice-cover melt-back of the Weichsel
glaciations is a still ongoing process. The sea level quickly rose to inundate large low-lying
areas. Later the land uplift kept pace with the sea level rise, and app. 18-15,000 yr. BP
the Yoldia Sea covered a large area of the Northern parts of Jutland, where ancient
marine coastal cliffs and plains today can be found at heights of 20-50 m. During the next
large transgression phase, the Litorina transgressions (9-6,000 yr. BP), large areas in the
northern parts of Denmark were once again inundated. Since then the land has
experienced a relative uplift in the northern and eastern parts of the country, Figure 2.2.
Figure 2.2 Isobases of the total uplift experienced since the Litorina transgressions (Mertz, 1924;
Reproduced from Noe-Nygaard and Hede, 2006).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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This means that remnants of Stone Age settlements can be found above present sea level
to the north of and under water to the south of the island of Funen, respectively, and
ancient costal cliffs may be found at elevations reaching 12-14 meters in the
northernmost part of Jutland. Towards the North Sea the land extended several
kilometres further west than today. Several authors, e.g. Mörner (1975, 1980), have
proposed sea level curves covering the postglacial period.
The still ongoing glacio-isostatic adjustment led Duun-Christensen (1990, 1992) to translate the impact of a climate change related sea level rise of 50 cm into a 33 to 46 cm rise
for the Danish coasts; most in the south-western parts in the Danish Wadden Sea. In the
period 1890-1989 the relative sea level change was between +13 and -2 cm. The author
based his results on comparisons of tidal gauge measurements from Denmark with
estimates of global sea level rise over the majority of the last century. The results of
Duun-Christensen are still the only serious attempt to estimate and communicate at the
national level climate change related sea level rise in Denmark. Pre-assumptions are that
the GIA will be of the same order of magnitude this century as in the last, which can be
justified, and that water levels in Denmark has risen by the same magnitude at all stations
in Denmark as the global mean, which is more doubtful. Furthermore, local subsidence
zones in tide gauge records are not taken into account. Still, principles used and
assumptions made, make the results of Duun-Christensen somehow valid. Unfortunately,
accompanying Duun-Christensen’s results is an otherwise unpublished figure (Figure 9.4 in
Duun-Christensen, 1992) showing current uplift rates in Denmark. This erroneous and
misleading figure does, so to speak, cancel out any absolute sea level rise experienced in
the 20th century until 1990 and it masks the sea level rise experienced over the last two
decades. Seemingly there has been no effort to correct this mistake in the general public
as several publications since then, presumably backed by this figure, have stated that in
Denmark we have not been able to measure any increased sea level rise.
Duun-Christensen (1992) found an accelerated sea level rise in the period 1970-1989 and
this acceleration was confirmed by Binderup and Frich (1993) for a variety of long gauge
data series from around Denmark and for individual series, e.g. in Esbjerg by Nielsen and
Nielsen (2002), see further below.
One problem in using tide gauge measurements is that they measure the relative sea level
changes, and thus it has been difficult to separate sea level from land movements on a
timescale longer than decades. Until recently models of GIA were using gauge
measurements as input. Now, however, with the results from approximately 10 year-long
time series of high precision satellite measurements in an international network, this
separation between sea and land movements is being resolved into much more detail.
Denmark lies at the margin of the latest glaciations and therefore only a few models seem
to have been able to yield satisfactory results (although still within ±2 mm/yr). DTU-space
is currently working on a national map of absolute land movements based on satellite
measurements that will add confidence to the sea level variations experienced. For the
German North Sea and Baltic Sea coasts the tectonic and isostatic movements are an
order of magnitude less than eustatic movements (Mudersbach and Jensen, 2006). For
Denmark this may be correct for the most southern parts, whereas going towards north,
this is not the case. Another problem is the relation between the sea level rise in different
parts of Denmark and their relation to regional and global estimates of sea level rise. The
current as well as the future rates of sea level rise may thus deviate both between Danish
water compartments as in relation to the averaged recordings of mean global sea levels.
This issue is discussed later in relation to prognoses for climate change related sea level
rise.
The ‘tilting’ of Denmark and the rise in the relative sea level led the Danish National
Survey and Cadastre to shift from the old datum, DNN, based on high precision levelling
and mean water levels in the late 19th century to the new, DVR90, being the outcome of
the Danish third precise levelling in the 1980s and the sea level observations performed by
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P16
the Danish Meteorological Institute since the early 1890s. The shift means that the new
datum reflects the Danish topography and corresponds again to the actual mean sea levels
in Danish waters (around 1990). Here, the local annual mean sea level heights from 10
tide gauges, each with more than 100 years of data, have been used to fix the zero
reference point of the DVR90 (Schmidt, 2000). From mapping, the overall pattern of the
net differences is easily discernible although some islands previously had their own local
datum and thus deviate from the pattern, Figure 2.3. The conversion of heights between
the two systems can easily be made for any location in Denmark using the ‘KMStrans2007’
program downloadable from the Danish National Survey and Cadastre homepage,
www.kms.dk. So far, no absolute values of uplift rates or, glacio-isostatic adjustments
exist for the entire country.
Figure 2.3 Differences (m) between the former Danish datum (DNN) and the new Danish Vertical
reference (DVR90). Source: www.kms.dk.
Medium term phenomena
On a shorter time scale the nodal tide with a period of 19 years is dominant in tide gauge
measurements. Periodicities in the sunspot activity of approximately 11 years have also
been proposed as a control over sea level variations (Binderup and Frich, 1993). Yearly to
decadal regional sea level variations have been coupled to the North Atlantic Oscillation
(NAO) relating to atmospheric high and low pressures of northwest Europe. Especially the
winter NAO-index has been used as a correlating factor to regional sea levels. According
to Wakelin et al. (2003), the mean sea levels at the Danish North Sea coast are positively
correlated to the NAO-index by raising the mean sea levels in an order of 10 cm for each
positive integer of the index. In general, positive values (0 – +3) have been registered in
recent years leading to higher mean sea levels on the North Sea coast. The Baltic Sea also
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P17
shows a correlation to the NAO. The inner Danish waters act as a transitional zone
between the Baltic Sea and the North Sea. In this area, covering the majority of the
coastline, the relation between mean sea levels and the NAO-index is unclear and needs
to be resolved into more detail.
Astronomical tides
Tides in Denmark are semi-diurnal. The tidal waves propagate anti-clockwise in the North
Sea and the mean tidal range at the German-Danish border is a little less than two
metres. A large friction energy loss occurs due to the presence of the large submerged
terminal moraine, Horns Rev, north of the Wadden Sea that reduces the tidal range to one
meter on the central parts of the Danish North Sea coast. The Kattegat region has a tidal
range of about 30-40 cm, decreasing towards the south, and the southern parts of the
inner Danish waters and the Baltic Sea are essentially without tides with a range <20 cm
(e.g. Huess, Nielsen and Nielsen, 2002). Thus when relating to the coastal impact, the
astronomical tides play a major role in forming the landscapes of the Wadden Sea but
bear no morphological impact on the coasts of the inner Danish Waters including the
Baltic Sea.
Several investigations in recent years have shown that the tidal range along the European
North Sea coasts have increased over the last 50 years. The reason for this increase,
however, is more uncertain. Mudersbach and Jensen (2006) investigated long-term water
level time series from the German North Sea and Baltic Sea coasts and found that “since
about 1950 an extraordinary increase in Mean Tidal Range at the German North Sea
coastline occurred because of an increase of Mean Tidal High Water and a smaller
decrease of Mean Low Water”. In combining gauge stations from the German North Sea
and the Baltic Sea, respectively, Mudersbach and Jensen were able to make more general
statements about the variations of water level based on the ‘mean gauges’ of Mean Sea
Level (MSL), Mean Tidal High Water (MHW), Mean Tidal Low Water (MLW) and Mean Tidal
Range (MTR). Whereas changes in the MSL showed to be fairly constant with only a small
increase in the trends in the period 1950-2004 compared to the entire times series, the
mean tidal range has increased with up to 10 %, Table 2.1.
Table 2.1
Secular trends of mean tidal gauges with RMSE investigated by Mudersbach and Jensen
(2006).
Mean Gauge North Sea Island
1894-2005
Trend
RMSE
[cm/100
years]
Mean gauge North Sea Coastline
1950-2005
Trend
RMSE
[cm/100
years]
1857-2005
Trend
RMSE
[cm/100
years]
1950-2005
Trend
RMSE
[cm/100
years]
MSL
12.5
5.2
18.5
5.3
15.1
5.2
14.4
6.0
MHW
19.1
5.5
32.8
4.9
23.5
6.2
40.7
6.3
MLW
5.2
5.7
2.5
6.0
5.4
6.2
-16.2
6.3
MTR
14.1
4.1
30.5
3.0
18.1
6.9
57.1
4.0
Mean Gauge Baltic Sea
1838-2004
Trend
RMSE
[cm/100
years]
MSL
10.5
1949-2004
Trend
RMSE
[cm/100
years]
3.8
12.5
3.3
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Klagenberg et al. (2008) investigated the mean tidal range in the Danish Wadden Sea in
the period 1970-2003 and found increases at Havneby on the island of Rømø, close to the
German border, of 13 cm (from 171 to 184 cm) and at the harbour of Esbjerg of 9 cm
(from 152 to 161 cm).
2.2 Storm surges and extreme water levels
Catastrophic events wiping out entire communities and being geographically extensive
have been reported from around the North Sea region throughout history. The probability
of future tsunamis in Danish waters cannot completely be ruled out (Buch et al., 2005).
Going back in time, alleged numbers of fatalities may be grossly overestimated. From the
various compilations of coastal flooding events in several European countries (for
Denmark, see Gram-Jensen, 1985; 1991) and from proxy data, two points are taken:
•
There has been severe events pre-dating gauge measurements,
•
Collective memory is short.
In the Wadden Sea area reports are many as these relative densely populated areas have
suffered from many storm surges throughout history; still with the 1364 and 1634 ‘large
mandrownings’ taken as two of the most extreme in terms of victims. Along the central
Danish North Sea coast the loss of life has not been as large as most people lived on higher
grounds with the possibility to escape. Here, reports of fishermen lost at sea and of entire
villages disappearing by the dozen tell the story of storm surges and of coastal erosion.
The breaches of the Lim Fjord barriers in 1825 and 1862 have gained attention over the
years and they did, indeed, have a large impact on the physical processes and on the
population along the Lim Fjord. Along parts of the Danish Baltic Sea coasts storm surges in
1760, 1825 and in 1872 were extreme and are fairly well documented. Especially the very
large storm surge of 1872, which affected the entire Baltic Sea coasts of Germany and
Denmark, stands out. The very thorough work carried out by Colding (1881) tells the
detailed story of both the atmospheric conditions that led to the storm surge and of the
surge levels reaching more than 3 metres. Whereas storm surges are a regularly occurring
phenomenon on the North Sea coast, they rarely occur in the Baltic Sea. What makes the
1872 Baltic Sea surge so interesting is the extremity of the water levels. Different
statistical methods have estimated return periods of the surge water levels of 700 to
10.000 years. It occurred only 135 years ago, however, and we do not know whether we
face another storm surge of the same magnitude next winter or never. As pointed out by
Nielsen and Huess (2007), an extreme water level is more likely to occur in the future if it
has already occurred.
Relating to the statement about the short collective memory, reports of “…the largest
surge ever experienced”, or, “…for as long as anyone remembers” from the same
localities only a few decades apart are common in the historic literature. In November
2006 parts of the Belts and Kattegat regions experienced the highest measured water
levels in the up to 120 years of gauge measurements. Dikes breached, coastal cliffs
receded, towns were flooded and more than 4,100 insurance claims were filed due to
flooding. At one particular location on Funen an old dike had been dug through by the
local municipality to make space for a road and parking space for tourists. The village
behind the dike was flooded. Had everyone forgotten why the dike was built in the first
place? Apparently so!
The November 2006 storm surge and another few events in later years have put to the
attention the forces of the sea in areas not normally experiencing large storm surges.
Focus on climate change in the public has followed, although extreme water levels are, of
course, of a rarely occurring nature that in itself does not have anything to do with
climate change. Extreme events will have a large impact on the Danish coasts and with
specific water levels being exceeded more often in the future with an increase in mean
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P19
sea levels. Water level extremes and the applied methodology are therefore dealt with
into more detail.
Surge levels at the Danish coasts
Surge levels at the coast depend on the atmospheric forcing, on coastal profile steepness,
on the orientation of the coast and of the water compartment, amongst other parameters.
During passage of an atmospheric low, depending on the intensity and the track, water
may be forced into the North Sea thereby raising the general water level. Onshore winds
may lead to storm surges in the Wadden Sea and along the central Danish North Sea coast.
In the Wadden Sea surge levels of more than 5 metres have been recorded and water
levels may reach more than three metres on the central parts of the North Sea coast.
Usually there is a fairly straightforward relationship between storm events and surge
levels with onshore winds leading to positive surges. Storms in the North Sea transport
water into both the Lim Fjord and the Kattegat. In the Lim Fjord extreme storm surges,
reaching +2 m, are experienced in situations with a combined high general water level
and local surges. In the Kattegat, on the northern Jutland east coast, high water levels
are thus often experienced in connection with offshore winds. Together with waves from
the North Sea refracted on the Skaw spit, inflows of water lead to extreme conditions.
Likewise, easterly winds usually lead to water transport from the Baltic and Kattegat to
the North Sea yielding negative surges. Incidences of coastal erosion under the combined
influences of waves due to onshore winds and high water levels, therefore, are rare.
In the Sound and in the Belts some of the most extreme surge levels have been measured
in relation to standing wave phenomena. Occasions leading to extreme water levels are
rare and are not always connected to storms in the Danish area. Contributions to wave
phenomena may originate in either the Kattegat, the Baltic, or, both. Individual events in
later years have been described in detail by the Danish Meteorological Institute (DMI at
www.dmi.dk). Comparisons between these incidences and an overall estimation of
individual incidences and their likelihood to occur in the future still need to be
performed.
Extreme events may act as a ‘joker’ in the impact assessments on coastal change due to
climate change. To what extend should we protect ourselves against extreme events? It
obviously makes a big difference in the statistics whether calculated return periods of
water levels for some areas are based solely on continuous series of water level
measurements or they incorporate historical evidence. DCA has produced extreme sea
level statistics based on the analysis of 2767 equivalent full years of gauge measurements
at 55 stations (avg. 50.3 years) (Sørensen and Ingvardsen, 2007). The statistics are based
solely on continuous series of data collection (manual readings, analogue and digital
measurements) and the lengths of individual data series vary between 11.2 and 133.8
years and with more than 50 years at 21 stations. All extreme sea level data are presented
as trend-free values related to the mean sea level of the year of the individual
measurements. The statistics do not take into account the future local glacio-isostatic
adjustment or the eustatic sea level rise due to climate change which has to be accounted
for e.g. in impact assessments or spatial planning.
The highest sea level during a storm is registered as an extreme if it exceeds a certain
threshold for the station. One storm may have a long duration with several peaks and a
minimum of 36 h is chosen as the time limit between two independent extremes. In cases
where extreme sea levels are registered only a few days apart, the time series are
evaluated manually to see if incidents can indeed be considered as independent. From the
series of extreme values, the statistics are calculated as points over threshold (POT) using
the X3M S-Plus software. The stations are grouped in water compartments (the Wadden
Sea, the West Coast, and the Lim Fjord etc.) where the extreme sea levels are assumed to
follow the same distribution within each compartment. For the Wadden Sea, the Lim
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P20
Fjord and Odense Fjord the Log-Normal distribution generally gives the best fit, whereas
the Weibull distribution is used for the remaining stations. At some stations the
Exponential distribution is not statistically different but, given the above reason, is not
preferred. However, this tends to give more conservative results. In finding the best fit,
the distributional functions, the extreme return levels and the standard deviations are
calculated for different thresholds, or, cut-off levels. Extreme sea levels can have
different meteorological and hydrographical causes and one distribution may not describe
data very well. An investigation of the couplings between the individual station extremes
and their meteorological causes has not been possible and no attempt is made in
separating the extreme sea levels into different distributions. Refer to Sørensen and
Ingvardsen (2007) for further information on the applied methodology and the measured
extremes from the gauge stations.
Maps of extreme sea level curves for different return periods may be approximated from
the statistics. As mentioned earlier, the maximum extreme water levels vary geographically; refer to Figure 2.4 for a plot of the statistical 100 years return levels. The produced
frequency functions, Figure 2.5, presented in more detail later in relation to the pilot
sites, give an overlook of the statistical return periods of extreme water levels. Further,
the lines give some information on the nature of extreme events, where a very flat curve
usually is found in areas with smaller water level variations and with no very extreme
events in relation to the normal variations for that station.
Figure 2.4 Map showing Station numbers, 100 years extreme return heights and Station Names for
the 55 gauge stations. Dark blue dots annotate stations with good statistics and the light
blue dots stations where statistics are less certain due to short measurement periods (<15
years in general) and/or have large gaps in the data set (Adapted from Sørensen and
Ingvardsen, 2007).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P21
Figure 2.5 Frequency functions for the five gauge stations in the Danish Wadden Sea.
One consequence of a sea level rise is that extreme water levels of a given magnitude will
occur more often in the future. If sea level rises by e.g. 40 cm in Esbjerg, the 100 years
return level will become 446 cm, compared to 406 cm today, and the level of 406 cm will
statistically be exceeded every 22 years instead of every 100 years (Figure 2.6), following
the methods of Lowe, Gregory and Flather (2001). Given the uncertainties in the statistics
and in prognoses for climate change, a simple parallel shift of the frequency distribution
is justified (Haigh and Nicholls, 2008). This decrease in return periods will have a large
impact on the Danish coasts.
Figure 2.6 Sea level rise will cause extreme water levels to occur more frequent in the future. A
water level statistically experienced once every century today at Esbjerg, will occur
every 22 years with a sea level rise of 40 cm.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P22
The Danish Storm Council
Although the economic impact of climate change in Denmark only recently has become a
part of the agenda, in relation to extreme water levels focus on the economics has been
intense in the public since the flooding of 1-2 November 2006 in the inner Danish waters.
Succeeding severe coastal flooding events in Denmark a flood insurance scheme was
established in 1990 in order to administrate an emergency fund for compensation
payments due to flooding. Since 2000 the Danish Storm Council (www.stormraadet.dk),
has also covered compensation payments for damage in forests (CPSL, 2001; CWSS, 2005;
Danish Parliament, 1991, 2000). The means for compensation payments are collected as a
€ 2.7 annual fee for each signed fire policy covering buildings and movable property
yielding total annual revenue of app. € 11 million.
Compensations are paid to private persons, companies and farms for losses due to storm
flooding generally meeting the following criteria:
•
Gale force winds;
•
Water levels statistically occurring less than once in 20 years;
•
Extreme wave conditions may play a role;
•
Flooding must occur in a wider geographical area.
The DSC board has 4 ministerial members (min. of Economy, Transport, Justice, and
Environment) and representatives from the public insurance and pension organisations,
the Danish Regions and the Local Government Denmark, and the Danish Consumer
Association, respectively, each appointed for four years by the Ministry of Economics. In
the case of a severe storm, DSC is advised by experts from the Danish Coastal Authority
(DCA), Danish Meteorological Institute (DMI) and the Danish Forest and Nature Agency
before deciding whether an area is legible for compensation (CPSL, 2005).
Since 1991, 20 accounts of storm related flooding have been acknowledged by the DSC,
Annex A. Most of these have covered fairly restricted geographical areas. The extent of
the 1-2 November 2006 flooding, and the fact that the winter 2006/07 experienced 4
storm events leading to compensations, led to a still ongoing evaluation and debate on
how to re-define the scheme. A map of areas previously pointed out as eligible to claim
compensation from DSC is presented in Figure 2.7.
2.3 Current rates and projections of sea level rise in Denmark
How to project a future climate change related global sea level rise is a matter of much
debate. Comparison of historical records with recent data and the modelling by
climatologists and other scientist give no clear answers as to which extent sea levels will
rise over the coming centuries. Regional and local variability add complexity to
projections. IPCC (2007) estimates of an 18-59 cm rise, although with some uncertainties
inherited and a possible arctic contribution from melting and increased run-off not
included, are opposed to much larger numbers anticipated by some scientists. Examples
are Rahmstorf (2007) who projects a sea level rise in 2100 of 0.5 – 1.4 meters above the
1990 level, Pfeffer, Harper and O'Neel (2008) who project 0.8-2.0 m with 0.8 m being the
most plausible sea level rise, and, Grinsted, Moore, and Jevrejeva’s (2009) projections in
the interval 0.7-1.6 m by 2100 for a range of scenarios, Figure 2.8. There is also the ever
ongoing debate on how large a part of the sea level rise is due to anthropogenic effects.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P23
Figure 2.7 Number of times the coastal stretches of Denmark have been acknowledged as storm
surge areas by the Danish Storm Council, 1991-2008 (Source: www.stormraadet.dk).
Although the impact of sea level rise until year 2050 is of main interest here, prognoses
from IPCC AR3 & AR4 (IPCC, 2007; McCarty et al., 2001) are extended to year 2100 and
used in the discussions and in relation to the pilot sites. The intention is thus to aim for a
simple and realistic method for the assessment of a future sea level rise in Denmark; a
method that later may be updated and incorporated as a dynamical tool for DCA and
others.
In AR4 (IPCC, 2007), the scenario dependent range of sea level rise (5 - 95%) until 20902099 (avg.) is 0.18 - 0.59 m compared to the 1980-1999 average as mentioned above. The
scenarios describe different societal developments and no one of the scenarios is the
preferred, or, stated as most likely. The report, however, states that there is some
uncertainty regarding the melt-off in Arctic regions that may lead to an increased sea
level rise and is unaccounted for in the scenario projections. AR4 summarises the
contributions to sea level rise, their uncertainties and the knowledge gaps regarding both
past and future global water levels.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P24
Figure 2.8 Reconstructed sea level curves since 200 AD and new projections for sea level rise until
2100 against the IPCC AR4 A1B (inserted). Shades show standard deviations (5-95
percentiles). From Grinsted, Moore, and Jevrejeva (2009).
AR3 (McCarty et al., 2001) projections of sea level rise were pictured as curves and with
slightly different start and ending intervals as in AR4, where only the intervals of
projected sea level rise over the century are shown. The AR4 (IPCC, 2007) states,
however, that “for an average model (the central estimate for each scenario), the
scenario spread in sea level rise is only 0.02 m by the middle of the century. This is small
because of the time integrating effect of sea level rise, on which the divergence among
the scenarios has had little effect by then. By 2090 to 2099 it is 0.15 m”. Further, the AR4
lists the minimum and maximum rates of sea level rise in 1980-1999 and 2090-2099. This
justifies the drawing of a curve for the estimation of sea level rise to a given year in this
century. AR4 points out that sea level change will not be geographically uniform, with
regional sea level change varying within about ± 0.15 m of the mean in a typical model
projection. Therefore projections of sea level rise may be conservative when used on a
local to regional scale.
Holgate and Woodworth (2004 cf. IPCC, 2007) concluded “that the 1990s had one of the
fastest recorded rates of sea level rise averaged along the global coastline (~4 mm/y),
slightly higher than the altimetry based open ocean sea level rise (~3 mm/y)”. However,
their analysis also shows that some previous decades had comparably large rates of
coastal sea level rise. White, Church and Gregory (2005 cf. IPCC, 2007) confirmed the
larger sea level rise during the 1990s around coastlines compared to the open ocean but
found that in some previous periods the coastal rate was smaller than the open ocean
rate, and concluded that over the last 50 years the coastal and open ocean rates of
change were the same on average.
For the Danish Wadden Sea (town of Esbjerg) recent investigations by Knudsen, Sørensen
and Sørensen (2008) from tide gauge measurements show an accelerated rate of sea level
rise of ~4 mm/yr from 1972-2007 and ~5 mm/yr from 1993-2003, compared to an 18892007 average of 1.35 mm/yr. Their results are in accordance with a sea level rise at the
North Sea coast of 4.5 mm/yr measured from satellite altimetry in the period 1993-2004.
This slightly lower rate may be ascribed to a local subduction effect at Esbjerg (Ole B.
Andersen, DTU-Space, pers. comm.) that are not accounted for in the former
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P25
investigation, where the assumption is made that glacio-isostatic adjustments are small in
the Danish Wadden Sea. Methodologically Knudsen, Sørensen and Sørensen (2008)
removed most of the meteorological effects on the water level variations (r = 0.92, 86 %
of the variance) by the variable (0.8 + 0.065 * Wind speed) * (Wind speed)2 * cosine (Wind
direction - 250°) that takes into account the wind friction and projects the power of the
wind on the direction 250°. A good correlation to the NAO-index was also found. Table 1
gives an overview of recent results on rates of sea level rise, where the short-term period
1993-2003 is chosen for a comparison to available global sea level measurements by
satellite at the time of analysis. The sea level rise in the Danish Wadden Sea is currently
larger than the global average, and a higher rate of rise is measured at the coast than is
apparent in the local North Sea trend.
Table 2.2
Rates of sea level rise in the Danish Wadden Sea from tide gauge measurements
compared to regional and global measurements from satellite altimetry.
Source
Location
Method
Period
Nielsen and Nielsen (2002)
Danish Wadden
Sea (Esbjerg)
Tide gauge, yearly
averages
18901998
1.26
Knudsen, Sørensen and
Sørensen (2008)
Danish Wadden
Sea (Esbjerg)
Tide gauge, yearly
averages
18892006
1.35
Nielsen and Nielsen (2002)
Danish Wadden
Sea (Esbjerg)
Tide gauge, yearly
averages
19731998
4.21
Knudsen, Sørensen and
Sørensen (2008)
Danish Wadden
Sea (Esbjerg and
Havneby)
Tide gauge, weekly
averages, met. effects
removed
19722007
4
Knudsen, Sørensen and
Sørensen (2008)
Danish Wadden
Sea (Esbjerg and
Havneby)
Tide gauge, weekly
averages, met. effects
removed
19932003
5
DTU-Space (unpublished)
Danish North
Sea Coast
Satellite altimetry
19932004
4.5
http://sealevel.colorado.edu/
wizard.php
North Sea
(4º lon., 56º
lat.)
Satellite altimetry
(Topex/Poseidon)
19932003
2.2
http://sealevel.colorado.edu/
presentations.php
Atlantic Ocean
Satellite altimetry
(Topex/Poseidon)
19932006
3.5
Global
Satellite altimetry
(Topex/Poseidon)
Seasonal signals
removed
19932006
3.0
http://sealevel.colorado.edu/
presentations.php
Rate
mm/yr
Projections used in the present study
For the projections of sea level rise the A2 scenario (IPCC, 2007) is used for at the
national level and for the pilot sites, Figure 2.9. The mean value from the projected rates
of rise has been applied into an approximated curve throughout this century. Further,
taking the middle of the starting (1980-1999) and ending (2090-2999) intervals in the IPCC
report, projections are fixed in time. It is generally referred to just as A2 elsewhere in
this report. The figure also shows a ‘maximum’ curve of sea level rise from IPPC AR4
taken as the maximum (averaged) rates of sea level rise for the A1FI-scenario. This serves
as a reference in discussions. These two scenarios, A2mean and A1FImax, are situated in
the upper end of the cluster of scenarios presented in the latest IPCC assessment report
from 2007.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P26
Besides the IPCC projections, various countries have decided on national sea level rise
scenarios for local impact assessments. In the UK, for example, a national sea level rise of
6 mm/yr has been defined. The 6 mm/yr sea level rise is used next to the IPCC A2
projection in agreement with the overall Safecoast project outline.
Additionally, at the two pilot sites situated on the North Sea coast a modified A2 scenario
is used that incorporates the 4 mm/y sea level rise experienced there since 1973
(Knudsen, Sørensen and Sørensen, 2008). Constant rates of sea level rise at 4 mm/yr and 6
mm/yr introduce higher rates of sea level rise than the IPCC scenarios in the forthcoming
decades. Three set-ups are generated: From a 1990 base (Figure 2.8a), from a 2008 base
(b), and, (c) with 4 mm/yr from 1990-2008 for the IPCC-scenarios. The last set-up (c) is
used for the North Sea coast only as the time-averaged water level curves have not been
resolved into detail for the inner Danish Waters. Further, due to variations in the Glacial
Isostatic Adjustment (GIA) and to local zones of subduction, local projections are
corrected accordingly. Still, as no firm results of the isostatic movements have been
published for Denmark and as the eustatic sea level changes may vary due to differences
in the hydrodynamic forcing from place to place, inferred projections are to be taken only
as a rough measure.
For the variety of needs in relation to the local assessments and in the general use by DCA
(future nourishment needs, assessment of dike strength, the calculation of future extreme
water levels, socio-economic investigations, calculation of future erosion rates etc.) the
projections must be flexible back and forth in time. Jensen and Sørensen (2008) use the
modified scenarios to estimate future nourishment needs and relate these to the yearly
amounts nourished over the last two decades with an averaged 4 mm/yr sea level rise.
Some models (DMI, 2008; Kaas et al., 2001) show that a climatic deterioration will lead to
more frequent and more intense storms in the future that may effect erosion rates. For
the North Sea coast this may add 0.3 m to extreme water levels due to changes in wind
directions and intensities (2071-2100). Downscaling for the inner Danish coasts has not
shown the same effect. The possible climatic deterioration is dealt with in some of the
pilot site studies.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P27
Figure 2.9 Projections of sea level rise used in the present study. The IPCC AR4 A2(mean) is used in
the study at both the national and local level. IPCC A1FI(maximum) is used as a reference
and both scenarios have been approximated to a single curve. Further, a 4 mm/yr and 6
mm/yr have been used in the local assessments. Projection are shown: (a) for the period
1990-2100, (b) for the period 2008-2100, and, (c) with IPCC projections modified to
reflect an already experienced sea level rise of 4 mm/yr in later years (North Sea only).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P28
3. Erosion atlas at a national level
The applied methodology to produce an erosion atlas is described step by step as
illustrated in Figure 3.1. Apart from aiming at producing this macro-scale erosion atlas,
the scope is to develop a methodology that can be downscaled to local surveys that may
bring in more information but still take advantage of the macro-scale methodology.
Task
Assessment of future coastal
erosion
Methodology
(section 3.1)
Bruun rule
"Experts
guess"
Projections based on
historical evidence
Numerical
modelling
Probabilistic
Application
(section 3.2 &
section 3.3)
Results
(section 3.4)
Wave energy
Matrix setup
Coastal classification
Evaluation
Coastal Erosion Atlas
Figure 3.1 Methodology applied to produce an erosion atlas at a national level.
Coastal erosion is widespread along major parts of the world’s coasts. At some coasts
erosion has occurred over millennia, but at others erosion has only been experienced in
recent years, or, an increase in erosion is experienced. Dependent on the coast, on
infrastructure, on the industrial and residential use of the coast, we often seek to
minimize the land loss by means of expensive coastal protection schemes. At many
locations these coastal protection schemes are fairly successful, and over the years
coastal engineers etc. have gained invaluable insights into the physical coastal processes
governing the erosion and have been able to mitigate the consequences. To foresee to a
reasonable accuracy, however, how a natural unprotected coast will develop over the
next 10, 20 or 50 years, with or without sea level rise, is not an easy task. No single model
has yet been developed to accommodate various types of coasts relating sea level changes
to coastal development, and little agreement exists on the subject on how to make
projections.
In section 3.1, a short discussion on available models and on the diverse opinions amongst
scientists leads to a set-up of the methodology chosen for the current study. For Denmark
we set out to estimate the additional coastal erosion by year 2050 due to a climatic
change related sea level rise. An argument is that one should not attempt to distinguish
between the natural and the additional erosion due to sea level rise in projections as we
are still unable to model coastal development properly. This is what this study, as well as
other studies do, set out to accomplish. At least to a first approximation we have to point
to possible consequences of climate change in order to assist decision makers and others
with a focus on the coastal environment, and we have to be honest about how we make
our projections and about their validity.
The rate of coastal retreat depends, amongst other factors, on the geology. A hard rocky
coast withstands energetic waves much better than an unconsolidated sandy beach where
erosion rates of 10 m/yr are not uncommon. To differentiate between the different types
of coasts in Denmark, a classification scheme is put forward in Section 3.2. In the
classification scheme simplifications have been made to account for the large diversity of
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P29
coasts with distinctive differences in the size of the bordering water body, coastal
orientation and hydrodynamic conditions. The classification scheme acknowledges windgenerated surface waves to be the main, but not only, agent shaping the coastlines, and a
coupling of coastal classes and energy levels (treated in Section 3.3) combines into the
Erosion atlas in Section 3.4.
3.1 Methodology
Coastal erosion occurs in a complex interplay between wind, waves, currents, geology and
the topography of the coast. Some results from climate change modelling of the North Sea
imply that sea levels rise and that storms will become both more frequent and more
intense. The increase in mean winds will lead to a larger erosion potential of incoming
waves, and the sea level rise is generally believed to lead to permanent inundation of
low-lying coastal habitats and to an increase in erosion as the line of attack of the waves
moves further inland. Only a few models to predict/explain coastal erosion are
theoretically based, whereas many models are empirically founded on local to regional
scale studies. Some developments do indeed explain local variations but cannot be
extended to account for the coastal evolution at other localities. This is pointing both to
the coastal complexity and to the fact that no single model can adequately describe
coastal evolution in general. Not only do the physical processes act differently but also
e.g. the abundance of sediment may be a controlling factor between deposition and
erosion in a sea level rise scenario on a natural coastal system. A short overview of
existing approaches to coastal erosion is given below with reflections to the Danish
coastline and the usefulness for this study. The overall methodology used in the study is
presented.
Work in line of the Bruun Model and the Bruun Rule
Bruun (1954) suggested a simple equation to describe equilibrium beach profile forms on
unconsolidated coasts and later extended his work to describe a simple conceptual model
of profile evolution and a relationship between sea level rise and coastal erosion (Bruun,
1962; 1983; 1988). These have become known as the Bruun model and the associated
Bruun rule.
The Bruun model has the assumptions that (cf. Davidson-Arnott, 2005) “it applies to a
two-dimensional profile normal to the shoreline so that all net sediment transfers are
onshore-offshore and no consideration is given to inputs or outputs alongshore; the profile
is assumed to be an equilibrium profile entirely developed in sand, with the mean profile
form reflecting the wave climate and the size of the sediment, and, the material
landward of the shoreline consists of easily erodible sand with characteristics similar to
those in the nearshore. A fourth implicit assumption is that the wave climate frequently
produces waves of sufficient size to erode, transport, and redistribute sediment over the
profile - clearly if there are no waves, sea level rise would simply result in inundation of
the landward profile”.
Based on the hypotheses that wave action will erode the upper beach as a result of a
translation of the profile due to sea level rise, that all eroded material will deposit in the
nearshore and the thickness of the layer deposited equals the sea level rise, thereby
maintaining the equilibrium form and nearshore depth, Bruun (1962; 1983) proposed the
intriguingly simple relationship between sea level rise and coastline retreat (see Figure
3.2):
s=
la
,
h
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P30
where s is coastline retreat, a is sea level rise, h is the height of the active profile, and l
is the ‘active’ profile width, i.e. the point where the profile levels out and no change in
height of the bottom is observed. This point is often referred to as the Depth of closure
(Doc) following the concepts of Hallermeier (1981). Typically, according to the Bruun
Rule, the rate of lateral erosion is typically two orders of magnitude greater than the sea
level rise (e.g. Leatherman, 2001).
os
Er
ion
Deposition
Figure 3.2 Sketch of the Bruun Rule.
Numerous field studies deal with the Bruun Rule and variations thereof. The rule has been
applied to coasts all around the world and, irrespective of its explicit limitations, not only
on sandy beaches but also on soft cliff shores. Without engaging too much into discussions
here, statements previously made in the literature are that the simplistic assumptions
made for the Bruun Rule are rarely (or never) met and generalisations upon its use
between different stretches of coast have been hard to establish. Criticism of its use for
planning purposes without consideration of its limitations has often been forwarded.
Nevertheless, the Bruun Rule has survived for almost 50 years and is still in use in spatial
planning in several countries, e.g. “to recommend additional coastal setback allowances
in relation to IPCC scenario projections” (Walsh et al., 2004).
Jensen and Knudsen (2006) used averaged profile steepness’ between -2 to -4 m for
protected, moderately exposed, and exposed coasts to evaluate erosion due to sea level
rise for the inner Danish coasts. Zhang, Douglas and Leatherman (2004) validate the two
orders of magnitude ‘rule of thumb’ recession in their study from the U.S. east coast. The
authors, emphasise, however, long-term studies/results must be applied to allow for
shorter term storm and post-storm recovery coastline displacements. Others like Cooper
and Pilkey (2004; 2004b) advocate the abandonment of the Bruun Rule to look instead for
alternatives to mathematical and numerical modelling of beaches.
In general, only some parts of the Danish coast facing the North Sea are made from
unconsolidated sandy materials. The abundance of sand varies and alongshore transport is
far from negligible. The Bruun Rule somehow could prove valid here, which has been
considered for the present study, but the choice is to look for alternatives as the rule
seems meaningless for application to the remainder of the Danish coast.
Davidson-Arnott (2005) proposed a conceptual model that, opposed to the Bruun Rule,
considers the dune sediment budget and processes associated with beach-dune
interaction. In the model of Davidson-Arnott no net transfer of sediment to the nearshore
profile is predicted and the foredune is preserved through landward migration. A further
development and testing of this model would be interesting for especially the Wadden Sea
island coasts towards the North Sea but this is beyond the scope of the present study.
Numerical and mathematical modelling approaches
The Bruun rule can, of course, be seen as the simplest form of 2D-modelling, and the
ideas and hypotheses of Bruun play a role in numerous more complex modelling suites
developed over the years to which profile development plays a central role. We here have
to distinguish between the Bruun Rule and a ‘Bruun(like) profile’ where the latter often is
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P31
used as a general term for equilibrium beach profile forms (Bruun, 1954; Dean, 1977;
1991, and many others) that, together with changes in the slope of the profile, is
described by fairly simple mathematical equations. Coastal evolution will to some extent
depend on the profile form which again is a function of wave energy levels, geology and
sediment availability.
Still, dependent on local conditions many other factors like rainfall, vegetation, offshore
shoals etc. may play a role. Problems with existing models, from a planner’s point of
view, are that they are relatively difficult to access and laborious and time consuming to
set up. Many models further contain ‘black box’-calculations that make results hard to
evaluate in relation to possible future coastal erosion. There also seems to be a lack of
models to assess coastal erosion for all coastal types and we do, in general, not have
enough data to feed the models. Another problem is that the priority of the parameters
(hydrodynamic forcing, geological conditions, morphological gradient of the seafloor etc.)
is unknown under a changing rate of sea level rise. Also, it is impossible to transfer data
and results from one coastal stretch to another due to differences in a lot of parameters.
“Coastal stretches should [therefore] always be regarded as ‘geomorphologic units’ which
means to take into account the whole coastal area from the erosion part via the transition
zone to the accumulation area” (K. Schwarzer, pers. comm.).
Much progress has been made in modelling in recent years to accommodate a larger
variety of coasts, however, there still is some way to go to calculate future coastal
erosion rates precisely with or without sea level rise. For the present study it soon
became apparent that modelling could not be performed within the timeframe of the
Safecoast project. Modelling can become a viable tool in future studies; validation of the
models still being difficult due to the scarcity of data. Of particular interest for the Danish
coastline is the modelling work on erosion of soft cliffs performed under the Tyndall
Centre for Climate Change Research (UK) (Dickson, Walkden and Hall, 2007; Walkden and
Dickson, 2008; Walkden and Hall, 2005; 2006).
Experts’ guess
Projections of future coastal erosion based on experience and scientific knowledge may
play a part of a ‘common sense’ conceptual approach framework as an alternative to the
mathematical modelling of beaches. Cooper and Pilkey (2004) proposed the following 7
alternatives:
•
Experience on the beach in question,
•
Experience on neighbouring beaches,
•
The global beach experience,
•
Local beach indicators and field studies,
•
Predict nothing,
•
Go slow – Go soft,
•
Use a composite approach.
The authors do, so to speak, advocate a more intuitive qualitative modelling approach. In
relation to this study, where an assessment for future (additional) erosion of 7,400 km
coastline is desired, we will have to make simplifications and have to extend ‘experience’
beyond the beach in question or neighbouring beaches to some form of ‘global beach
experience’, or, national beach experience.
For the composite approach the authors use the British project ‘Futurecoast’ (Cooper and
Jay, 2002) as an example of an investigation that “employs a combination of
geomorphological expert opinion, historical change records, wave modelling,
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P32
morphological measurements and bathymetric changes” to assess the likely future
behaviour of the coast for different scenarios. According to Nicholls and Stive (2004)
“more than ‘expert judgement’ and extrapolation of past rates is necessary in the analysis
of future erosion”, and “…such analyses generally cannot be deferred in the face of
population growth and development on coasts affected by rising sea levels” (Cowell et al.,
2006, cf. Walkden and Dickson, 2008). Discussions in literature, as to which methods to
apply, are intriguingly interesting to the question on how to predict future erosion.
Uncertainties will be inherent in all projections, but problems are a large natural
variability in nature and climate, and the fact that most field studies and historical
records do not cover periods long enough to really be able to do more than general
predictions.
Moreover, in the case of an accelerated sea level rise, we simply do not have correlations
between erosion rates and rates of sea level rise back in time. Still the qualitative
modelling approach has some advantages to the trained eye, especially in local
assessments as it may build on estimation of many physical parameters; adding e.g.
extreme water level statistics and climatic records to the parameters of Cooper and Jay
(2002) mentioned above. An assessment of this kind cannot be objective and will depend
on the eyes that see and, further, it will be difficult to make e.g. for a planning officer in
a local municipality. On the other hand, if arguments for the assessment and for the
priorities made in choice of the parameters included stand clear, and if the
communication of the results of the assessment within its given limitations is honest, a
qualitative modelling method can prove a viable dynamical tool.
Projections based on historical evidence
Projections based on historical evidence only have been used in several studies
worldwide. Most often the historical rate of erosion has just been used to estimate future
set-back lines. This method presupposes that no changes occur in wave energies,
bathymetry, the geology into which the erosion occurs etc. Most often this is not the case
in nature and, particularly in relation to an accelerated sea level rise, this may prove
erroneous. One way to deal with this is to add a contribution (positive or negative) to the
future rate. The most precise way is to measure the cross shore profile regularly at
constant intervals along the coast over e.g. several decades. In this way knowledge is
attained of both the long-term averaged erosion and where in the profile erosion occurs.
DCA has carried out yearly profile measurements along the North Sea coast for many years
(at some stretches since 1874) and thus keeps a unique record of profile retreat (DCA,
2008). Most often long beach profile records do not exist and investigations based on
aerial photography (for Denmark dating back to 1945 at approximately 5 year intervals)
and maps can be performed. Coastal erosion does not occur linearly. An example from
Denmark is a soft cliff (till) coast on the north-western part of the island of Funen that
experienced a recession of approximately 10 m during a single storm with associated
extreme water levels in November 2006. This recession exceeds the total recession in the
past 40 years. Care must therefore be taken in evaluation of the results and, if possible, a
correlation between episodes of erosion and specific climatic occurrences can be helpful
in the assessment of future erosion.
Probabilistic methods
The above leads to the last suite of methods presented here, the probabilistic. Instead of
linear projections Hall et al. (2002) applied an episodic stochastic simulation model to a
coastal site in the UK. The use of this probabilistic method demonstrates the potential
variability in predictions of costal cliff recession. Also does the statistical methods allow
for inclusion of expert geomorphological assessment of the local landslide characteristics
and cliff fall measurement in combination with historic records of cliff recession.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P33
Various probabilistic models/approaches have been developed in relation to coastal
erosion, that deal e.g. with statistical techniques, structural use of experts guesses and
simulations in process-based setups.
Methodological set-up
Now, at the national level for Denmark we seek to estimate the additional erosion in 2050
due to an accelerated sea level rise at a spatial resolution of 5-10 km along the coastline.
The projected sea level rise over the next 40-45 years is ‘only’ about 15 cm, its fairly
difficult to make precise predictions of coastal change and ever harder, if not impossible,
to differ between erosion and additional erosion, and, previous investigations in Denmark
are few and scattered in time and place. Still, we want to give an overview. The choice
has been to make an ‘expert judgement’ based on the authors and on two of our
colleagues who have been working with coastal engineering (processes and design) for
more than 30 years.
In acknowledging short comes of the approach in relation to the methodology, to the
natural variability and the coast’s different exposure to extreme water levels, and, the
diversity in coasts (geology, morphology, processes), we emphasise that results may be
merely an overall reflection of the erosion potential: If we look at a specific stretch of
coast, say 30 km, we can get a hint to the order of future erosion, whereas if we look at 3
km a site-specific study is needed to evaluate on that coast.
The expert judgement (presented in Section 3.4) is based on the experience in
combination with a coastal classification scheme (Section 3.3) and estimations of wave
energy levels on the coast (Section 3.2) as mentioned in the beginning of this chapter.
Experiences from the Danish coasts are that central factors to the understanding of
coastal erosion are the exposure together with the geological and morphodynamic
conditions. In the evaluation of results, comparisons with historical erosion rates are
performed where possible.
3.2 Wave energy at the coast
One way to parameterize the wave energy levels at the coast is by applying the CERCformulas (CERC, 1984). The CERC-formulas calculate the energy-flux pr. unit wave length
using fetch and wind speed. In applying the CERC-formulas, we disregard local effects of
the beach profile, e.g. refraction, diffraction and energy dissipation as waves approach
shallow waters but to a first approximation the method seems valid. With more than 7,400
km of coastline we have to find a simplified method on how to do so.
Fetch calculations
The fetch, F, describes the distance over water over which the wind may blow to generate
waves. F may be calculated in a variety of ways depending on its use, and on the accuracy
and number of calculations desired. Traditionally F is measured manually every 5 5/8
degrees on topographic maps and a mean F is calculated for each of the eight major
compass directions. Here, the choice has been to calculate F for each 10 degrees.
Lundqvist et al. (2006) use a raster-based GIS method to calculate the maximum F in
different compass directions at sea for cells with a spatial resolution of one square
nautical mile in the North Sea - Baltic Sea transition and apply their methodology to the
re-suspension potential frequency for sediments at the sea floor. Ekebom, Laihonen, and
Suominen (2003, 2003b) and Puotinen (2005) use a vector-based GIS method of calculating
F for a coastline. The coastline is divided into a number of points and at each point, the
vector F is measured in a number of directions radiating out from that point. For the
production of the erosion atlas, calculations of F are automated in a GIS-environment set
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P34
out in accordance with the methodology of Ekebom, Laihonen, and Suominen (2003,
2003b).
GIS calculations are performed in ArcView 3.2 and with user developed extensions
applied. From an area information system coastline theme of Denmark (AIS, 2006) a
polygon theme is created to get an exact match between the two. The coastline is divided
into a number of regularly spaced points representing the individual coastline stretches
using the Poly to Points function of Huber (2002). The distance between points, which
may be set at any value in weighing accuracy against computing capacity for the further
calculations, is set at 5 km. The distance of 5 km is evaluated to give a good
representation of the conditions on the Danish coastline at macro-scale level. A few points
e.g. in harbour basins are not representative for the coastline and are deleted manually.
Then radiating lines are drawn from each point in ten degrees intervals and with a length
of 750 km using the Radiating Lines and Points 1.1 (Jenness, 2006). To cut off the lines, a
polygon of the sea around Denmark was made from inversion of a polygon map of Europe
(Mapinfo, 2006) where the Danish coastline was substituted with a polygon theme created
from the Danish coastline theme to obtain a perfect match between the two.
The lines were then cut using the ArcView processing tool thereby removing line segments
that are not placed over the sea. Many line segments then show, e.g. when a line crosses
an island or a fjord, which do not originate in a point. To remove these line segments the
start and end coordinates of all lines as well as their lengths were calculated by the
function Point & Polyline Tools (Alsleben, 2002). By comparing the coordinates of the
points with those of the line segments, the lines that do not originate in a point can be
discarded. The lengths of the remaining line segments now represent mean F for the
different compass directions in each point, Figure 3.3, and with F representing 2.5 km of
coastline on either side of that point. Each point thus now holds values of fetches for the
different directions as well as values of maximum and mean onshore fetch.
Figure 3.3 Line segments representing the fetch for different compass directions at points 5 km
apart. Points that are not representative of the coastline, e.g. in harbours, are deleted
manually.
The methodology sketched for calculating the mean and maximum F has then been
automated to allow for future calculations of F at the local level, i.e. with points
representing shorter stretches of coastline.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P35
Wind data
Statistically wind data is often used in ‘normal periods’ of 30 years, e.g. 1961-1990, that
acknowledges natural variability in regional wind climate. Here the choice is to use recent
(1995-2005) 10-11 years data sets of wind measurements. A 10 year period does not have
the same statistic significance as a 30 year period but it may yield a better picture of the
current conditions and better reflect the actual wind climate at the coasts where onshore
winds are the main interest. Further does the shorter data series allow for a closer
relationship between wind measurements and a given stretch of coastline as only a few
stations keep a long record, and as large local variations in winds occur regionally (e.g.
see Jensen (2007) for an analysis of the correlation between some of the stations). Also,
in a climate change scenario shifts in the dominant meteorological conditions may occur
over a period of less than 30 years and possibly with an even larger natural variability than
has been experienced in the past. As such, the use of the last ten years of recordings may
be justified: they show the current climatic conditions but do not tell anything about
changes that may be experienced in the near future. Raw data from 16 stations with good
data series, courtesy of DMI (2006), are selected to cover the Danish coastline (see Figure
3.4).
Figure 3.4 The 10 year data series representation on the Danish coastline of wind climate
measurements from 16 stations.
Wind measurements are often affected by the surrounding topography and a considerable
variation may be found over short distances. The division into the 16 sectors was
performed to best represent the different coastal stretches and surrounding water bodies
and no interpolation between stations was performed. Generally, the data series are of a
very good quality. Erroneous data have been omitted, but missing periods in the data have
not been substituted. This may lead to a slight under-representation of high wind speeds
if malfunctioning occurs during storm events.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P36
All wind data were adjusted to 10 m above ground according to CERC (1984), and weighing
of data has been performed at stations where the measurement frequency has changed
over the period of 10 years. The wind data was sorted by direction in 30 degree intervals
and wind speed after the Beaufort scale (12 by 13 interval matrix). The Beaufort scale
may seem outdated but gives the possibility to compare with former investigations of wind
climate. A presentation of wind distributions is given in Figure 3.5.
Wave energy levels
The wave energy levels at the coast are then calculated based on the wind and fetch
calculations by use of the aforementioned CERC-formulas. Some pre-assumptions had to
be made regarding the waves and energy levels: First, the wave energy in individual waves
depends on their size. Often the energy levels are calculated from the significant wave
heights, defined as the average of the largest 1/3 of the waves. This method is a
conceptually simple distributional representation of the wave energy (In the CERCformulas wave period and height are given by T1/ 3 = 0,95 * Tm and HS ≈ Hm o making the
relationship straightforward).
Secondly, water depth may limit wave growth and where calculations of wave parameters
depend on water depth. As the wave energy levels have to be calculated for the entire
Danish coastline, as waves form and propagate differently in deep and shallow water, and
as data on local bathymetry are not readily available, wave energy levels have been
calculated as deep water waves. This will lead to an overestimation of the wave energies
actually reaching the coast.
Thirdly, wave formation depends on time. For the inner Danish coast the wave formation
is assumed to depend only on the fetch as the limiting factor and not the duration of the
wind. As fetches are relatively short (<150 km) this may be justified. On open coasts the
wave formation may be duration limited. This will lead to overestimation in the
calculations. The governing formulas in the calculations thus are (CERC, 1984):
Hmo = 5,112 E − 4 * U A * F 1/ 2 ,
Tm = 6,238 E − 2 * (U A * F )1/ 3 ,
with fetch (F) and the adjusted wind velocity ( U A ) calculated form the measured wind
velocity (U) by: U A = 0.71 * U 1,23 .
The averaged energy flux pr. unit wave is calculated as: P = EC g =
ρ * g * H2
8
* Cg ,
with the density of water (ρ), gravity (g) of 9.8 m/s2, and group velocity of the waves
( C g ). The salinity, and thus the density of the water, varies in the Danish waters. To
simplify calculations, however, a constant density of 1030 kg/m3 is used. Further, in deep
water Cg = C0 / 2 and C0 = 1.56 * T .
The wave energy flux may then be calculated using the fetch (F) and wind velocity ( U A ):
P=
1030 * 9.8 * H 2 1.56T
*
⇔
8
2
P=
1030 * 9.8 * 5.112 E − 4 * U A * F 1/ 2
8
(
)
2
*
1.56 * 0.95 * 6.238 E − 2 * (U A * F )1/ 3
⇔
2
P = U A7 / 3 * F 4 / 3 * 1.524118 E − 5 .
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P37
Anholt
Hvide Sande
S12
S12
S11
S11
S11
S10
S10
S10
S9
S9
S9
S8
S8
S8
S7
S7
S7
S6
S6
S6
S5
S5
S5
S4
S4
S4
S3
S3
S3
S2
S2
S1
1
2
3
4
Rømø/Juvre
0
5
6
7
8
9
10
11
Blåvandshuk Fyr
S12
12
1
2
Skagen
0
4
5
6
7
8
9
10
11
1
2
3
4
5
6
7
8
9
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11
0
2
3
5
6
7
8
9
10
11
1
2
3
5
6
7
8
9
10
11
2
Figure 3.5
4
5
6
7
8
9
10
11
12
S10
S9
S9
S9
S8
S8
S8
S7
S7
S7
S6
S6
S6
S5
S5
S5
S4
S4
S4
S3
S3
S3
S2
S2
S2
S1
0
12
1
2
3
4
5
6
7
8
9
10
11
12
S1
0
1
2
3
4
5
6
7
8
9
10
11
12
Omø Fyr
S12
S12
S12
S11
S11
S11
S10
S10
S10
S9
S9
S9
S8
S8
S8
S7
S7
S7
S6
S6
S6
S5
S5
S5
S4
S4
S4
S3
S3
S3
S2
S2
12
S2
S1
0
1
2
3
4
5
6
7
8
9
10
11
12
S1
0
1
2
3
4
5
6
7
8
9
10
11
12
Gniben
S12
S12
S12
S11
S11
S11
S10
S10
S10
S9
S9
S9
S8
S8
S8
S7
S7
S7
S6
S6
S6
S5
S5
S5
S4
S4
S4
S3
S3
S3
S2
S2
S2
S1
S1
12
12
11
S11
0
1
2
3
4
5
6
7
8
9
10
11
12
Kastrup
3
10
S10
S1
0
1
2
3
4
5
6
7
8
9
10
11
12
Hammer Odde Fyr
S12
S12
S12
S11
S11
S11
S10
S10
S10
S9
S9
S9
S8
S8
S8
S7
S7
S7
S6
S6
S6
S5
S5
S5
S4
S4
S4
S3
S3
S3
S2
S2
S1
1
9
S10
Møns Fyr
0
8
S12
Nakkehoved Fyr
4
7
S11
Røsnæs Fyr
0
6
S12
Albuen
4
5
S11
S1
1
S1
4
S12
Kegnæs Fyr
0
1
2
3
Thyborøn
12
S1
0
S2
S1
3
S2
S1
S1
0
1
2
3
4
5
6
7
8
9
10
11
12
Distribution of winds on the Beaufort scale in
directional sectors (S1-S12) for the 16
measurement stations shown in Figure 3.3. Each
sector represents a 30°-interval, where S1 is 346°15°, S2 is 16-45° etc. 10 year datasets 1995-2005.
0
1
2
3
4
5
6
7
8
9
10
11
12
Hanstholm
S12
S11
S10
S9
S8
S7
S6
S5
S4
S3
S2
S1
0
1
2
3
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
4
5
6
7
8
9
10
11
12
P38
The mean value in the intervals from the Beaufort scale has been used. In the matrix,
with onshore winds sorted by direction and speed, the wave energy is calculated for each
cell by multiplying with the corresponding fetch and multiplied by the share (in %) for that
cell. The total wave energy is then found by summing up the contributions.
Results show, Figure 3.6, the highest wave energy levels on the Danish North Sea coast.
The intervals were chosen to give the best representation (within a limited number of
intervals) of the energy levels. Relatively high energy levels (red and grey colours) are
also seen for parts of the coastlines facing the Baltic Sea and the Kattegat, whereas
sheltered conditions prevail in the belts and in the fjords. As mentioned above, wave
energy calculations are based on the fetches, and as duration limitation may occur with
long fetches, some overestimation may occur at the open coasts facing the North Sea and
the Baltic Sea. For comparison, measurements of the Fjaltring wave gauge on the Danish
North Sea coast show wave energy levels of 11-17,000 W/m averaged over the year (DCA,
2003). Of some importance are also local deviations at the coastline where a point may
not be very representative of that stretch of coast. Still, the results should only be
evaluated at a macro-scale level and not for individual points.
Figure 3.6 Wave energy levels at the Danish coast calculated for every 5 km. See text for further
explanation.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P39
Inferred alongshore transport directions
As wave energies have been calculated for all measured directions, the resultant vectors
may be used to infer net alongshore transport directions, Figure 3.7. Results are divided
into a significant transport towards left or right along the coast, or, no significant
transport when wave energies are equally distributed around the coast-normal direction,
or, when wave energies are very low. There may still, of course, occur net local
alongshore transport. The results do not take into account currents and tides and physical
phenomena related to the local bathymetry.
DCA (2000) investigated alongshore transport directions from visual inspections (aerial
photography and film) of groins and marine landforms. Their results (refer to Annex B)
coincide reasonably well with Figure 3.7. Actually, on the open coasts the nodes between
upstream and downstream transport show a striking agreement, irrespective that
investigations are from different years/periods and neither have measured the actual net
transport directions. In the inner Danish water the comparison is not as convincing as local
bathymetry and currents have a larger importance, and where the scale is smaller. For a
first macro-scale evaluation the results are useful as indicators of the net transport
directions.
Figure 3.7 Inferred alongshore transport directions based on the resultant wave energy.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P40
3.3 Coastal classification schemes
The Danish coast consists of a large variety of marine coastal landforms in different
energetic environments and a classification of the coast may be performed differently
depending on the scope of study. This sections deals with the classification used and
begins with a short overview of previous studies on coastal erosion in Denmark.
Coastal erosion studies in Denmark
Previously, the only study to evaluate stretches of erosion and deposition along the entire
Danish coastline was performed by Bird (1974). The author presents an overlook of the
state of the coasts and concludes that substantial erosion is found on major parts of the
west coast of Jutland (pre-dating nourishment). Other smaller areas of substantial erosion
are found, whereas areas of deposition relate to marine forelands and beach ridge
systems in relation to longshore sediment transport.
DCA keeps a more than one hundred year long record of the coastal erosion from annual
profile measurements along the west coast of Jutland. Along parts of the coast profile
measurements started in 1874. Numerous studies have dealt with the erosion, and both
natural erosion rates and natural variability are fairly well described. The coast has been
allowed to retreat on some parts, whereas on others defence measures have been
applied. North and south of the Lim Fjord barriers, respectively, groins were built from
1875 onwards. Since the beginning of the 1980s a large stretch has been sand nourished
and today the central 100 km is annually being fed with app. 3 mio. m3 sand. For a review
it is referred e.g. to DCA (2008) or Fenger et al. (2008).
Local areas of nourishments and areas with hard structures are found along the North Sea
coast and infested areas and jetties are found in relation to harbours, e.g. at the towns of
Hvide Sande, Thorsminde, Thyborøn, Hanstholm and Hirtshals. Today the aim essentially is
to maintain the coastline at its current position along the central part of the west coast.
Numerous studies have dealt with coastal development and coastal erosion on local and
regional scales. The many local studies are not mentioned here but have been registered
for future local level assessments. DCA has registered all coastal defence measures from
aerial photographs and video and produced a map of inferred littoral transport directions
based on e.g. accumulations behind groins and marine depositional landforms (see e.g.
DCA, 1999) as mentioned in the previous section.
From the presence of coastal structures, that in most places may be assumed to be a
consequence of actual erosion problems (although, going back in time, groins were out of
misconception built for prevention, or, to gather sand) it appears that coastal erosion is a
potential problem along the majority of coasts. On the west coast of Sealand, the former
county of West Sealand carried out a fairly detailed coastal registration of areas of erosion
and deposition along 600 km of coast. Also, at the western and northern parts of the coast
of Funen investigations have been carried out that also involve considerations to wave
energy levels (Anthony, 1998; Binderup and GIS-laboratoriet, 1997). Coastal erosion is by
far more common than deposition. A registration of low-lying areas and reclaimed areas
has previously been carried out (Jakobsen, Andersen and Laustrup, 1992). The low-lying
areas, as well as narrow zones along many coastlines, are today at risk of being flooded
during storm surges and/or inundated if dikes breach.
Coastal classification
Many coastal classification schemes have been proposed and used over the years. Some of
these are very complex and some deal with subjects that are inferior to our use with a
focus on the geology in relation to the morphodynamics. For the erosion atlas we seek a
fairly few classes that in a generalised way describe the Danish coastline. The following
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P41
section discusses a few classification schemes and presents the classification scheme used
in this study.
The Eurosion study (www.eurosion.org; Eurosion, 2004) divided the Danish coastline into
coastal types (hard rock coasts, soft rock coasts, beaches (sand or gravel), muddy coasts,
and artificial coasts) and classes based on the geology (plutonic, volcanic, metamorphic or
sedimentary rock stratum, and, continental, lacustrine or marine sedimentary deposits,
respectively). The geological classification is very specific in relation to the materials
whereas the typology is more in the direction of a classification scheme useful for this
study. The Eurosion study is made on a larger scale and focuses more on the already
experienced erosion. The study contains many sub-classes that contain equivalent
resistance to erosion as does the study of Leatherman (2001). Leatherman examines the
characteristics of a coast by dividing it into a number of classes based on geomorphology,
coastal protection, areal use and geology. Subsequently the geomorphology is divided into
a further number of classes of which each is then further divided into sub-classes. With
app. 50 sub-classes this gives the possibility for a very precise description of the coast but
some classes are very close to each other and complicated to apply, for this study in
particular due to number of classes to distinguish between.
Mangor (2004) extends his coastal classifications based on profile and geology
(exposed/moderately exposed littoral dune or cliff coast; protected or marshy coast; tidal
flat coast; others not relevant to present study) to include wave parameters. The
classification acknowledges that waves and alongshore transport are essential in
evaluating the coast stability. The coast is further divided into near straight coasts, and
special coastal form elements like deltas, spits, barriers etc. For the nearly straight
coasts, Mangor (2004) operates with 15 types based on the exposure to waves and angle of
incidence (3 by 5) and well as other coasts are subdivided. The scheme of Mangor provides
a good tool for management purposes and for evaluating natural coastal development,
and, it is oriented towards the coastal dynamics by including wave characteristics. It is
very useful in evaluating beaches on a local and regional scale. It is, however, too
complicated for practical use in relation to this study. The idea of incorporating wave
energies has been adopted.
Nielsen and Binderup (1996) made a costal classification for the coasts of Funen. Coasts
are, to the first order, divided into soft cliff coasts, littoral coasts and protected coasts.
The soft cliff coast is characterised by compact sediments (till, gravel, sand) that
depending on the content of e.g. clay may be vertical or show a flatter cliff profile. The
littoral coasts consist of loosely consolidated sediments, are most often formed by waves,
and are often characterised by beach ridges and bar systems. The protected coasts lie in
the lee of waves and are to a larger degree formed in relation to water level variations
due to storm surges. With only little wave energy, these protected coasts sometimes have
vegetation extending to the coastline.
Nielsen and Binderup (1996) further make sub-divisions by combining the three classes and
differentiating between coasts that have experienced a relative sea level rise and a sea
level fall, respectively. Combined coasts exist (see also Anthony, 1998), where e.g. a
littoral coast lies in front of a protected coast, or, a coast is divided in height relating to
different transgression periods. The classification of Binderup and Nielsen is used and
expanded to cover the entire coastline of Denmark. Still for simplicity, we aim at only a
few classes.
Set-up for the Danish coastline
The Danish coastline is divided into five coastal classes based on the three classes by
Nielsen and Binderup (1996) and extended to cover also the tidal flat/marsh coasts of the
Wadden Sea in south western Jutland and rocky coasts of Bornholm, cp. Table 3.1. At the
nationwide scale, the former three classes show a larger diversity than the coasts of
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P42
Funen. At the tidal flat/marsh coasts often the tidal currents and water level variations
are more important than waves in forming the landscape. At the rocky coasts the erosion
processes are much slower and a sea level rise will not in the short term yield any
measurable increase in the erosion.
Table 3.1
Coastal classifications used in producing the Erosion Atlas.
Coastal classification
Properties/characteristics
Rocky coast
Consists of hard rock stratum
Soft cliff coast
Consolidated sediments. Glacial tills, meltwater deposits, calcareous
cliffs, moler etc.
Tidal flat/marsh coast
Dominated by tidal processes
Protected coast
Dominated by water level variations and sheltered from wave impact
Sandy/littoral dune coasts
Loosely to unconsolidated sediments under the influence of waves
Figure 3.8 The Danish coastline divided into coastal classes.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P43
Complex coasts are considered as belonging to either of the classes dependent on the
individual coastal stretch. In relation to coasts that are superficial or heavily modified
(harbours, revetments, nourishment etc.) these constructions are not considered at the
national level. For e.g. the nourished coast, where zero recession is allowed, the problem
is more to calculate the additional amount of sand necessary to compensate the loss due
to sea level rise. Likewise, in connection to a harbour the climate impact assessment will
focus more on upgrading/renewal of protection measures that may or may not have to do
with the coastal classes. In any case, as performed at the pilot sites, specific
investigations will have to take a closer look at the coastal stretch in focus and, if
necessary, expand the methodology applied here.
The division into classes was performed by inspection of aerial photographs, combined
with general knowledge of the coast and information from Binzer and Marcussen (2001).
At some stretches this has not been sufficient and a classification has been taken to be
representative of a given stretch, e.g. with combined coasts or if several classes are
represented on very short stretches, Figure 3.8. This gives uncertainties in the division
into classes but, considering the national perspective, is acceptable.
3.4 The Danish erosion atlas
Based on the methodology sketched above, the coastal classifications and the calculated
wave energies are combined to form the erosion atlas. The results are presented and
evaluated. The wave energy levels (7 intervals) and coastal classification (5 classes) have
been transformed into a matrix. The matrix has then been subjected to an expert’s
judgement on how large additional total erosion may be expected until 2050 for the IPCC
(2007) A2 scenario, Table 3.2.
Table 3.2
Matrix combining energy classes (w/m) and coastal classes for the expert’s judgement of
additional coastal erosion (in m) until 2050.
0-300
300-600
600-1200
1200-3000
600012000
>12000
0.0
0.0
1.5 - 2.5
1.5 - 2.5
2.0 - 3.0
1.0 - 2.0
1.0 - 2.0
1.0 - 2.0
2.0 - 3.0
5.0 - 7.0
3000-6000
Rocky
Soft cliff
0.0 - 1.0
0.5 - 1.5
0.5 - 1.5
1.0 - 2.0
Tidal/marsh
1.0 - 2.0
1.0 - 2.0
Protected
0.5 - 1.5
0.5 - 1.5
1.0 - 2.0
1.5 - 2.5
1.5 - 2.5
Littoral/dune
0.5 - 1.5
0.5 - 1.5
1.0 - 2.0
1.5 - 2.5
2.0 - 3.0
Only 9 of the 35 combined classes are not represented (grey shading). The erosion
potential generally increases with increasing wave energy level and, for example, it is
seen how the additional erosion at soft cliffs is evaluated at 0-3 m, whereas at the tidal
flat/marsh coast it is constant at 1-2 m.
Based on the scatter of the results, these have been ‘tested’, or, verified against existing
studies and knowledge of specific coastal stretches, and corrections have been made. Still
the number given may at some locations be overestimated as fetch duration limitations
have not been applied. For the low energy interval the differences between the additional
erosion for soft cliff and littoral/dune coasts is evaluated to be small and therefore no
attempt has been made to make further divisions in the coastal classes for some of the
coastal stretches in the inner Danish waters.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P44
At high energy coasts, however, more erosion is anticipated on littoral/dune coastal than
at other coasts. The results, when applied to the coastline of Denmark, show the macroscale pattern of additional erosion until 2050, Figure 3.9.
The atlas is evaluated to give a good estimation of the potential additional erosion at the
Danish coast by 2050 due to a climate change related sea level rise. Still, the atlas is only
meant to give an overview and it is worth remembering that erosion today may vary
considerably between similar coasts only a few kilometres apart. Also, in the
interpretation it is important to note that today some coasts are experiencing high erosion
rates whereas others may be stable. To the former, e.g. at locations on the North Sea
coast, an additional erosion of 5-7 m only constitutes a few percentages of the total until
2050, whereas at other locations 5-7 m of erosion by far will the exceed erosion as
experienced in the last 50 years. By stating that it is the potential additional erosion that
is investigated, we may also add: At some places the erosion may exceed these numbers
and at other locations the additional erosion may be considerably smaller or absent.
Figure 3.9 Erosion atlas of Denmark showing the potential additional coastal erosion until 2050 due
to a climate change related sea level rise.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P45
4. Local assessments of four pilot sites
One objective of Safecoast Action 5A is to analyse how to assess the potential
consequences of climate change at different geographic scales. At the local level four
pilot sites are selected along the Danish coastline. These are selected to reflect the
variability of the Danish coastline and the different challenges facing the communities in
the year 2050, i.e. that the various consequences of climate change (coastal erosion,
coastal flooding, etc.) will have different impacts on the respective pilot sites.
Selection of the four pilot sites is based on parameters such as the type of coast (e.g.
exposed littoral coast, tidal coast, fjord coast, etc.), the type of existent coastal defence
structures and the degree of hinterland occupation by housing and infrastructure.
Moreover, former events of hinterland flooding or large rates of coastal retreat are also
considered in selecting the four pilot sites.
Figure 4.1 shows the location of the four Danish pilot sites, which are:
•
Ballum-Koldby, on the Danish Wadden Sea coast of Jutland, is a low-lying
agricultural area protected only by a summer dike. The hinterland is flooded
during severe winter storms affecting a small number of farms.
•
Houvig, on the open North Sea coast of Jutland, contains many holiday houses.
The coast is sand nourished and managed dunes protect the hinterlands from
flooding during storm surges. In general, coastal erosion and flood protection are
the main concerns.
•
Løgstør, a small town in the Lim Fjord of which a large part is liable to flooding
during storms. Coastal erosion is currently experienced at the stretches adjacent
to the town and harbour. Flooding of the town is, however, the main concern.
•
Aabenraa, a town situated in a fjord in southern Jutland towards the Baltic Sea,
is prone to flooding but is currently experiencing relatively little coastal erosion
along the fjord.
Figure 4.1 Location of the four Danish pilot sites.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P46
For the pilot sites Ballum-Koldby, Houvig and Løgstør detailed studies concerning the
consequences of climate change have been carried out, which are available as separate
reports. For Aabenraa an assessment based on GIS-map information, sea level gauge data
and local knowledge has been performed.
In the following sections 4.1 to 4.3, a summary of the respective studies concerning the
consequences of climate change in the pilot sites is given. Section 4.4 includes the brief
assessment for the pilot site of Aabenraa. All four sections comply with the same
structure including a brief presentation of the pilot sites and available data, a description
of the applied methods for assessing climate change impacts, and the results of the
impact assessment. For more detailed information about the impact assessments of the
four Danish pilot sites, refer to references listed in Table 4.1.
Table 4.1
Separate studies regarding the four pilot sites.
Pilot site
Reference
Section
Ballum-Koldby
Schlappkohl, S. (2008): Effekte des Klimawandels auf
Küstenschutzanlagen am Beispiel des Sommerdeiches in BallumKoldby, Dänemark. Student research work, Institute of Geography,
Christian-Albrechts-University, Germany (In German).
4.1
Houvig
Pförtner, S. (2008): Auswirkungen des Klimawandels auf die
dänische Westküste am Beispiel des Untersuchungsgebietes
„Houvig“. Student research work, Leichtweiß-Institute, Technical
University of Braunschweig, Germany (In German).
4.2
Løgstør
Jensen, B.E. (2007): Klimaændringernes betydning for kysten ved
Løgstør. Master Thesis, Physical Geography, Aalborg University,
Denmark (In Danish).
4.3
Aabenraa
No report. Brief assessment based on GIS-map information, sea
level gauge data and local knowledge
4.4
4.1 Pilot site of Ballum-Koldby
The pilot site of Ballum-Koldby is located in the Danish part of the Wadden Sea about 30
km north of the Danish-German border. The pilot site comprises the area on both sides of
the Ballum-Koldby dike. On the seaward side of the dike, the pilot site is delimited by the
outer foreland edge. On the landward side, the pilot site is defined by the +5 m contour
line. The area within this contour line represents the potential flood-prone hinterland at
very high storm surge events. The potential flood-prone area accounts to 379 ha. The pilot
site is part of the municipality of Tønder. Figure 4.2 gives an overview of the BallumKoldby pilot site.
Physical settings
The coast at Ballum-Koldby can be described as tidal marsh coast. The coastal
development is governed by the twice-daily tide and periodic storm surge events. The
coast is characterised by a very wide and gently sloping foreshore, which normally
implicates accumulative areas where suspended sediment, brought in by the flood
currents, settle. The accumulative processes at Ballum-Koldby are enhanced by
sedimentation fields that are situated in front of the foreland along the entire dike line
(Figures 4.3 and 4.4).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P47
Figure 4.2 Location and extent of the Ballum-Koldby pilot site.
The southern part of the barrier island of Rømø is located offshore and protects the pilot
site coast against large wave impacts. Different to the flood-prone areas north and south
of the pilot site is the fact, that the pilot site of Ballum-Koldby is elevated 2.5 m higher
on average than the neighbouring flood-prone areas.
The hinterland at Ballum-Koldby has a low occupation and is mainly used for agriculture. A
small number of farmsteads are located in the hinterland (Figures 4.3 and 4.4). All except
one lie at an elevation of 2-2.5 m. A rural road runs in south-north direction through the
hinterland from the villages Koldby to Østerende Ballum. Cross-connections to the coast
exist at several locations and are normally farm tracks.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P48
Figure 4.3 The foreland and sedimentation fields at Ballum-Koldby (left), and a farmstead located in
the hinterland (right).
Figure 4.4 The Ballum-Koldby pilot site (from south to north) (Photo: Hunderup Luftfoto, 2008).
The hinterland at Ballum-Koldby is protected against flooding by a summer dike. The
summer dike stands by itself as a single flood defence component due to the low
concentration of inhabitation and the dominantly agricultural activities in the hinterland.
In the summer time the hinterland is protected against high tide events by the summer
dike, whereas flooding of the area due to winter storm surges is accepted.
The crest of the summer dike is 3.81 m high on average, which is considerably lower than
the neighbouring sea dikes. The Ballum-Koldby summer dike was built in 1978 and has a
total length of 6,043 m (see Figure 4.5). The seaward slope is 1:10 and the landward slope
is 1:5. The width of the dike crest averages 2 m. The summer dike consists of sandy soil
core covered by a 75 cm thick layer of clay. The outer slope foot is reinforced by a
revetment of filled rock armour. The revetment has to protect the dike foot against
erosion due to wave attack and currents. At four locations, outlets crossing the dike body
allow for drainage of the hinterland of inland water as well as sea water in the case of
that the hinterland has been flooded during a winter storm surge.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P49
Figure 4.5 Foreland and seaward slope of the Ballum-Koldby summer dike.
Data and methods
For the impact assessment of the Ballum-Koldby pilot site valid and sufficient data and
information about (i) the hydrodynamic regime (wind, sea level, waves), (ii) the coastal
bathymetry/topography and (iii) the existent coastal flood defence have to be available.
In the following, an overview of available data and information is given followed by an
introduction of the methodological approach for assessing the impact on the coastal
system at Ballum-Koldby due to climate change.
Hydrodynamic online data are available as water level, wind speed and wind direction.
Wave data for the Ballum-Koldby pilot site result from a numerical study.
The nearest tidal gauge is located at the harbour of Havneby at the southern end of the
barrier island of Rømø, approximately 6 km North West of the pilot site. Sea level data
have been recorded since 1972. Until 1990 water levels were registered every 30 minutes.
From 1990 to 2001 the recording interval was 15 minutes, which was reduced to 10
minutes in 2001.
In the period 1990-1995 a measuring error of about 12 cm occurred at the tidal gauge at
Havneby. Sea level data have been corrected and verified against the corresponding time
series from the tidal gauge at Esbjerg harbour. Wind data are measured at the northern
end of the barrier island of Rømø.
Wave heights and wave periods in front of the Ballum-Koldby summer dike during storm
surges are also important parameters. Besides the surge level, wave run-up and wave
overtopping are crucial for the stability of the flood defence and the extent of hinterland
flooding. In order to assess the potential extent of flooding due to wave overtopping at
the Ballum-Koldby summer dike, wave data have been used from numerical model
computations performed by DHI (1998). The wave model MIKE 21 NSW was used for
numerical modelling of wave heights and wave periods for specific points 100 m apart
along the coastline as well as a distance from the toe of the summer dike of 50 m and 300
m, respectively.
Data concerning the bathymetry/topography of the tidal basin and the foreland in front of
the Ballum-Koldby dike are from studies by DCA (1999b,c) that comprised a detailed
analysis of the morphological development in the Lister tidal basin. The morphological
analysis is based on two surveys that were conducted in 1968 and 1994 covering the entire
area of the Lister tidal basin. Both surveys comprise the foreland area in front of the
Ballum-Koldby dike and refer to the same fix-point system, i.e. bathymetric or
topographic changes between both surveys can be regarded as erosion or accumulation.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Erosion and accumulation processes on the foreland at Ballum-Koldby have therefore been
investigated based on the two surveys of the Lister tidal basin.
First, assessing the impact on the coastal system of Ballum-Koldby due to climate change
includes an analysis of the coastal development. Based on these results, estimates for
climate change impact that affects the coastal system slowly and on the long-term, such
as enhanced coastal erosion and loss of land due permanent flooding, are made. Next, the
future increase of flooding of the hinterland is assessed.
The Ballum-Koldby dike was surveyed in 1997 and 2007. Both surveys included longitudinal
and cross sectional profile measurements. The longitudinal profiles are compared in order
to inspect the summer dike for possible settlements between 1997 and 2007. A lowering
of the dike affects the ratio between water levels and crest height during storm surges.
The cross section profiles are used to investigate the geometry of the summer dike and,
together with the foreland survey of 1994, to calculate mean slope values of the foreland
along the summer dike. The foreland slope is calculated between elevation lines 0.7 and
1.7 m for a number of coastal foreland profiles. The slope values are averaged to a mean
foreland slope along the entire dike line.
The method to assess increased impact due to climate change at the Ballum-Koldby
summer dike includes also the analysis of relevant dike failures. Generally, dike failure
modes are divided into (Kortenhaus, 2003):
•
Global failure modes which may lead to direct failure of the dike cross-section;
•
Failure modes at the seaward slope may result in failure of the seaward slope
and subsequently to breaching;
•
Failure modes at the seaward slope may lead to failure of the shoreward slope
and then to breaching of the dike;
•
Failure modes in the dike core describe mechanisms which lead to inner erosion
of the core and thus provide the basis for breaching of the dike.
Only global failure modes have been studied for the Ballum-Koldby sea defence, such as
wave overtopping and overflow. Due to the fact that the sea defence is being a summer
dike, the incidence of the failure mode of overflow has been considered as most relevant
when assessing future climate change impact. In this respect, the question of more
frequent overflow events per year is of major interest. The single overflow event, as such,
is already known and accepted since the Ballum-Koldby sea defence is defined as a
summer dike.
Coastal development
In order to assess climate change impact at the pilot site of Ballum-Koldby by 2050, the
coastal development has to be known. For this, the available hydrodynamic and
geomorphodynamic data (see above) have been analysed.
Over the past 30 years, the annual mean high tide has increased by approximately 11 cm.
Figure 4.6. The number of events where the water level exceeds two threshold values of
342 and 381 cm has been calculated for Havneby from 1972 to 2007. The threshold value
of 342 cm represents the lowest crest height along the entire summer dike implicating the
beginning of dike overflow at this water level. The threshold value of 381 cm is the
average crest height of the summer dike.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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110
High tide [cm DVR90]
100
90
80
70
60
Mean high tide
Trend line, mean high tide
50
1975
1980
1985
1990
1995
2000
2005
2010
Year
Figure 4.6 Annual mean high tides at the tidal gauge at Havneby from 1977 to 2007.
In 1990, the threshold value of 342 cm was exceeded 3 times, which is the maximum
number in the period 1977-1997. The threshold value of 381 cm was exceeded in 1981 and
in 1991, Figure 4.7.
3
Surge level >= 3.42 m DVR90
Surge level >= 3.81 m DVR90
Number of events
2
1
2008
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
1986
1984
1982
1980
1978
1976
1974
1972
0
Year
Figure 4.7 Number of surge events exceeding the threshold values of 342 and 381 cm.
Extreme sea level statistics for Havneby have been produced based on trend-free values
related to the mean sea level of the year of the individual measurements. The statistics
do not take into account the future local glacio-isostatic adjustment or the eustatic sea
level rise due to climate change (Sørensen and Ingvardsen, 2007).
The highest sea level during a storm is registered as an extreme if it exceeds a threshold
value of 303 cm. The best fit to the distribution function was the Log-Normal distribution
with parameters α = 5.735 and β = 0.158. Statistical extreme return heights of 20, 50 and
100 years were calculated to VS20 = 394 cm, VS50 = 421 cm and VS100 = 440 cm. The
frequency function graph for extreme water levels at Havneby is plotted in Figure 4.8.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Sea level [cm DVR90]
600
580
560
540
520
500
480
460
440
420
400
380
360
340
320
300
280
1
10
Frequency function, LogNormal distribution
100
Standard deviation
1000
Return period [year]
Figure 4.8 Frequency function graph for extreme water levels at Havneby.
The wave height and wave period at the seaward dike foot are important parameters that
give information about the wave loads on the sea dike. Wave loads at sea dikes comprise
wave run-up and run-down flow in the seaward slope, water flow on the landward slope
due to wave overtopping and wave impact on the seaward slope due to wave breaking. As
mentioned above, wave data from numerical model computations (DHI, 1998) are
available. Computations were performed, amongst others, for three main wave directions
(250, 270, 290 deg. N) and 7 different water levels. The seven input water levels were
selected with constant increase starting at 3.70 m. This water level is only 11 cm lower
than the average crest height of the Ballum-Koldby summer dike. At a sea level of 3.70 m,
the summer dike is close to being overflowed. Since the numerical wave modelling study
only considered water levels above 3.70 m, and because of the fact that wave loads at
lower water levels make a minor hazard to sea dikes as the different wave loads correlate
strongly with the water level, only the failure mode of overflow has been considered in
the impact assessment of the Ballum-Koldby dike. For the failure mode of overflow, the
water level in front of the sea dike and the crest height of the flood defence has to been
considered.
In 1997, the mean crest height of the summer dike was 3.89 m. The lowest crest height
was measured to 3.51 m. In 2007, the mean crest height was 3.81 m and the lowest crest
height was 3.42 m, Figure 4.9 and Table 4.2.The mean crest height is thus 8 cm lower in
2007 than in 1997 and can be explained by:
•
Settlements of the dike construction over 10 years;
•
Errors due to a divergent measuring track or instrument setting in 2007.
The latter explanation has not been further investigated as DCA (1999b) predicted a
future settlement of the Ballum-Koldby dike of 5-10 cm in analysis of all flood defences in
the Danish Wadden Sea.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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5,50
1997
2007
5,00
Crest height [m]
4,50
4,00
3,50
3,00
2,50
5.350
4.598
3.778
3.012
2.249
1.476
723
0
Length [m]
Figure 4.9 Longitudinal profile measurements of the summer dike in 1997 and 2007.
Table 4.2
Dike crest height based on longitudinal profile measurements in 1997 and 2007.
Year
1997
2007
Mean crest height
3.89 m DVR90
3.81 m DVR90
Lowest crest height
3.51 m DVR90
3.42 m DVR90
The dike crest settlements 30 years after construction (1978) are generally defined as
secondary settlements and originate in the distinct creep behaviour of cohesive soils such
as clay. In which way the Ballum-Koldby dike will settle in the future is difficult to say.
The secondary settlement of the Ballum-Koldby summer dike will, however, decrease to a
rate smaller than 8 mm/yr.
Considering the extreme water levels, Figure 4.8, the mean crest height of 3.81 m
corresponds to a statistical return period of 7.5 years for the failure mode of overflow. At
the lowest crest height of 3.42 m, overflow will occur every 3 years. Overall, the return
period for flooding of the hinterland is below 10 years at present.
The development of the foreland in front of the summer dike influences the safety of dike
and consequently the flooding frequency of the hinterland. The geomorphologic analyses
by DCA (1999c) showed that the area in front of the summer dike is characterised by
accumulation. Figure 4.10 shows the development in elevation in one cross-sectional
profile approximately 300 m south of the summer dike (green line in Figure 4.11),
stretching east-west across the tidal basin. The illustrated erosion/accumulation applies
for the period 1966-1994. Sediment accumulation has dominated between stationing
+5000 and the mainland at stationing +8672 m.
The sediment map in Figure 4.11, showing erosion and accumulation in the Lister tidal
basin, confirms this development for the period 1966-1994 in front of the summer dike.
The sediment accumulation in front of the Ballum-Koldby dike results from the seaward
sedimentation fields and the shallow foreshore. Only at on location along the summer dike
(red line), erosion up to 0.6 m occurred (blue colour) due to the discharge at one outlet
crossing the dike line.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Change in sedimenation [m] (Scale 1:40)
Stationing [m] (Scale 1:10,000)
Figure 4.10 Coastal profile across the Lister tidal basin showing erosion and accumulation from 1966
to 1994.
Furthermore, the development of sediment accumulation in front of the summer dike is
confirmed by the profile geometry of the foreland. Overall, two general geometrical
formats can be distinguished:
•
A stable foreland including a constant transition from the tidal flats to the
foreland platform. At the transition the foreland is not exposed to erosion,
whereas foreland accretion may occur. The transition area being exposed to the
daily tide will not be covered by extensive vegetation.
•
A foreland under retreating conditions at the seaward border. A cliffed foreland
edge of several decimetres may occur.
The foreland at the summer dike is characterised by the first geometrical format as
illustrated in Figure 4.12.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Figure 4.11 Map of the erosion/accumulation development in the Lister tidal basin between 1966 and
1994.
Figure 4.12 Foreland and sedimentation fields in front of the Ballum-Koldby summer dike.
Impacts due to climate change
As introduced in Section 2.4, the assessment of climate change impact for the pilot site of
Ballum-Koldby bases upon two scenarios:
I.
Sea level rise of 6 mm/yr,
II.
IPCC A2 scenario, adjusted to the regional conditions along the Danish North Sea
coast (see Table 4.3).
Table 4.3
Projected sea level rise for Ballum-Koldby based on IPCC A2 scenario.
Year
Sea level rise
2025
+ 8.5 cm
+ 4.5 mm/yr
2050
+ 21.7 cm
+ 5.3 mm/yr
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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For the IPCC A2 scenario, adjusted to the prevailing regional conditions, accounts an
exponential rise of sea level in the future. Sea level rise will therefore accelerate over the
time period being dealt with in the impact assessment.
Overall, the consequences of climate change for the area of Ballum-Koldby will mainly
result in (i) the loss of foreland due to permanent flooding due to sea level rise and (ii)
more frequent loading of the summer dike by overflow implicating concurrent flooding of
the polder area behind the summer dike.
The loss of land due to permanent flooding depends on the foreland slope, the projected
sea level rise and the ability of the foreland to grow in height by sedimentation as sea
level rises. The latter is a rather complicated issue and depends on the erosion and
accumulation pattern. It is still unclear to which degree foreland erosion will occur. The
geomorphological processes at the foreland in front of the summer dike are at present
dominated by net sedimentation caused by a very flat foreshore, and on the other hand,
by offshore sedimentation fields.
Permanent flooding of the upper foreland due to sea level rise will implicate drowning of
marsh vegetation and cause landward retreat of the salt marsh vegetation as it is not able
to withstand permanent flooding. The loss of the vegetation cover will then allow for
erosion of the marsh soil. In this impact assessment, it has been assumed that foreland
growth cannot keep up sea level rise and permanent flooding of the foreland will occur.
Based on the scenarios A2 and 6mm/yr the loss of foreland due to sea level rise has been
calculated in total to 0.36 km2 and 0.41 km2 up to 2050, respectively. Considering the
entire length of the pilot site of 6,050 m, this equals 59.5 m2/m and 67.8 m2/m,
respectively, Table 4.4. More than 60 m foreland in front of the summer dike will thus
become permanently flooded at both sea level rise scenarios.
Table 4.4
Loss of land due to permanent flooding for scenarios I and II.
Scenario I (6 mm/yr)
Year
Sea level rise
Loss of land
[m2/m]
Loss of land, entire pilot site
[km2]
2005
3.8 mm/yr
3.4 cm
8.26
0.05
2025
6 mm/yr
12 cm
26.45
0.16
2050
6 mm/yr
27 cm
33.06
0.20
Σ = 0.41
Scenario II (modified IPCC A2)
Year
Sea level rise
Loss of land
[m2/m]
Loss of land, entire pilot site
[km2]
2005
3.8 mm/yr
3.4 cm
8.26
0.05
2025
4.5 mm/yr
8.5 cm
19.83
0.12
2050
5.3 mm/yr
21.7 cm
31.40
0.19
Σ = 0.36
The most severe consequence of climate change for the pilot site Ballum-Koldby is,
however, the increase in the frequency of hinterland flooding. As the coastal flood
defence is only composed of a summer dike that protects the hinterland against sea water
up to a surge level of approximately 3.42 m (lowest point of the dike crest height),
flooding of the polder area is expected more often in future. Considering the modified
IPCC A2 scenario, the surge level of 3.42 m, which has a return period of 3 years today,
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P57
will increase to 2 years in 2050. The 6 mm/yr-scenario will even result in a return period
of 1.8 years, Table 4.5.
Table 4.5
Change in return period for dike overflow considering scenarios I and II
Surge level
Return period
Today
Return period
Scenario I
[6 mm/yr]
Return period
Scenario II
[modified IPCC A2]
3.42 mDVR90
3 yr
1.8 yr
2 yr
3.81 mDVR90
7.5 yr
4.5 yr
5.5 yr
Considering the mean crest height of 3.81 m, the return period will change from 7.5 years
today to 4.5 years assuming a sea level rise of 6 mm/yr up to 2050. For the modified IPCC
A2 scenario the return period will decrease to 5.5 years.
Today, when flooding of the polder area has occurred, the drainage of the polder area can
take some time. When considering more frequent floods in the future due to sea level
rise, the period of having flood water standing in the polder area will increase. This will
clearly complicate the agricultural use of the area and may cause destruction of the
foundation of the existing and partly old building structures.
Summing up, the Ballum-Koldby summer dike will be exposed to more frequent overflow
events and subsequent flooding of the hinterland. This future impact due to climate
change can be solved in a cost-efficient way by not developing the area further. On the
other hand, site-specific measures for flood protection of the five farms should be
considered in the long run, such as minor ring dikes around the farms or movable flood
protection barriers.
4.2 Pilot site of Houvig
The pilot site of Houvig is located at the open North Sea coast of Jutland north of the
village Søndervig, Figure 4.13. Survey lines, which are part of the official West coast
survey system for annual survey programs carried out by the Danish Coastal Authority, are
used to delimit the area of investigation. The coastal stretch of interest starts at survey
line 5440 and ends at line 5540 and has a total length of 10 km.
Figure 4.13 Location and extent of the Houvig pilot site.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Physical setting
The coast can be described as a highly exposed littoral dune coast. Dune formations of up
to 3 dune rows are found at Houvig. The hinterland is characterised by approximately
2000 holiday houses and a large number of tourist activities. Larger towns or industrial
areas are non-existent in the area. The largest density of holiday houses can be found
around the village of Søndervig. In many places holiday houses are placed on dune crests
and in dune slacks, Figure 4.14. Some holiday houses are located less than 150 m from the
coastline. These houses are particularly threatened during storm surges, since dune front
recession of up to 25 m has occurred in the past during individual storm events.
Figure 4.14 The village of Søndervig with holiday houses placed on dune crests and in dune slacks
(Photo: Hunderup Luftfoto).
The southern end of the pilot site expands to the northern end of the Holmsland barrier
(Sandene) which separates the fjord of Ringkøbing from the North Sea, Figure 4.15. At this
location, the width of the barrier is only app. 1,200 m. The Ringkøbing Fjord is a shallow
body of water that is fed by tributaries from the inland and exchanges water with the
North Sea through a narrow sluice regulated channel at the village of Hvide Sande, at the
coast south of Houvig.
Figure 4.15 Southern end of the Houvig pilot site where a narrow barrier separates the Ringkøbing
Fjord from the North Sea. Photo: Hunderup Luftfoto.
In case of a breach of the barrier, or, the failure of the sluice at Hvide Sande, sea water
from the North Sea will flow into the fjord and cause major flooding at the fjord coasts.
Especially the town of Ringkøbing with 20,000 inhabitants would be at risk of flooding as
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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the town has no major flood defences. In this way coastal defence measures at Houvig are
not only serving as protection against coastal erosion and flooding at Søndervig and the
nearby holiday houses, but are also very important for flood protection reasons further
inland at e.g. the town of Ringkøbing.
Data and methods
For the impact assessment of Houvig pilot site valid and sufficient data and information
about (i) the coastal topography and bathymetry, (ii) the hydrodynamic regime (wind, sea
level, waves) and (iii) the existent coastal protection schemes have to be available. An
overview about available data and information is given followed by an introduction of the
methodological approach for assessing the impact on the coastal system at Houvig due to
climate change.
In the process of data gathering it has been noticed that a comprehensive data set is
available for Houvig compared to other pilot sites. This originates in the fact that the pilot
site of Houvig is located in the central part of the open North Sea coast of Jutland, which
is under state administration by the Danish Coastal Authority (Ministry of Transport).
Coastal protection schemes and sand nourishments along this coastal stretch are, to a
large extent, funded by national government implying many years of data gathering and
monitoring.
For data concerning both the coastal topography (dune landscape) and bathymetry a
consistent set-up of survey lines is available. Survey lines are located perpendicular to the
coastline and start at the seaward dune top and reach offshore to a water depth of about
-15 m. The distance between the survey lines is about 1,000 m. The length of each survey
line varies between 4 and 9 km according to its location. The coastal profile (crosssection) is measured annually at each survey line.
The first measurements in the pilot site were performed back in 1957. Since then, the
measurement accuracy (vertically) has changed due to various survey methods being
applied, Table 4.6. The largest inaccuracy occurred during the period from 1963 to 1976.
Measurements referring to this period are ignored in the present analysis of the coastal
development.
Table 4.6
Measurement accuracy for different time periods.
Time period
Measurement accuracy
1957 - 1962
± 10 cm
1963 - 1976
± 20 cm
1977 - 1983
± 10 cm
1984 - 2008
± 5 cm
Since the dune barrier has to have a minimum cross-section together with a minimum
volume to conduct its function as a flood defence, topographic data based on airborne
laser surveys are collected to complement the line-based measurements of the dune
landscape. Dune erosion of up to 25 m during one surge has been registered several times.
However, in analysing the strength of the dune barrier at Houvig, only a few locations of
the dune topography turn out to be homogeneous. Laser surveys of the dune landscape
from 1999 and 2007 are used to investigate the dune profile for each survey line. For the
1999 survey, measurement data are available up to survey line 5500, whereas in 2007 the
laser survey covered the entire area of the pilot site.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Hydrodynamic data are available as water level, wind speed, wind direction, wave height,
wave period and wave direction. Table 4.7 gives an overview about measured parameters
at the three gauging stations Thorsminde, Hvide Sande and Ferring which are closest to
the pilot site.
Table 4.7
Measured parameters at three gauging stations.
Type of gauging station
Parameter
Tidal gauge
Sea level
Wind gauge
Wind speed
Wind direction
Atmospheric pressure
Wind gusts
Wave gauge
Significant wave height Hs
Characteristic wave height Hm0
Maximum wave height Hmax
Significant wave period Ts
Maximum wave period Tmax
Wave direction d
Tidal gauges are located in the villages of Thorsminde and Hvide Sande. Hvide Sande is
located approximately 9.5 km south of the pilot site and Thorsminde is situated 20 km
north of Houvig. At both locations sea level has been recorded every 15 minutes since
1981 and 1979, respectively. Wind data are also measured in Hvide Sande and
Thorsminde. Both wind gauges are measuring wind speed, atmospheric pressure and wind
direction since 1982. Since 1985 both gauges also allowed recording of wind gusts.
For the further study at Houvig, only one measuring location has to be applied. From a
perspective of distance, Hvide Sande is closest. From the perspective of available data,
annual mean values of the monthly highest water levels have been plotted for the period
1981-2006 for both tidal gauges. The same has been done for both wind gauges in the time
period 1983-2006. Comparisons of the time series showed marginally higher values for
water level and wind speed in Hvide Sande for the period 1995-2008. Due to this, the
shorter distance to the pilot site, and equal quality of data, data from Hvide Sande are
used. Wave data are only measured at three locations along the Danish West coast. The
closest wave gauge is located at the village of Ferring about 30 km north of Houvig. The
wave gauge is located offshore at a water depth of about 17.5 m. Wave data have been
recorded since the beginning of 1992.
The methodological approach to assess the climate change impact on the coastal system
at Houvig comprises two steps: First step includes the analysis of impact due to climate
change that affects the coastal system slowly and on the long-term, such as enhanced
coastal erosion and permanent flooding. Second step investigates climate change impact
based on a single flood event with a time frame of 2 days. Second step assesses future
impacts on the coastal system by short and severe surge events.
To analyse climate change impact on coastal erosion and permanent flooding at Houvig,
the effect of sand nourishments has to be considered. Sand nourishments were started in
1992 and were carried out as beach nourishments. The nourished sand volumes at Houvig
are shown in Figure 4.16.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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1000
900
Nourished sand volume [1000 m³]
800
700
600
500
400
300
200
100
0
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
Year
Figure 4.16 Nourished sand volumes at the pilot site from 1992 to 2007.
Regular beach nourishments since 1992 implicated that the coastal development was no
longer autonomous as the nourished sand volumes influenced the development of the
coastal profile. The autonomous coastal development is defined as the geomorphologic
development of the coast unaffected by any kind of coastal protection measures, i.e. the
natural geomorphologic development of the coast.
Hence, the coastal profiles measured after 1992 are not representing the natural coastal
development at Houvig. To analyse the autonomous coastal development after 1992, a
method was chosen which adds the nourished sand volume to the measured coastal
retreat for each coastal profile line, Figure 4.17.
Original coastal profile
Autonomous coastal retreat
Current coastal profile
Nourished sand volume
Figure 4.17 Calculation of the autonomous coastal retreat for profile measurements after 1992.
To investigate and illustrate the development of the coastal profile in each survey line,
two methods have been applied:
I.
Measured profile lines of different years are plotted for each survey line in one
plot. Each annual profile measurement refers to the same base point, Figure
4.18.
II.
The coastal profile is divided into different sections, which allow for a more
differentiated analysis of the development of the coastal profile.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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With respect to the second method, four sections are defined:
•
dune profile,
•
beach profile,
•
inner profile, and
•
offshore profile.
These sections are defined by contour lines, as shown in Figure 4.19. It is noted that the
inner profile covers also the beach profile.
20
1977
1990
15
1996
2000
2006
Elevation [m DVR90]
10
5
0
950
1150
1350
1550
1750
1950
-5
-10
Distance [m]
Figure 4.18 Measured profiles of survey line 5440.
Figure 4.19 Definition of coastal profile sections
Besides the assessment of long-term changes of the coastal processes at Houvig, impacts
in terms of more frequent and intensive storm surges were also considered in assessing
the consequences of climate change by 2050. The storm surge of 8/9th January 2005 was
selected as a reference storm surge to investigate the consequences of such an event in
2025 and 2050, respectively.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Wind and water level data are taken from the respective gauges in Hvide Sande. The wave
gauge at Fjaltring was out of duty because of a technical error during the event. Wave
data from the wave gauge close to the village of Nymindegab, located about 32 km south
of the pilot site, was therefore analysed to determine the wave attack during this storm
surge. The wave gauge at Nymindegab is located offshore at a water depth of about 20 m.
Geomorphologic changes of the coastal profile due to the extreme sea level and wave
attack are well documented by bathymetric and topographic measurements before and
after the event.
Coastal development
In order to assess the climate change impact at the pilot site of Houvig by 2050, the
coastal development up date in the area has to be known. The available hydrodynamic
and geomorphodynamic data have been analysed with focus on the coastal development
up to 2006.
Figure 4.20 shows accumulated numbers of sea level events that exceed the three
threshold values of 125, 150 and 175 cm. The analysis covers the period 1981-2006 and
represents data from the Hvide Sande(sea) tidal gauge. Trend lines have been plotted for
each of the three threshold values. The trend lines show an increasing tendency of
exceedance that is most noticeable for the threshold value of 125 m DVR90.
Extreme sea level statistics for the tidal gauge at Hvide gauge has also been produced
based on trend-free values related to the mean sea level of the year of the individual
measurements. The statistics do not take into account the future local glacio-isostatic
adjustment or the eustatic sea level rise due to climate change (Sørensen and Ingvardsen,
2007).
The highest sea level during a storm is registered as an extreme if it exceeds a threshold
of 207 cm. As best fitting distribution function, the Weibull distribution was calculated
with parameters α = 1.089 and β = 237.752. Statistic extreme return heights of 20, 50 and
100 years were calculated to VS20 = 290cm, VS50 = 313 cm and VS100 = 330 cm. The
frequency function graph for the sea level gauge at Hvide Sande is plotted in Figure 4.21.
35
Sea level >=125 cm DVR90
Sea level >=150 cm DVR90
30
Sea level >=175 cm DVR90
Trend line, sea level >=125 cm DVR90
Trend line, sea level >=150 cm DVR90
25
Number of days
Trend line, sea level >= 175 cm DVR90
20
15
10
5
0
1981
1983
1985
1987
1989
1991
1993
1995
1997
1999
2001
2003
2005
Year
Figure 4.20 Number of events and trend for sea levels exceeding threshold values of 125, 150 and 175
cm, respectively.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P64
Sea level [cm DVR90]
500
480
460
440
420
400
380
360
340
320
300
280
260
240
220
200
1
10
100
Frequency function, Weibull distribution
1000
Return period [year]
Standard deviation
Figure 4.21 Frequency function graph for extreme sea levels at Hvide Sande.
The time series of wave gauge measurements cover the period from 1992 to 2006 and are
thus 10 years shorter than the period of tidal gauge measurements at Hvide Sande. Figure
4.22 shows the number of days of significant wave heights above threshold values of 2.5
m, 3 m, 4 m and 5 m together with trend lines. However, it is important to notice that
due to the short time series of 14 years, the tendencies of the four trend lines are less
significant compared to the sea level trend lines (see Figure 4.20).
120
Wave height Hs >= 2.5 m
Wave height Hs >= 3.0 m
Wave height Hs >= 4.0 m
100
Wave height Hs >= 5.0 m
Trend line, wave height Hs >= 2.5 m
Trend line, wave height Hs >= 3.0 m
Trend line, wave height Hs >= 4.0 m
Trend line, wave height Hs >= 5.0 m
Number of days
80
60
40
20
0
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
Year
Figure 4.22 Frequencies and trends of wave heights exceeding threshold values of 2.5, 3.0, 4.0 and
5.0 m (Fjaltring).
In order to investigate the geomorphologic development of the coastal profiles, the four
pre-defined coastal profile sections are plotted in relation to the baseline (0 m DVR90).
This has been done for all 11 coastal profiles for the period from 1977 to 2006. For the
coastal development of profile 5440, Figure 4.23, a tendency towards accumulation at the
coastline and the inner profile is seen. The beach profile is stable and coastal retreat is
recorded at the dune profile and the offshore profile. However, the development of
profile 5440 is not representative of the overall coastal development of the area.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P65
2600
2400
2200
Coast line
Dune profile
Beach profile
Inner profile
Offshore profile
Depth contour -6 m DVR90
Depth contour -8 m DVR90
Depth contour -10 m DVR90
Depth contour -12 m DVR90
Depth contour -14 m DVR90
2000
1800
Distance to baseline [m]
1600
1400
1200
1000
800
600
400
200
0
-200
1976
1979
1982
1985
1988
1991
1994
1997
2000
2003
2006
Year
Figure 4.23 Coastal development of coastal profile 5440 from 1977 to 2006
Table 4.8 summarises the coastal development of the four profile sections and the
coastline for all 11 profiles. A mean value for each profile section and the coastline is
calculated representing the coastal development of the entire pilot site.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P66
Table 4.8
Coastal retreat of coastline and profile sections [m/yr] in the period 1977-2006.
Profile
Coastline
Dune profile
Beach profile
Inner profile
Offshore profile
5440
0.71
-0.83
0.0
1.38
-1.40
5450
-0.06
-1.27
-0.66
0.67
-2.52
5460
-0.55
-0.82
-0.92
0.43
-0.11
5470
-1.18
-1.18
-1.44
-0.09
0.02
5480
-0.62
-0.94
-1.11
-1.14
-2.39
5490
0.10
-0.19
-0.40
0.87
-1.16
5500
-1.53
-1.09
-1.31
-0.28
-3.26
5510
-1.59
-0.61
1.26
0.37
-1.86
5520
-1.66
-0.67
-1.25
-0.28
-5.01
5530
-1.52
-0.54
-1.65
-4.00
-4.57
5540
-1.36
-0.40
-1.55
-1.18
1.24
Mean
-0.84
-0.78
-0.82
-0.29
-1.91
In general, the profile sections and the coastline are retreating. The largest rate of
retreat is registered for the offshore profile. At the inner profile, coastal retreat is
smallest accounting to 0.29 m/yr. This low rate, compared to the other profile sections,
can be ascribed to the annual sand nourishments performed since 1992.
In a next step, the autonomous coastal development has been calculated in order to
identify coastal profile changes if no coastal protection measures (sand nourishment) were
carried out. Calculations of the autonomous profile development are based on the profile
measurements in the period 1977-1996. As sand nourishments started in 1992 and
comprised small sand volumes over the area at first (Figure 4.16), profile nourishments up
to 1996 may reflect an unaffected development of the coastal profiles. The mean values
for coastal retreat of the four profile sections and the coastline for the period 1977-1996
are listed in Table 4.9.
Table 4.9
Coastal retreats of coastline and profile sections for the time period 1977-1996
Coastal retreat [m/yr]
Coastline
-1.57
Dune profile
-0.96
Beach profile
-1.57
Inner profile
-0.26
Offshore profile
-1.07
Coastal retreat rates of the coastline and all profile sections, besides the inner profile,
are larger than for the entire period 1977-2006. Further investigations show that the
coastal development of the inner profile have varied considerably over the period. This
can be explained by a number of governing morphodynamic processes between the
depth/height contours -8 m and +4 m, including the surf and swash zone. In order to
compensate for these variations of the inner profile, the time period 1977-2006 was
extended with survey data from 1957-1962. Profile measurements between 1963 and 1976
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P67
were not used due to the low measurement accuracy, as previously mentioned. The
results of the coastal retreat calculations of the inner profile for the extended time period
(1957-1962 and 1977-1996) are listed in Table 4.10. The mean value for the pilot site is
calculated to -1.35 m/yr, which is 1.09 m/yr more than the actual coastal retreat rate of
the inner profile (Table 4.9).
Table 4.10 Autonomous coastal retreat of the inner profile for the extended time period (1957-1962
and 1977-1996).
Line
Inner profile [m/yr]
5440
-1.06
5450
-0.11
5460
-0.21
5470
-1.96
5480
-2.27
5490
-1.51
5500
-1.27
5510
-0.73
5520
-3.10
5530
-0.62
5540
-2.00
Mean
-1.35
To illustrate the difference between the autonomous coastal development and the actual
development influenced by periodic sand nourishments, these are plotted against each
other based of coastal profile 5540, Figure 4.24. The impact of periodic sand nourishments
is clearly seen. Without sand nourishments the inner profile of Line 5540 would have
retreated about 13.5 m in relation to its present position.
240
220
Abstand Küstenlinie
200
180
160
140
120
100
1976
1979
1982
1985
1988
1991
1994
1997
2000
2003
2006
Jahr
Ist-Zustand
autonome Entwicklung
Figure 4.24 Autonomous against actual coastal development of profile 5540.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P68
Reference storm surge
On 8/9th January 2008 a severe storm surge occurred along the Danish West coast. The
return period of the surge level has been calculated to 50 years.
The maximum sea level of 303 cm was reached around 1500 hours, Figure 4.25. A second
peak surge level occurred a few hours later between 2130 and 0100 hours on 8th January
2005.
350
300
Surge level d [cm DVR90]
250
200
150
100
50
0
06-01-2005
00:00
06-01-2005
12:00
07-01-2005
00:00
07-01-2005
12:00
08-01-2005
00:00
08-01-2005
12:00
09-01-2005
00:00
09-01-2005
12:00
10-01-2005
00:00
10-01-2005
12:00
11-01-2005
00:00
Date and time
Figure 4.25 Surge levels during the storm surge 8/9th January 2005 at Hvide Sande.
The significant wave heights, recorded by the wave gauge at Nymindegab, increased to
between 5.21 and 5.91 m between 0630 hours on 8th January and 0100 hours on 9th
January, Figure 4.26.
7
6
Wave height H s [m]
5
4
3
2
1
0
07-01-2005
00:00
07-01-2005
12:00
08-01-2005
00:00
08-01-2005
12:00
09-01-2005
00:00
09-01-2005
12:00
10-01-2005
00:00
10-01-2005
12:00
11-01-2005
00:00
Date and time
Figure 4.26 Significant wave heights measured at Nymindegab during the storm surge 8/9th January
2005.
Geomorphologic changes of the coastal profile occurred as shown for the coastal profile of
survey line 5510, Figure 4.27. The dune profile and the beach profile have been eroded
significantly. To quantify the coastal retreat more precisely, the profile positions before
and after the surge at elevation heights of 0 m, 4.40 m and 10 m were compared, Table
4.11.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P69
Table 4.11 Coastal retreat in survey line 5510 due to the storm surge 8/9th January 2005.
Elevation [m DVR90]
Coastal retreat [m]
0
30.95
4.80
22.27
10.0
13.72
The largest retreat of the coastal profile occurred at an elevation of 0 m where the
coastal profile retreated about 30 m. At higher elevation the coastal profile was eroded
less, however a retreat of nearly 14 m still occurred at an elevation of 10 m of the dune
profile, Figs. 4.27 and 4.28.
20
15
2004
2005,01
Elevation [m DVR90]
10
5
0
-5
-10
-15
950
1050
1150
1250
1350
1450
1550
Distance [m]
Figure 4.27 Coastal profile of survey line 5510 before and after the storm surge on 8/9th January 2005
Figure 4.28 Seaward dune top at Søndervig before (top) and after (below) the storm surge on 8/9th
January 2005.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P70
Impacts due to climate change
The assessment of climate change impact for the pilot site of Houvig is based on two
scenarios, as introduced in Section 2.4:
I.
Sea level rise of 6 mm/yr,
II.
IPCC A2 scenario, adjusted to the regional conditions along the Danish North Sea
coast, Table 4.12.
Table 4.12 Projected sea level rise for Thorsminde based on IPCC A2 scenario.
Year
Sea level rise
2025
+ 12.1 cm
+ 4.4 mm/yr
2050
+ 24.9 cm
+ 5.1 mm/yr
Besides the two scenarios of sea level rise, an increase of the significant wave heights, Hs,
at Thorsminde is assumed for the impact assessment, Table 4.13.
Table 4.13 Assumed increase of the significant wave height Hs at Thorsminde.
Year
Wave height Hs
2025
+1%
2050
+2%
An increase in wave heights affects the geomorphologic coastal development in the area.
Concerning the retreat of the coastal profile, the impact assessment has analysed the
future coastal retreat under the assumption of no sand nourishments being carried out to
counteract coastal retreat. In this way, the performed impact assessment shows the
prospective retreat of the coastal profiles at Houvig and clarifies the need for sand
nourishments also in future.
In order to assess the future coastal retreat without sand nourishments at Houvig, the
calculations of the autonomous coastal retreat are applied with two general assumptions:
•
constant autonomous coastal retreat,
•
accelerated autonomous coastal retreat.
Values for the constant autonomous coastal retreat refer to the calculated autonomous
coastal retreat of the inner profile up to 1996, see Table 4.14. Regarding the accelerated
autonomous coastal retreat, the calculated autonomous coastal retreat up to 1996 is
modified by the estimated future increase in significant wave heights, Hs: 1 % for 20062025 and of 2 % up to 2050. The autonomous coastal retreat from 1996 to 2005 is
presumed as the constant autonomous coastal retreat, Table 4.14.
Table 4.14 Assumed autonomous coastal retreat values for the impact assessment.
Autonomous coastal retreat
Constant
Accelerated
Up to 2006
1.35 m/yr
1.35 m/yr
Up to 2025
1.35 m/yr
1.35 + 1 % m/yr
Up to 2050
1.35 m/yr
1.35 + 2 % m/yr
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P71
The climate change impact assessment at the pilot site of Houvig bases upon the three
aforementioned changes in coastal processes: (a) permanent flooding and loss of land, (b)
coastal retreat due to increased erosion, and (c) increased hazard of flooding.
The loss of land due to permanent flooding depends on the slope of the beach profile and
the assumed sea level rise. For a description of the calculations of mean slope values for
each coastal profile at Houvig, refer to Pförtner (2008).
For both scenarios (adapted IPCC A2 and 6 mm/yr) the loss of land due to permanent
flooding has been calculated first for the time period 1996-2005, since the coastal profiles
and hence the slope of the beach profiles have been affected by the regular sand
nourishments. Sea level rise for the time period 1996-2005 has been calculated to 3.8
mm/yr (DCA, 2008).
Considering scenario I (6 mm/yr), permanent flooding due to sea level rise by 2025 will
result in the loss of land of 5.4 m2/m. In total, an area of 0.107 km2 will be permanently
flooded at the pilot site of Houvig (length of 10 km) by 2050 assuming a sea level rise of 6
mm/yr (see Table 4.15). For scenario II (adapted IPCC A2), the permanent flooded area by
2050 is calculated to 0.088 km2 over the entire length of the pilot site, Table 4.15.
Table 4.15 Loss of land due to permanent flooding for scenarios I and II.
Scenario I (6 mm/yr)
Year
Sea level rise
Loss of land
[m2/m]
Loss of land, entire pilot site
[km2]
2005
3.8 mm/yr
3.4 cm
1.20
0.0120
2025
6 mm/yr
12 cm
4.21
0.0421
2050
6 mm/yr
27 cm
5.27
0.0527
Σ = 0.1068
Scenario II (modified IPCC A2)
Year
Sea level rise
Loss of land
[m2/m]
Loss of land, entire pilot site
[km2]
2005
3.8 mm/yr
3.4 cm
1.20
0.0120
2025
4.4 mm/yr
8.8 cm
3.09
0.0309
2050
5.1 mm/yr
12.8 cm
4.48
0.0448
Σ = 0.0877
Mean values of future coastal retreat for the entire pilot site at Houvig are listed in Table
4.16. An increase in wave height by 1 and 2 % implicates only a minor increase in future
coastal retreat rates. In fact does the inner profile of the coastal profiles at Houvig
retreat more than 70 m until 2050.
Table 4.16 Average coastal retreat of the inner profiles at Houvig assuming constant and accelerated
(autonomous) coastal retreat.
Year
Constant coastal retreat
Accelerated coastal retreat
2006
13.49 m
13.49 m
2025
39.13 m
39.13 + 1 % = 39.52 m
2050
72.86 m
72.86 + 2 % = 73.59 m
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P72
The assessment of the future flooding hazard depends on the retreat of the coastal profile
and especially of the dune profile, as the dune barriers protects the hinterland against
flooding. Decisive in this respect is, in which way and at which speed the dunes and inner
coastal profiles will retreat up to 2050. If the dune cross section and the inner profile
move landwards with same velocity, the future flood hazard will mainly be governed by
the potential appearance of more frequent and stronger storm surges.
However, if the retreat of the inner coastal profile is faster than the retreat of the dune
cross section, the dune cross-section will be reduced implicating an increase in the flood
hazard. Investigations concerning the landward movement of the dune barrier have not
been undertaken. The dune movement and growth in the landwards direction is mainly
dependent on the landward aeolian sand transport. Due to the lack of knowledge, the
landward dune movement is ignored in the assessment of the future flood hazard.
Assuming a future retreat of the seaward dune slope of 0.96 m/yr, the actual widths of
the first dune row will decrease according to Table 4.17. In order to compare the
calculated dune width values, all values refer to the dune width at an elevation of +5 m
DVR90.
Table 4.17 Width of the first dune row at an elevation of +5 m DVR90 in 2025 and 2050.
Coastal
profile
Actual dune width [m]
(at elevation + 5 m
DVR90)
Dune width in 2025
[m]
Dune width in 2050
[m]
5440
91
63
39
5450
69
41
17
5460
39
11
-12
5470
41
13
-10
5480
293
265
241
5490
173
145
121
5500
130
102
78
5510
367
339
315
5520
43
15
-8
5530
804
776
752
5540
524
496
472
According to Table 4.17, the first dune row will have eroded in three coastal profiles
(5460, 5470, 5520) by 2050 under the assumption of an autonomous coastal development
of the coastal profiles. At coastal profiles 5470 and 5520, a second dune row would still
protect the hinterland against flooding under these assumptions. At profile 5460,
however, no second dune row exists which would implicate a direct flood hazard of the
hinterland assuming no dune management until 2050.
Referring to the reference storm surge of 8/9 January 2005, calculations have been made
to assess the flood hazard in case of the occurrence of this reference storm surge in 2050.
It is assumed that the reference storm surge occurs in 2050 under unchanged conditions,
such as the recorded retreat of the seaward dune slope of about 30 m. Assuming the
occurrence of this reference storm surge in 2050, the first dune row would be eroded in a
fourth coastal profile (5450) and very close to in profile 5440, see Table 4.18.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P73
Table 4.18 Width of the first dune row after an assumed occurrence of the reference storm surge in
2050.
Coastal
profile
Dune width after
storm surge [m]
(at elevation + 5 m DVR90)
5440
9
5450
-13
5460
-42
5470
-40
5480
211
5490
91
5500
48
5510
285
5520
-38
5530
722
5540
442
The potential dune retreat up to 2050 clearly shows that an increase of the flood hazard
has to be anticipated, especially in the northern part of the pilot site, as the first dune
row would erode completely in three coastal profiles (assuming no future sand
nourishments). At these locations flood protection would have to be taken over by a
second dune row, if existent.
Finally, it is important to notice that the results represent an assessment of climate
change impact under the assumption of an autonomous coastal development up to 2050,
without the performance of coastal protection measures (e.g. sand nourishment).
4.3 Pilot site of Løgstør
In the central part of the Lim Fjord lies the harbour town of Løgstør (population 4434;
Statistikbanken, 2007). The town, founded in 1514, owes its existence to transport, trades
and fishing for herring. To the west of the town the fjord is open and relatively shallow
and to the east it narrows into a single channel towards the Kattegat, Figure 4.29.
Immediately off the town are the shallow Løgstør Shoals.
Since 1825, where breaches of the Lim Fjord barriers occurred, there has been an open
connection to the North Sea. Today the opening is maintained at Thyborøn. The landscape
has developed over the millennia and the Lim Fjord barriers have formed from eroded
glacial material transported along the west coast of Jutland. Although presumably being
navigable at some stages during the pre-medieval, the fjord has been characterised by
calm conditions, as witnessed in the sedimentary record, since the Litorina transgressions
9,000-6,000 years B.P.
The more permanent opening to the North Sea has had a large societal, environmental
and, most important with regards to this study, physical impact on the coastal processes
and water level variations in the Lim Fjord. Westerly storms may raise the general water
level in the Lim Fjord by more than a meter and Løgstør face surge levels that, by far, are
more severe than when the town was built. The town currently faces risks of both flooding
and erosion of its surrounding coastlines.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P74
Kattegat
North Sea
Figure 4.29 The Lim Fjord and the location of Løgstør in the Løgstør Bredning.
Physical Setting
The investigated coastline constitutes the harbour front of Løgstør and app. 1 km to SW
and E of this, Figure 4.30. The surface topography in the area is dominated by the highly
irregular Pre-quaternary surface consisting of calcareous soft rocks, reformed by glacial
and postglacial processes and in places overlain by glacial till. Since the Litorina
transgressions the relative land uplift has been approximately 4 m (Mertz, 1924). The
ancient coastline, in front of which marine accumulations are found, is very pronounced in
the area.
Figure 4.30 The investigated stretch of coast at Løgstør (top left), the surface topography in the area
(bottom left), the Pre-quaternary surface topography (top right), and the bathymetry of
Løgstør Bredning (bottom right) (Jensen, 2007, with data from DGU, 1994; KMS 1998,
2005; FRV, 2007).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P75
A dredged channel goes through the Løgstør Shoals. The channel has been maintained
since 1901 where it replaced the ‘Frederik the VII’th canal’ built in 1861 (Lindhard, 1988,
1996). This canal immediately SW of the town is now closed at the distal end and serves
as a marina towards Løgstør, Figure 4.31.
Figure 4.31 The Frederik the VII’th canal SE of Løgstør with channel profile inserted.
Figure 4.32 Løgstør divided into areas of different functionalities: Residential areas (B), centre
functions, shops and housing (C), light industry (E), parks and recreational areas (G), and
public services such as sports facilities, campgrounds, cemeteries etc. (O).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P76
The town may be divided into different areas of functionality, Figure 4.32. Many of the
centre functions and public service facilities as well as many private houses are placed in
the very low-lying central parts of the town. Also, to the east of the town areas of light
industry, including the water treatment plant, are situated in low-lying areas close to the
coastline. The investigated area thus consists of the abovementioned stretch of coast and
includes the low-lying areas up to approximately 4 m above mean water level.
Data and methods
For the impact assessment at Løgstør valid and sufficient data and information about (i)
the coastal topography and bathymetry, (ii) the hydrodynamic regime (wind, sea level,
waves) and (iii) the existent coastal protection schemes have to be available. An overview
about available data and information is given followed by an introduction of the
methodological approach for assessing the impact on the coastal system at Løgstør due to
climate change.
Still, the forecasting and hind casting systems regarding high water levels in the Lim Fjord
are not very well developed and precise. It has proven difficult to model water levels in
the fjord due to combinations of regional and local forcing and the highly irregular
topography. This means that extreme water levels may vary considerably from one
location to another. Water levels are measured from tide gauge stations at Løgstør as well
as at approximately 10 other locations in the Lim Fjord. The length and quality of the
data series vary. Combined with water level measurements at the North Sea and at the
Kattegat coasts, respectively, fairly detailed pictures of the water level variations in the
fjord can be deducted and related to individual storm events.
Measurements of wind speed and wind direction are made at several locations regionally;
see Chapter 3 and www.dmi.dk for an overview. For the pilot site of Løgstør, wind
measurements were correlated between three stations in the region. Furthermore,
geostrophic wind fields have been calculated for the Danish area by others and may be
used in future assessments.
No continuous current and wave measurements are made in the area. Data from field
campaigns in the 1970s of the currents to the east of Løgstør has yielded some
information on the currents pattern in the fjord.
Bathymetric maps, aerial photographs, and a digital map of the area constitute the core
material for evaluating the coastal development together with information from the local
authorities. Although the photos and digital elevation maps do not have the desired
resolution they still are of a fair quality for the purpose of this assessment.
The local information on the existent coastal protection was somewhat scattered and took
quite some effort to gather. Work still persists in order to make a more detailed
assessment that convincingly takes into account the existing coastal protection and its
efficiency. In this study several short cuts were taken as it was not possible to yield
reliable information on the current safety level. This, of course, is a focus point for the
local authorities in their ongoing work relating to climate change.
Methodologically the current conditions and the coast’s vulnerability to flooding,
inundation and erosion was evaluated and compared to historic data and where the
January 2005 storm surge is used as a ‘worst case’ flooding scenario of today. The future
development of the coasts is then evaluated together with an assessment of the future
flooding risks given today’s safety levels for the IPPC A2 and 6 mm/yr scenarios for year
2050, as introduced in Section 2.4.
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Coastal protection and coastal development
The coastal erosion and flood protections at Løgstør, Figure 4.33, show a variety of
measures built over the years. Upgrading of the coastal protection has been made in
recent years due to renovation of the harbour area (2001) and following a major storm
surge in January 2005. The breakwaters were built between 1984 and 1995 to prevent
erosion immediately south of the harbour entrance. The revetment has a height of
2.0-2.5 m. Although giving some protection from the waves, the water still enters the
harbour and erodes the banks of the canal during high water levels, see Figure 4.34.
Breakwater
Flood protection wall
Quay
Revetment
Jetty
Figure 4.33 Coastal protection scheme at Løgstør (top) with photos of breakwaters west of,
revetment and flood protection wall in front of, and revetment east of the harbour,
respectively.
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Figure 4.34 Erosion of the canal bank.
Figure 4.35 Flood protections with sand bags.
Finalisation of the flood protection wall in front of the harbour was due after the site
study in 2007. At the time of study gaps in the flood protection wall were closed by
sandbags, see Figure 4.35, when warnings for high water levels were issued. The level of
the wall is 2.16 m yielding an effective flood protection up to a water level of 1.90 m
according to the local authorities. Behind the wall, surface drain systems collect water to
a strong pump that returns it to the fjord. To the east of the harbour the revetment, built
1992-1995, has a height of 2.0 m. This light industry area is given a low priority by the
local authorities and is little developed.
The coastal erosion to the west and east of the harbour since 1968, investigated from
aerial photos and digital data (COWI, 1995, 1999, 2005; DCA, 1969; 1984; KMS, 2005), is
relatively large with rates of 0.29 m/yr (west) and 0.67 m/yr (east), respectively, Figure
4.36 and Table 4.19. The entire stretch of coast is thus currently undergoing erosion and
where areas of deposition are very local and in later years mainly related to construction
work on the coastal protection. The general littoral transport direction is from west to
east and with the western part being the most exposed to wave action. Explanations to
the fairly large erosion rates to the east are that erosion is occurring on very low-lying
meadows, the littoral sand transport is obstructed by the harbour, and the presence of
the dredged canal very close to the coastline acting as a sink for the transported sand. On
average, 50,000 m3/yr are being dredged from the canal, and succeeding the January
2005 storm the amount was 160,000 m3.
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West
East
Figure 4.36 Coastal development in different periods since 1968 west (top) and east of the harbour.
Table 4.19 Calculated erosion and deposition at Løgstør, 1968-2005.
East
Erosion
[m2]
Deposition
[m2]
Total erosion
[m2]
Erosion rate
[m2/y]
Erosion rate
[m/y]
1968 - 1995
16,585
50
16,535
612
0.56
1995 - 1999
6,628
47
6,581
1,645
1.50
1999 - 2005
4,509
418
4,091
682
0.62
1968 - 2005
27,722
515
27,207
735
0.67
1968 - 1984
1,383
1,787
-404
-25
-0.02
1984 - 1995
8,309
385
7,924
720
0.60
1995 - 1999
2,050
375
1,675
419
0.35
1999 - 2005
3,996
151
3,845
641
0.53
1968 - 2005
15,738
2,698
13,040
352
0.29
West
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Water level variations and current flooding risks
As already mentioned, the general water level in the Lim Fjord may be raised by more
than one meter during atmospheric low pressure passages when water is transported in
from the North Sea. An additional meter of storm surge height may be added due to local
wind forcing within the fjord and associated waves. The water level variations at Løgstør
have been investigated with special emphasis on the January 2005 surge that lead to
severe flooding of the low-lying parts of the town.
Reliable tide gauge measurements have been carried out at Løgstør since 1930 and the
time series reveal an increased number of incidences over time with water levels above
0.85 m DVR90, Figure 4.37. Dividing these incidences into 5 years intervals and into 4
interval heights, Figure 4.38, this reveals an increasing tendency in both the numbers and
heights of incidences from 1950 to 1994, and where the period 1990-1994 experiences the
largest number of incidences. After 1994 the number decreases but is still larger than is
the pre 1990s. The development over time of extreme water levels reflects the climate
variability as well as it may be a change in the physical properties within the fjord that
affect the water levels towards a more extreme nature. 6 out of 10 extremes have
occurred after 1990 and only two before 1980.
Figure 4.37 Independent incidences with water levels exceeding 0.85 m DVR90.
Figure 4.38 Number of incidences with water levels exceeding 0.85 m divided into 5 years intervals
(6 years for the most recent).
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The extreme sea level statistics based on data from 1930-2006 reveal expected return
levels of 164 cm (20 yr), 181 cm (50 yr), 194 cm (100 yr) and 207 cm (200 yr),
respectively, Figure 4.39.
Sea level [cm DVR90]
260
240
220
200
180
160
140
120
100
80
60
1
10
Frequency function, LogNormal distribution
100
Standard deviation
1000
Return period [year]
Figure 4.39 Frequency function graph for extreme water levels at Løgstør (From Sørensen and
Ingvardsen, 2007).
The highest ever recorded water level at Løgstør occurred on the evening of 8 January
2005 reaching 2.05 m (the water level may locally have been up to 15-20 cm’s higher
according to some reports) and large parts of the town were flooded and the inhabitants
were evacuated. Also, several other locations along the fjord were flooded as the water
rose and dikes breached. In the afternoon of that day a deep low pressure system moved
over Norway towards Denmark and winds increased to gale force from a westerly
direction, Figs. 4.40 and 4.41.
Figure 4.40 Water levels at Løgstør 1-16 January 2005.
Figure 4.41 Local wind speeds and wind directions 1-16 January 2005 (Data from DMI).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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The wind was approximately 10 m/s from westerly directions for six days prior to the main
event and this persistent wind forcing lead to high general water levels of 70-100 cm in
the fjord. The narrowing of the fjord east of Løgstør Bredning meant that the water could
not be transported away fast enough. Conditions were ideal for a major storm surge at
Løgstør with a large wind set-up over the shoals and an atmospheric pressure that
dropped from 1015 hPa to 980 hPa as the storm peaked. Furthermore, local wave set-up
added centimetres to the unusually high water levels.
The flood protection wall was overtopped at a water level of 190 cm, and even before
then some flooding had occurred. At the peak of the storm the water level in the town
also reached approximately 205 cm and after the peak it took a while for the water to
recede due to the persistent wind force on the water. No exact levelling of the water
level in the streets was carried out but from the extent of the flooding and from local
observations of both the maximum water level and of the water flows in the narrow
streets; a detailed picture of the incident has been gained. In the lower-lying parts of the
town the water level reached 60-100 cm in the streets. Refer to Jensen (2007) for a more
detailed description and mapping of the flooding.
Impacts due to climate change
The assessment of impact due to climate change for the pilot site of Løgstør is based on
the IPCC A2 and 6 mm/yr scenarios.
Regarding the extreme water levels Jensen and Knudsen (2006) made a general evaluation
of the importance of increased storm intensity as modelled by Kaas et al. (2001).
According to the authors, an increase in the 50 yr return water level on the North Sea
coast of 4.6% and an increase of 4 cm on the inner Danish coasts may be anticipated in
2050. At Thyborøn this will amount to approximately 11 cm. As Løgstør is especially
exposed to westerly winds and as the water levels at Thyborøn are important for the
general water levels in the Lim Fjord, the impact of a climatic deterioration is evaluated
at 10 cm at Løgstør. These extra 10 cm on the extreme water levels may very well be
exaggerated, other models do not show an increase in future storm intensities, but for the
purpose of the calculations and as water levels will rise also after 2050, in any case, this
scenario will then occur sometime after 2050.
By raising the frequency function graph, shown in Figure 4.42, by 19.5 cm (A2) and 46 cm
(6 mm/yr) relative to a trend free 1990 mean water level, this dramatically reduces the
safety level of the current coastal protection at Løgstør. Return times are here presented
for water levels of 190 cm (the current level of flood protection) and of 205 cm (maximum
surge height 8 January 2005), see Table 4.20. The water level of 205 cm in January 2005
corresponds to a statistical return period of 179 years and the corresponding water levels
are thus 225 cm (A2) and 251 cm (6 mm/yr) in 2050. It is also apparent that, whereas
water levels statistically exceed the flood protection level every 83 years today, flooding
will occur every 28 years and 6 years in 2050.
Besides the more frequent flooding, larger areas will be flood prone in the future. From
the digital height models potentially flooded areas in 2050 have been calculated for 50 MT
and 100 MT return periods, Figure 4.43 and Table 4.21, together with the areas liable to
flooding in the town. As the low-lying coastal area east of the town is almost entirely
flooded at a water level of 2.01 m, the area increase is minimal with higher water levels.
The large area to the southeast of the town is connected to the Lim Fjord by a canal and
will probably not get flooded. At the town, however, only a minor rise in the extreme
water levels will increase the area liable to flooding considerably. The area shown does
not necessarily become flooded, as this also depends on other factors, but the potential
consequences are clear.
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For the A2 scenario, for example, the area increases by 1/3 to 400.000 m2 between the
50 MT and 100 MT return water levels. A large part of this increase occurs in the light
industrial area towards the east, Figure 4.32, which currently is not very developed. In
the older parts of the town with residential areas, this area is also being flooded at a
water level of 2.01 m; consequences are more frequent flooding with more water in the
streets. The current flood protection does indeed seem insufficient under the sketched
climate change scenarios. More work is needed in coupling the defence measures and
desired levels of safety for the town in an assessment that could incorporate
differentiated adaptation strategies for the different parts of the town depending on their
functionalities.
Figure 4.42 Frequency function graph for extreme water levels today and in 2050 due to sea level
rise and a climatic deterioration.
Table 4.20 Return periods and water levels in years (MT) in 2050 for climate change scenarios.
2007
A2 2050
6 mm/yr 2050
190 cm
83 yr MT
28 yr MT
6 yr MT
205 cm
179 yr MT
63 yr MT
15 yr MT
179 yr MT
205 cm
225 cm
251 cm
Table 4.21 50 and 100 yr. return water levels and areas liable to flooding in the infested areas of
Løgstør by 2050.
Scenario
Return period (yr)
Datum (m)
Area (*103 m2)
A2
50 MT
2.01
313.5
100 MT
2.14
400.9
50 MT
2.27
474.9
100 MT
2.40
540.5
6 mm/yr
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Figure 4.43 Potentially flooded areas in Løgstør at 50 MT and 100 MT return water levels in 2050
(Background map and digital elevation data by courtesy of COWI).
Regarding the permanently flooded areas in 2050 due to a general sea level rise of 9.4 cm
(A2) and 36 cm (6 mm/yr) the resolution and precision of the digital elevation data has
hindered exact calculations. The potentially most vulnerable areas are the meadows east
of the town with a general height of 0.6-1.0 m. A minor part of these may be inundated
and the drainage conditions will become poorer. West of the town, the land surface is
situated at higher grounds and the area inundated will be marginal. For very flat lowenergy coasts it may be difficult to distinguish between coastal erosion and inundation
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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under a sea level rise scenario. For the calculated coastline development, inundation at
the coastline is therefore evaluated at 2 m east of and 1 m west of the town,
respectively, by year 2050.
For future coastal erosion without any further coastal protection, it is highly relevant to
consider the current high erosion rates in this fairly low wave energy environment that,
however, has the highest energy levels in the Lim Fjord and possesses the largest erosion
potential due to climate change according to the erosion atlas (Chapter 3). If the current
erosion rates persist, this will lead to an additional erosion of (0.5 m/yr) 22.5 m and 27 m
(0.6 m/yr) by 2050. A rule of thumb, following a Bruun rule approach, the future erosion
will amount to 50-100 times the sea level rise (Zhang et al., 2004) yielding a total erosion
of 5-10 m (A2) and 18-36 m (6 mm/yr), that latter being within the limit of the above
projection.
This approach is debatable, but considering the current erosion rates and the fact that an
accelerated sea level rise most likely will increase the erosion rates, an ‘expert
judgement’ is made for the increased erosion until 2050. The extra erosion for the A2
scenario is set at 4 m and for the 6 mm/yr at 12 m. Again, these numbers may be
exaggerated but still seem reasonable and the projected total erosion until 2050 amounts
to between 33-41 m east of the town and to 27,5-35,5 m west of the town, respectively,
including the aforementioned contribution due to permanent flooding, Table 4.22.
Table 4.22 The projected averaged total coastal erosion until 2050 at Løgstør based on the current
rate and contributions from permanent flooding and additional erosion due to sea level
rise.
East of town
West of town
Current rate
27.0 m
22.5 m
A2 scenario
33.0 m
27.5 m
6 mm/yr scenario
41.0 m
33.5 m
The coastal erosion is assumed constant along the coastline and areas eroded are shown in
Figures 4.44 and 4.45. Local variations over time will occur but these are impossible to
predict. To the east of the town (Figure 4.44), erosion into the low-lying meadows does
not pose a threat to infrastructure or residential housings as these are +200 m away from
the coast. The erosion, however, may potentially increase the risk of flooding of the area.
Towards the light industrial area erosion will yield a need for further coastal protection,
as well as differential coastal erosion west and east of the revetment may deteriorate its
safety level.
Figure 4.44 Total erosion until 2050 for different scenarios east of Løgstør (background map by
courtesy of COWI).
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To the west of the town (Figure 4.45), even with the current rate of erosion, the coast
towards the canal may be threatened. A breach through to the canal will alter both the
nature of coastal erosion and the flooding protection scheme: Waves will attack the
landward banks of the canal and not allow for simple flood protection due to the
combined forces of high water levels and waves. The recreational facilities situated
immediately behind the canal are exposed and the flood protection of the town is
insufficient. Thus, even if the current erosion rates persist an upgrading of the coastal
protection is needed in the near future.
Figure 4.45 Total erosion until 2050 for different scenarios west of Løgstør (background map by
courtesy of COWI).
Summing up, Løgstør is currently experiencing both flooding of the town and coastal
erosion of the adjacent coastlines. Even with the current rates of erosion, problems are
foreseen in the near future as breaches at the coast toward the canal behind the coastline
may lead to new points of attack from the waves and to reduced effectiveness of some of
the current natural flood protection.
4.4 Pilot site of Aabenraa
The pilot site of Aabenraa is located on the south east coast of Jutland. The pilot site is
concentrated around the town of Aabenraa in the bottom of Aabenraa Fjord, Figure 4.46.
Aabenraa is a medieval town dating back to the 13th century and owes its existence to
trades, fishing and shipbuilding. Today the town has a population of around 16,000. Parts
of the old town are low-lying and face risks of flooding, whereas coastal erosion in
relation to a climate change related sea level rise is inferior to the flooding risks.
The east-west oriented Aabenraa Fjord opens towards the fairly sheltered southern part of
the Lillebælt. The fjord is 10 km long, 2-4 km wide and has a maximum depth of more
than 20 m. The maximum fetch of 40 km towards ENE means that wave erosion along the
coastline can occur. Storms from this direction are rare, however, and are often
associated with low water levels in the inner Danish waters. Incidences of extreme sea
levels at Aabenraa therefore are most often associated with phenomena originating
outside the Aabenraa Fjord/Lillebælt area.
Physical setting
The investigated area consists of the parts of the town of Aabenraa facing the fjord and a
couple of kilometres along the northern and southern shores, respectively. The fjord was
formed during the Weichsel where a glacier tongue formed the basin where the fjord now
is found as a drowned glacial valley.
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Jutland
Funen
Lillebælt
Aabenraa Fjord
Als
Figure 4.46 Overview of the town of Aabenraa in Aabenraa Fjord and the surrounding waters
(Courtesy of COWI and Google Earth with bathymetry from Danish Maritime Safety
Administration, www.frv.dk).
As noticed in Figure 4.46, no sill is found at the entrance to the fjord. Parts of the town
lie at high ground. From only a few kilometres inland, coinciding with the main stationary
line of the Weichsel glaciations, the drainage pattern is towards west and the Wadden
Sea. The coastal landscape thus exists of low coastal cliffs into glacial tills in combination
with a few small marine accumulation forms. The relatively large water depths in
combination with a steep beach profile, and the aforementioned rare episodes of wave
occurrences in relation to high water levels, assist in maintaining active cliff profiles along
the majority of the coastline. The erosion rates are limited, however.
An investigation of aerial photographs shows only little coastline variation since 1945 and
only a few locations show erosion of more than a couple of metres in the period in the
inner parts of the fjord. Little change shows on the marine accumulation forms, as
noticed on the ‘Skarrev’ (top right in photos in Figure 4.47 showing the coastline in 1954
and 1993). Also, the bars seem to be very constant in both position and form between
individual photos (1945, 1954, 1964, 1968, 1973, 1975, 1084, 1989 and 1993) and the most
recent photo on Google Earth (app. 2007-2008). Although extreme events occur, the
longshore sediment transport is small along the coastline and the cross-shore variability is
also small. Historical erosion rates are low although erosion has been experienced at the
majority of the coastline since 1945. Especially at the southern coastline, as witnessed by
small groins being built, erosion may be a potential problem now and in the future.
The difference between the former datum DNN and DVR90 is 12 cm at Aabenraa. This
difference corresponds fairly close to the sea level rise experienced over the last century
and the area is not expected to currently experience any noteworthy uplift or subduction
due to glacial recovery processes. Locally subduction may occur due to compaction of
sediments used e.g. in harbour fillings. Harnow et al. (2008) mention that at Aabenraa it
takes up to 100 years for an area to settle and that in the period 1920 to 1950 a newly
built harbour area had to be elevated five times.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Figure 4.47 The study site in the inner part of Aabenraa Fjord. The mosaics of aerial photographs
from 1954 (top) and 1993 show little change in coastline position in the period (Photos
from www.aabenraa.dk).
At the most western part of the fjord, several new harbour areas have been filled up
and/or extended over the last half a century. Most pronounced is the ‘Enstedværket’ from
1958, Figs. 4.47 and 4.48. Here, a reclaimed area of 29 ha with fill of sand imported from
elsewhere in the fjord was used to make new harbour facilities disconnected to the
harbour of Aabenraa. The power plant and the reclaimed area are dominant on the
coastline of the southern outskirts of Aabenraa and contain also facilities for deposition of
coal, oil and ashes. The quays at Enstedværket facilitate ships with a draught of up to 18
m and the harbour of Aabenraa has depths of 4-11 m. The harbour of Aabenraa has three
basins named Gammelhavn, Sydhavn and Nyhavn. Together the harbours, Enstedværket
and Aabenraa harbour, have more than 2 km of quays and handle about 7% of the goods in
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Danish harbours (www.aabenraaport.dk). In recent years also a new marina has been built
together with harbour front facilities for leisure. Restaurants, a promenade and an
artificial sandy beach markedly increase the recreational value of the coastline at
Aabenraa, Figs. 4.49 and 4.50.
Figure 4.48 ‘Enstedværket’; Power plant on the southern coastline of Aabenraa Fjord (Photo: Port of
Aabenraa, www.aabenraaport.dk).
Figure 4.49 The beachfront at Aabenraa on a typical Danish summer’s day.
Figure 4.50 The recreational value of the harbour front at Aabenraa has increased in recent years by
the construction of a new marina, an artificial beach and a harbour-front promenade
(Photo: Google Earth/COWI).
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Data and methods
For an impact assessment of a coastal site valid and sufficient data and information about
(i) the coastal topography and bathymetry, (ii) the hydrodynamic regime (wind, sea level,
waves) and (iii) the existent coastal protection schemes have to be available. The impact
assessment of Aabenraa has, however, not been performed to the same level of detail as
the other investigated pilot sites and is based on readily available data. Apart from a brief
visit to the site no field investigations have been carried out.
Nevertheless, an overview about available data and information is given followed by an
introduction of the methodological approach for assessing the impact on the coastal
system at Aabenraa due to climate change. To our knowledge no continuous
measurements of waves and current are made in the area. Tide gauge measurements have
been carried out since 1980. The data series contain only minor gaps and provide a good
means of evaluating water level variations at the site. Aerial photographs, bathymetric
mapping and a low-resolution height model of the area constitute the core of study. The
DCA (1999) has registered the coastal protection measures at Aabenraa that, although not
entirely updated, provide a foundation on which to assess future risks of flooding. Also,
local municipality and harbour employees as well as construction architects have provided
information on local matters for the study.
Information on the historical evolution of the coast and the town since 1945 is deduced
from aerial photographs provided by an Aabenraa municipality homepage entrance at
www.aabenraa.dk. Bathymetric data is provided by the Danish Maritime Safety
Administration (Da: Farvandsvæsenet) at www.frv.dk and presented in Google Earth
format. Google and COWI are acknowledged for the use of photographic material. As no
explicit calculations on erosion rates and wave energy levels have been carried out, it
must be emphasised that in the discussions and presentations of results regarding future
climate impacts on the coasts of Aabenraa, these are kept in general terms as there may
locally be areas where erosion may pose a problem already today.
The available data are sufficient at the current level of investigation; however for a
detailed investigation more specific information on heights of sea defences etc., elevation
of streets in the town, flooding scenarios, and data on the local climatic variations must
be available.
Coastal protection
The coastal protection at Aabenraa consists of a variety of defence measures, Figure 4.51.
Regarding the flooding defences, the town is well protected by sea walls along the
coastline to the south of the harbour itself. In front of the sea wall a few isolated groins
protect the beach against erosion. At the north eastern coastline of the pilot site coastal
protection measures against erosion and flooding also consists of sea walls and groins. The
main risks of flooding are at the harbour where a water level of 1.8 m means that harbour
areas around the three basins are flooded.
In conjunction to a recent construction of an entrance road into the harbour area the
height of this road has been increased by about 0.5 m to correspond with other
constructions in protecting the town centre of Aabenraa against flooding. Flooding of the
town centre will happen at water levels of 2.0 m or more according to the port
authorities.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Figure 4.51 Coastal protection measures at Aabenraa.
Impacts due to climate change
The major coast related impact due to climate change will become the increased risks of
flooding. Here, the town and the harbour face serious challenges in order to adapt to even
higher extreme water levels in the future. At numerous occasions in the past, the town
has experienced flooded streets in especially the older parts of the town due to extreme
water levels in the fjord. The storm surge in 1872 reached a height of up to 3.3 m and is,
by far, the most severe. The water levels of this surge as well as a storm surge in 1904
have been marked at the wall of the custom office. Also, in 1931 the streets were
flooded, Figure 4.52. Based on app. 27 years of reliable tide gauge measurements,
Sørensen and Ingvardsen (2007) have produced extreme water level statistics at Aabenraa.
Expected return levels are 154 cm (20 yr), 165 cm (50 yr) and 173 cm (100 yr),
respectively. One of the highest measured surge levels occurred 1-2 November 2006
around midnight.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Figure 4.52 1931-floodings in Aabenraa (Photo: Peter Clausen, www.museum-sonderjylland.dk).
In the statistics, this water level of 170 cm was estimated as the harbour witnessed a total
power failure a couple of hours before the peak of the surge, which made the protection
struggle on the harbour against the water masses somewhat more difficult.
One consequence of a climate related general sea level rise is more frequent storm surges
in the future. A statistical 100 yr (173 cm) event of today is thus expected to be reached
or exceeded every 27 yr for the A2 and every 13 yr for the 6 mm/y scenarios in 2050,
Figure 4.53. Accordingly a water level of 200 cm will have return periods of app. 100 yr
(6 mm/y) and 250 yr (A2) in 2050 as opposed to a return period today in the order of 1000
yr according to an extension of the frequency function graph of the statistics.
Water level (cm)
260
Frequency function, Weibull distribution
240
A2 year 2050
220
6 mm/y year 2050
200
180
160
140
120
100
80
60
1
10
100
1000
Return period (year)
Figure 4.53 Frequency function graph for extreme water levels at Aabenraa today and in 2050 for the
A2 and 6 mm/y scenarios.
Considering that the town today may be secured to this water level of 2 m, flooding may
occur today, however, a marked increase in the flooding risk will occur in the future. As
the area susceptible to flooding is quite large, Figure 4.54, and as water levels may stand
high in the streets, an adaptation strategy should be made or updated for the entire town.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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Such an adaptation strategy should also incorporate infrastructure and possibly be divided
into different zones depending on their liability to flooding and the expected water levels
in the streets. Depending on the meteorological causes of flooding, the corresponding
surge may have a long duration which, in combination with slowly receding water from
the town, may become a costly affair.
Figure 4.54 Low-resolution mapping of areas in Aabenraa situated below 2 m that are liable to
flooding if extreme water levels exceed the current safety level of 2 m. (From: Google,
flood.firetree.net).
Regarding the coastal erosion, sediment transport rates have been small in the fjord over
the last couple of centuries but may increase considerably at some locations in the future.
As the fjord has witnessed an absolute sea level rise in the order of 12-15 cm since the
1890s this rise will somehow relate to the current erosion rates. Still, large erosion events
may correlate to individual extreme water levels in conjunction with wind waves. With an
accelerated sea level rise due to climate change the coastline is expected to show more
erosion in the future. At some places the erosion may be more severe than at others
dependent on the very local conditions. The changes and the exact points of major
recessions compared to today, however, are hard to assess from the present level of
investigation. To the local municipality future erosion should be given consideration in
relation to potential further developments along the coastline of the fjord.
4.5 Findings of all four pilot sites
For the assessment of future impact due to climate change at four coastal pilot sites in
Denmark, the IPCC A2 scenario (IPCC, 2007) and a scenario of future sea level rise by 6
mm/yr were applied. From Danish tidal gauges it is observed that the rate of sea level rise
in the Danish Wadden Sea is currently larger (4 mm/yr since 1973) than corresponding
rates projected by IPCC. Therefore, a modified IPCC A2 scenario was applied at the two
pilot sites in the Danish Wadden Sea and at the North Sea coast to account for the specific
sea level rise recorded in this region since 1990. At all pilot sites the rates of sea level rise
were corrected consequently to compensate for local isostatic movements.
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Furthermore, attempts were made to assess the consequences on coastal erosion of an
increase in storminess shown by some models. Finally, extreme water level statistics of
tide gauges were projected based on different scenarios in order to assess future flood
risks and the combined effects of flooding and surge levels on coastal erosion at the four
pilot sites.
Ballum-Koldby
At the pilot site of Ballum-Koldby an assessment of the consequences of climate change
has been performed for a tidal marsh coast. In this connection, the impact assessment
showed a lack of data and knowledge concerning the growth of the foreland under
different rates of sea level rise. At the moment the foreland in front of the Ballum-Koldby
dike is characterised by sediment accumulation. The question is, however, will
accumulation keep up with sea level rise, or will present net accumulation change into
net erosion at a certain time?
The progression of possible erosion of the Ballum-Koldby foreland is unclear. Erosion
models for tidal marsh coasts are rare and a suitable model for the Ballum-Koldby
situation could not be found in the literature. In the case that foreland growth cannot
keep up with sea level rise, the loss of foreland due to permanent flooding is remarkable
due to the very flat coastal profile. In fact the question: ‘Does sea level rise proceed
faster than sedimentation processes?’ is relevant for other areas in the tidal basins, such
as the tidal flats.
The second main impact due to climate change in the Ballum-Koldby area is the change
towards more frequent overflow events at the summer dike and subsequent flooding of
the hinterland. Since the hinterland is only occupied by a few farms, site-specific flood
protection measures should be considered in the long run at the five farms, such as minor
ring dikes around the farms or movable flood protection barriers. These measures are
assessed more cost-efficiently than heightening of the summer dike.
More frequent overflow events together with high water level events where the sea
reaches the dike will implicate more wetting of the dike foot. In which way and to which
extent enhanced wetting of the dike foot will affect the dike stability is still unclear and
has to be investigated further.
A future site-specific problem may turn out to be the drainage of the hinterland. More
frequent flooding of the hinterland together with a general increase in precipitation may
require new measures to drain the hinterland fast in order to minimize the impact for the
agricultural use of the hinterland.
Houvig
At the sandy North Sea coast at Houvig, the regular sand nourishment measures had to be
included in the assessment of future impact due to climate change as they influence the
coastal development in the area. The autonomous coastal development up to 1996 had to
be calculated and was enabled by long series of coastal profile surveys.
Also the assessment of changes due to (a) permanent flooding and loss of land, (b) coastal
retreat due to increased erosion, and (c) increased hazard of flooding was practicable due
to the available and sufficient data at the pilot site. Based on the two scenarios the loss
of land due to permanent flooding was calculated in total to 8.8 m2/m (A2 modified) and
10.7 m2/m (6 mm/yr) along the coastline up to 2050, respectively.
The autonomic coastal retreat rate, based on the average of 1.35 m/yr from 1957 to 1996,
will lead to a land loss of 730.000 m2 in 2050 relative to 1996. An assumed increase in
wave height by 1-2 % due to climate change results only in a minor increase in coastal
retreat per year up to 2050. This makes clear that (i) differentiation between the
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‘natural’ coastal erosion and the additional erosion due to climate change is difficult to
assess, and (ii) future coastal retreat at Houvig is mainly governed by a prospective
autonomous coastal development and regular nourished sand volumes. However, even
with an increase in the amount of sand being nourished, the pilot site will be under
pressure in the future.
Besides coastal retreat, erosion of the first dune row has to be considered when discussing
consequences of climate change at this pilot site. The safety criteria for the first dune
row for protection against flooding of the hinterland will not be met at several locations
along the 10 km. In addition, the flood hazard for the holiday houses located in the dune
slacks will increase even more as the first dune row may erode and cause flooding of
holiday houses in adjacent dune slacks. This may, however, not be equivalent to the
failure of the entire dune barrier and the subsequent flooding of the hinterland.
In this respect, the impact assessment of the pilot site at Houvig showed a lack of
knowledge regarding the future movement of the dune rows. In order to assess the future
protection against flooding into more detail, further information about the aeolian sand
transport and the dune dynamics have to be investigated.
Løgstør
At the pilot site of Løgstør an assessment of the consequences of climate change has been
performed for a fjord coast connected to the North Sea. Overall for the pilot site accounts
that the town has an erosion problem that especially towards west of the town has to be
dealt with before 2050; even with a simple projection of the current erosion rates.
Whereas the height model has a high resolution that allows for a detailed analysis of the
flooding potential now and in the future, the bathymetric data from Løgstør Bredning are
of a much poorer quality that makes estimations of offshore depths difficult. A more
detailed mapping that showed alongshore and cross-shore variability would allow for a
more detailed interpretation of the coastal evolution.
The data availability on the hydrodynamic forcing parameters was sufficient in relation to
water levels and wind measurements but poor in relation to waves and current
measurements. The produced extreme water level statistics allowed for fairly detailed
projections of future extreme water levels. The water levels in the fjord are governed
both by local and regional hydrodynamic forcing and resolving the impact of climate
change on a regional perspective has been beyond the scope of the present study. A
better understanding of how climate change will affect water levels in the fjord will
improve projections of future extreme water levels at Løgstør. Here the projected
statistical extreme water levels for 2050 also included a contribution from a general
climatic deterioration that may not occur. Therefore estimates of future return water
levels may be exaggerated.
The investigation of aerial photography has proven a valid tool for estimating the natural
coastal development at the site as well as documenting harbour works since the 1950s.
Although being of a varying quality, ortophotos taken approximately 5 years apart have
proven a valid tool in estimating historical and current rates of erosion at Løgstør.
As concluding remarks, it may be added that the local municipality has to make an effort
in relating the land-use and infrastructure to a climate change adaptation strategy that
accounts for the entire town. Today’s safety standards are at a minimum and reflect both
an ad hoc approach to the erosion and flooding protection so far, diverging matters of
interests in e.g. keeping the sea view, economy etc. One step ahead could be made by
gathering all relevant information into a single planning tool that may be used for
discussions in the public and political decision-making on subjects like: How often will we
allow flooding at different locations in the town in the future? Can some houses become
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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flood-proof? To which extent do we want to keep the existing appearance of the harbour
front?
Aabenraa
At the pilot site of Aabenraa, concentrated around the town of Aabenraa in the bottom of
Aabenraa Fjord, the impact assessment has not been performed to the same level of
detail as the other three investigated pilot sites. The assessment bases mainly on readily
available data from aerial photography, tide gauge measurements and previous studies.
This approach has been chosen in part to the lack of data information compared with the
other three pilot sites and in. Neither continuous measurements of waves and currents
were available nor detailed bathymetric data.
The Aabenraa pilot site is currently experiencing sea level rise and will under a climate
change scenario be even more vulnerable to flooding in the lower-lying parts of the town.
Regarding the coastal erosion, sediment transport rates have been small in the fjord over
the last couple of centuries but may increase considerably at some locations in the future.
The susceptibility to flooding, however, is the main issue here. As extreme water levels in
Aabenraa are a result of rarely occurring wave phenomena in the Baltic Sea – North Sea
transition, a future climatic deterioration may have larger implications on the surge levels
and the associated consequences, than sea level rise alone. It is, however, important to
state that in relation to flooding the infrequent occurrences of very high water levels
‘overrule’ sea level rise in a short time frame. As such the question may rather be to
which level of protection the town will adapt to. Discussions like this may therefore play a
role in a long term adaptation strategy for the area of Aabenraa.
Data availability
Central for all four pilot sites is the fact that data availability is scarce and varying. The
availability of different data types (concerning the coastal topography and bathymetry,
the hydrodynamic regime, the existent coastal protection schemes) varies considerably
between the pilot sites. Systematic data collection, e.g. within a program for data
collection, could not be detected at any pilot site. Available data and data quality seems
somehow coincidental in the four pilot sites.
The four pilot site assessments further showed that programs for data gathering and
monitoring would require a site-specific set-up and specification of data types needed due
to the large variability along the Danish coastline. Moreover, a specification of needed
data types has to include not only traditional data types such as bathymetric data and
hydrodynamic data. In fact, it has to be ascertained which data types are needed for
future impact assessments. For example, socio-economic data may get as important as
bathymetric and hydrodynamic data.
Besides gathering data of different types, data allocation, also for public information
initiatives (e.g. at pilot site of Løgstør), has to be improved in order to allow for faster
data access.
Models
The impact assessments at four different coastal sites showed also that there is not one
single model that can accommodate parameterizations for all different types of coast.
Further, it is unknown in detail how to prioritize the different parameters (e.g.
hydrodynamic forcing, seafloor gradient, morphological and geological conditions)
controlling coastal erosion, making the assessments difficult. Different methods were thus
applied at each of the four pilot sites.
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First, historic evolution at the sites was investigated from old charts, aerial photographs
and from satellite photos. Together with investigations of the physical appearance of the
coastline, morphological features, the degree of coastal protection and infrastructure,
this forms the basis for the local assessments. Secondly, all available data on
hydrodynamic forcing, depth measurements etc. were gathered and analysed in relation
to current sediment budgets and the vulnerability to flooding, and a first approximation of
the impact of climate change is assessed.
As coastal erosion rates are not linear in time in an ever evolving coastal landscape with a
large natural climatic variability, the assessment of it is not an easy task. Where possible,
the coastal erosion is looked into in morphological units that take into account the coastal
areas of erosion, transitional areas and areas of deposition but, even so, this may not
suffice. One example is the pilot site at Løgstør where the breach towards the North Sea
of the Lim Fjord barriers in 1825 had a large impact of storm surge water levels and
coastal erosion rates, and another is Ballum-Koldby where it is still unclear if
sedimentation processes in the tidal basins can keep up with sea level rise or shifting of
sand further out in the Wadden Sea may lead to a different hydrodynamic forcing in the
future.
Coastal erosion is a naturally occurring process that is a prerequisite in the formation of
new land, and three out of the four pilot sites owe their existence to coastal erosion in
the first place as being situated on marine accumulation forms. Valid models for assessing
coastal erosion at different types of coasts are still missing. In the vast literature on
coastal profile development and coastal erosion in relation to sea level rise, no one model
seems to satisfactorily describe and predict coastal development.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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5. Conclusions and recommendations
The main objective of this study is to analyse how approaches for impact assessments of
the consequences of climate change can be realised at different scales for complex
coastlines. It is also the intention to investigate the informative value and quality of
impact assessments at different scales. To achieve these objectives, an assessment of the
additional coastal erosion by the year 2050 due to sea level rise has been produced.
Moreover, to cover impact assessments at a local level, four pilot sites along the Danish
coastline have been investigated concerning potential impacts due to climate change.
In general, from the pilot sites and elsewhere along the Danish coastline, it may be
concluded that assessments of future coastal erosion and coastal flooding even at local
levels may be a difficult task especially in countries with a diverse coastal landscape such
as Denmark. In this study impact assessments at a local level refer to a geographic
extension of few kilometres of coastline depending on the respective definition. The
assessment at a nation level in form of an Erosion Atlas comprised the entire Danish
coastline of 7,400 km.
5.1 Conclusions
The production of the national erosion atlas shows that impact assessments at a national
level aim at simplistic approaches due to the tremendous lack of data and appropriate
models to calculated future coastal developments. Simple approaches often go along with
a number of assumptions which have to be decided to implement national impact
assessments that again include considerable uncertainties in the results. The quality of
these results will often be insufficient for planning and design due to the imperfect
information.
The method applied in the erosion atlas is simplified in relation to both the physical and
hydrodynamic conditions. Although it includes wave energy at the coast, still, in the
context of the division into 5 km stretches of coast, much of the potential change that
will occur in the future is unaccounted for. Small changes in the directions of storm tracks
will radically alter sediment transport patterns at some locations. Also, the offshore
bathymetry with shoals and future erosion into marine accumulations not experiencing
erosion today are examples of parameters that cannot at this level of study be included in
the assessment. Furthermore, natural variability and the spatial variation of e.g. coastal
stretches only a few kilometres apart, mean that future changes at the coast must be
looked upon at a smaller scale. The division into ‘normal’ erosion and the additional
erosion due to climate change also is somehow dubious. Nevertheless, it puts to the
attention potential effects of climate change and to certain issues regarding assessment
needs. E.g. on the North Sea coast, the additional erosion due to sea level rise until 2050
only constitutes a small percentage of the current natural erosion, whereas on some of
the inner coasts a marked increase may be expected. Still, it is also worth remembering
that coastal evolution in Denmark over the last century has, except for the northernmost
parts of Denmark, indeed occurred under a relative sea level rise. We are not that
unfamiliar with sea level rise and coastal erosion.
Impact assessments at the national level may therefore be seen as suitable for raising
national initiatives which contribute to improve impact assessments at other spatial
scales. These national initiatives may comprise programs for methodological data
collection for relevant data types or give an overall impression about coastal
developments in the future; such as whether coastal erosion may become a major threat.
Impact assessments will at the national level, however, not be suitable for planning and
design in the coastal zone in Denmark.
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According to Klein et al. (1999) (Figure 1.4) coastal adaptation processes consist of four
basic and iterative steps, where the first step comprises information collection and
awareness raising. This first step may establish the basis for the next step of planning and
design. In this respect, we conclude that impact assessments are needed at a local level.
Moreover, the results of impact assessments at a local level allow for a more targetoriented communication to the public and to decision-makers, which will raise the
awareness and understanding of potential impacts in the coastal zone due to climate
change.
Concerning the impact assessments at a local level, rates of impact of climate change on
coastal erosion, inundation and flooding were found to vary greatly between the pilot
sites relating to the varying coastline classifications, profile steepness, exposure and
orientation of the coastline etc. as well as within different morphological units at the
individual pilot sites. Essential to the great variation between the pilot sites is also the
different availability of data types and data volumes. The lack of models to assess coastal
developments at different coasts complicates the performance of impact assessments at
any spatial scale. The expansion and improvement of these models still represents a huge
challenge in the coming decades, since the improvements of the models go along with
better information, including less uncertainty about further coastal impacts.
Attention must also be given subjects regarding both the methodology and the way impact
assessments are incorporated in the coastal management at all levels. For existent or
intended coastal protection strategies, the question arises about the spatial scale they
should refer to: Are national high-level strategies to be preferred, or, should coastal
protection strategies take origin at a local level and how can this be achieved? From the
results in this report, it is concluded that national coastal management strategies should
consider site-specific coastal criteria in their objectives to be applicable along all coasts
with a large variability of their characteristics.
In more detail, the following basic principles may be considered for impact assessments:
•
The coastal type including the geomorphologic characteristics of the coastal
stretch should be defined clearly.
•
Models concerning coastal erosion and flooding that correspond best with the
coastal characteristics should be applied. For many types of coasts appropriate
models are still missing.
•
The future scenarios for potential impacts in the coastal zone due to climate
change that correspond in time and space with scenarios for other potential
stresses, such as changes in land-use and an increased settlement at the coasts
and in harbour areas, should be defined.
•
All uncertainties in the performance of the impact assessments should be dealt
with. This implicates also a target communication of the uncertainties to
decision-makers and the public.
In summary, the approaches used for producing the national erosion atlas compared to
those used for the local assessments at the four pilot sites show, that on the local level
assessments are adequate if credible results are available to allow for sustainable
decisions to be made. The reasons are:
•
Impacts of climate change along the Danish coastline vary and have to be
considered in decision making processes and also be reflected in decisions made
on a national level.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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•
Decisions regarding long-term adaptation measures should be made on the
regional or local level, i.e. they should account for local variations in the coastal
zone.
•
Communicating adaptation measures, public perception on climate change and
on its consequences must relate to local and thus well-known conditions and
measures.
•
The availability and quality of data is crucial in the impact assessment. On a local
level data are easy to acquire or generate. The intention of a detailed impact
assessment at a national level to allow for sustainable decisions considering local
variations is onerous due to the very high workload. On a local level, the
performance of impact assessments including the generation of necessary data is
more manageable.
5.2 Recommendations
The study showed that the availability of data differs from location to location. However,
adequate data are needed to perform impact assessments in the coastal zone to help
make sustainable decisions. We therefore recommend the initiation of a Danish national
data collection program to collect data on local level to allow for local impact
assessments. Within this program the type and quality of the data should be defined, as
well as the general framework for impact assessments to allow for the integration of
neighbouring local impact assessments into one regional assessment should be set.
Further work is needed on coastal models that accommodate the coastal variability and
incorporate climate change. These models have to consider all potential parameters that
may be affected by climate change. Furthermore, methods to adjust the models in time
and space are indispensable. Tools for the assessment of impacts of climate change are
very much in demand by local authorities and the results of the work carried out on the
four pilot sites are currently in the process of being transformed into guidelines and tasks
by the Danish Coastal Authority for utilisation at a broader scale.
Sharing knowledge about climate change and climate variability and the related
consequences has to be extended to yield awareness about realistic scenarios of the
future coastal changes. At this, the Danish Coastal Authority has already implemented a
first idea to assist and work together with municipalities in initiating coastal adaptation
processes: The form chosen is to visit relevant employees from all coastal municipalities
for a one day seminar to establish the dialogue and discuss issues relevant to the
information collection, the planning and management of future challenges facing their
coastlines to climate change. Further actions, also involving other relevant coastal
players, are needed to improve the common understanding of the complex interrelations
in the coastal zone.
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
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List of figures
Figure 1.1
Different levels of climate change impact in the coastal zone. .................8
Figure 1.2
Changes in coastal processes due to sea level rise: (a) permanent flooding
and loss of land, (b) coastal erosion, (c) increased hazard of flooding.........9
Figure 1.3
Denmark and the Danish coastline. ................................................ 10
Figure 1.4
Coastal adaptation process to climate variability and change including four
basic and iterative steps (Klein et al., 1999). .................................... 11
Figure 2.1
The Danish waters..................................................................... 14
Figure 2.2
Isobases of the total uplift experienced since the Litorina transgressions
(Mertz, 1924; Reproduced from Noe-Nygaard and Hede, 2006). .............. 15
Figure 2.3
Differences (m) between the former Danish datum (DNN) and the new Danish
Vertical reference (DVR90). Source: www.kms.dk............................... 17
Figure 2.4
Map showing Station numbers, 100 years extreme return heights and Station
Names for the 55 gauge stations. Dark blue dots annotate stations with good
statistics and the light blue dots stations where statistics are less certain due
to short measurement periods (<15 years in general) and/or have large gaps
in the data set (Adapted from Sørensen and Ingvardsen, 2007). .............. 21
Figure 2.5
Frequency functions for the five gauge stations in the Danish Wadden Sea. 22
Figure 2.6
Sea level rise will cause extreme water levels to occur more frequent in the
future. A water level statistically experienced once every century today at
Esbjerg, will occur every 22 years with a sea level rise of 40 cm. ............ 22
Figure 2.7
Number of times the coastal stretches of Denmark have been acknowledged
as storm surge areas by the Danish Storm Council, 1991-2008 (Source:
www.stormraadet.dk). ............................................................... 24
Figure 2.8
Reconstructed sea level curves since 200 AD and new projections for sea
level rise until 2100 against the IPCC AR4 A1B (inserted). Shades show
standard deviations (5-95 percentiles). From Grinsted, Moore, and Jevrejeva
(2009). .................................................................................. 25
Figure 2.9
Projections of sea level rise used in the present study. The IPCC AR4
A2(mean) is used in the study at both the national and local level. IPCC
A1FI(maximum) is used as a reference and both scenarios have been
approximated to a single curve. Further, a 4 mm/yr and 6 mm/yr have been
used in the local assessments. Projection are shown: (a) for the period 19902100, (b) for the period 2008-2100, and, (c) with IPCC projections modified
to reflect an already experienced sea level rise of 4 mm/yr in later years
(North Sea only). ...................................................................... 28
Figure 3.1
Methodology applied to produce an erosion atlas at a national level. ....... 29
Figure 3.2
Sketch of the Bruun Rule............................................................. 31
Figure 3.3
Line segments representing the fetch for different compass directions at
points 5 km apart. Points that are not representative of the coastline, e.g. in
harbours, are deleted manually. ................................................... 35
Figure 3.4
The 10 year data series representation on the Danish coastline of wind
climate measurements from 16 stations. ......................................... 36
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Figure 3.5
Distribution of winds on the Beaufort scale in directional sectors (S1-S12) for
the 16 measurement stations shown in Figure 3.3. Each sector represents a
30°-interval, where S1 is 346°-15°, S2 is 16-45° etc. 10 year datasets 19952005. .................................................................................... 38
Figure 3.6
Wave energy levels at the Danish coast calculated for every 5 km. See text
for further explanation............................................................... 39
Figure 3.7
Inferred alongshore transport directions based on the resultant wave energy.
........................................................................................... 40
Figure 3.8
The Danish coastline divided into coastal classes................................ 43
Figure 3.9
Erosion atlas of Denmark showing the potential additional coastal erosion
until 2050 due to a climate change related sea level rise. ..................... 45
Figure 4.1
Location of the four Danish pilot sites. ............................................ 46
Figure 4.2
Location and extent of the Ballum-Koldby pilot site. ........................... 48
Figure 4.3
The foreland and sedimentation fields at Ballum-Koldby (left), and a
farmstead located in the hinterland (right)....................................... 49
Figure 4.4
The Ballum-Koldby pilot site (from south to north) (Photo: Hunderup
Luftfoto, 2008). ....................................................................... 49
Figure 4.5
Foreland and seaward slope of the Ballum-Koldby summer dike. ............. 50
Figure 4.6
Annual mean high tides at the tidal gauge at Havneby from 1977 to 2007. . 52
Figure 4.7
Number of surge events exceeding the threshold values of 342 and 381 cm.52
Figure 4.8
Frequency function graph for extreme water levels at Havneby. ............. 53
Figure 4.9
Longitudinal profile measurements of the summer dike in 1997 and 2007. . 54
Figure 4.10 Coastal profile across the Lister tidal basin showing erosion and accumulation
from 1966 to 1994..................................................................... 55
Figure 4.11 Map of the erosion/accumulation development in the Lister tidal basin
between 1966 and 1994. ............................................................. 56
Figure 4.12 Foreland and sedimentation fields in front of the Ballum-Koldby summer
dike...................................................................................... 56
Figure 4.13 Location and extent of the Houvig pilot site. .................................... 58
Figure 4.14 The village of Søndervig with holiday houses placed on dune crests and in
dune slacks (Photo: Hunderup Luftfoto)........................................... 59
Figure 4.15 Southern end of the Houvig pilot site where a narrow barrier separates the
Ringkøbing Fjord from the North Sea. Photo: Hunderup Luftfoto. ............ 59
Figure 4.16 Nourished sand volumes at the pilot site from 1992 to 2007................... 62
Figure 4.17 Calculation of the autonomous coastal retreat for profile measurements
after 1992. ............................................................................. 62
Figure 4.18 Measured profiles of survey line 5440.............................................. 63
Figure 4.19 Definition of coastal profile sections............................................... 63
Figure 4.20 Number of events and trend for sea levels exceeding threshold values of 125,
150 and 175 cm, respectively. ...................................................... 64
Figure 4.21 Frequency function graph for extreme sea levels at Hvide Sande. ........... 65
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Figure 4.22 Frequencies and trends of wave heights exceeding threshold values of 2.5,
3.0, 4.0 and 5.0 m (Fjaltring). ...................................................... 65
Figure 4.23 Coastal development of coastal profile 5440 from 1977 to 2006 .............. 66
Figure 4.24 Autonomous against actual coastal development of profile 5540. ............ 68
Figure 4.25 Surge levels during the storm surge 8/9th January 2005 at Hvide Sande. .... 69
Figure 4.26 Significant wave heights measured at Nymindegab during the storm surge
8/9th January 2005. ................................................................... 69
Figure 4.27 Coastal profile of survey line 5510 before and after the storm surge on 8/9th
January 2005 ........................................................................... 70
Figure 4.28 Seaward dune top at Søndervig before (top) and after (below) the storm
surge on 8/9th January 2005. ........................................................ 70
Figure 4.29 The Lim Fjord and the location of Løgstør in the Løgstør Bredning........... 75
Figure 4.30 The investigated stretch of coast at Løgstør (top left), the surface
topography in the area (bottom left), the Pre-quaternary surface topography
(top right), and the bathymetry of Løgstør Bredning (bottom right) (Jensen,
2007, with data from DGU, 1994; KMS 1998, 2005; FRV, 2007)................ 75
Figure 4.31 The Frederik the VII’th canal SE of Løgstør with channel profile inserted... 76
Figure 4.32 Løgstør divided into areas of different functionalities: Residential areas (B),
centre functions, shops and housing (C), light industry (E), parks and
recreational areas (G), and public services such as sports facilities,
campgrounds, cemeteries etc. (O). ................................................ 76
Figure 4.33 Coastal protection scheme at Løgstør (top) with photos of breakwaters west
of, revetment and flood protection wall in front of, and revetment east of
the harbour, respectively. ........................................................... 78
Figure 4.34 Erosion of the canal bank. ........................................................... 79
Figure 4.35 Flood protections with sand bags................................................... 79
Figure 4.36 Coastal development in different periods since 1968 west (top) and east of
the harbour............................................................................. 80
Figure 4.37 Independent incidences with water levels exceeding 0.85 m DVR90. ........ 81
Figure 4.38 Number of incidences with water levels exceeding 0.85 m divided into 5
years intervals (6 years for the most recent). ................................... 81
Figure 4.39 Frequency function graph for extreme water levels at Løgstør (From
Sørensen and Ingvardsen, 2007). ................................................... 82
Figure 4.40 Water levels at Løgstør 1-16 January 2005........................................ 82
Figure 4.41 Local wind speeds and wind directions 1-16 January 2005 (Data from DMI). 82
Figure 4.42 Frequency function graph for extreme water levels today and in 2050 due to
sea level rise and a climatic deterioration........................................ 84
Figure 4.43 Potentially flooded areas in Løgstør at 50 MT and 100 MT return water levels
in 2050 (Background map and digital elevation data by courtesy of COWI). 85
Figure 4.44 Total erosion until 2050 for different scenarios east of Løgstør (background
map by courtesy of COWI). .......................................................... 86
Figure 4.45 Total erosion until 2050 for different scenarios west of Løgstør (background
map by courtesy of COWI). .......................................................... 87
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Figure 4.46 Overview of the town of Aabenraa in Aabenraa Fjord and the surrounding
waters (Courtesy of COWI and Google Earth with bathymetry from Danish
Maritime Safety Administration, www.frv.dk).................................... 88
Figure 4.47 The study site in the inner part of Aabenraa Fjord. The mosaics of aerial
photographs from 1954 (top) and 1993 show little change in coastline
position in the period (Photos from www.aabenraa.dk). ....................... 89
Figure 4.48 ‘Enstedværket’; Power plant on the southern coastline of Aabenraa Fjord
(Photo: Port of Aabenraa, www.aabenraaport.dk). ............................. 90
Figure 4.49 The beachfront at Aabenraa on a typical Danish summer’s day. .............. 90
Figure 4.50 The recreational value of the harbour front at Aabenraa has increased in
recent years by the construction of a new marina, an artificial beach and a
harbour-front promenade (Photo: Google Earth/COWI). ....................... 90
Figure 4.51 Coastal protection measures at Aabenraa......................................... 92
Figure 4.52 1931-floodings in Aabenraa (Photo: Peter Clausen, www.museumsonderjylland.dk)...................................................................... 93
Figure 4.53 Frequency function graph for extreme water levels at Aabenraa today and in
2050 for the A2 and 6 mm/y scenarios. ........................................... 93
Figure 4.54 Low-resolution mapping of areas in Aabenraa situated below 2 m that are
liable to flooding if extreme water levels exceed the current safety level of 2
m. (From: Google, flood.firetree.net)............................................. 94
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List of tables
Table 2.1
Secular trends of mean tidal gauges with RMSE investigated by Mudersbach
and Jensen (2006)..................................................................... 18
Table 2.2
Rates of sea level rise in the Danish Wadden Sea from tide gauge
measurements compared to regional and global measurements from satellite
altimetry................................................................................ 26
Table 3.1
Coastal classifications used in producing the Erosion Atlas. ................... 43
Table 3.2
Matrix combining energy classes (w/m) and coastal classes for the expert’s
judgement of additional coastal erosion (in m) until 2050. .................... 44
Table 4.1
Separate studies regarding the four pilot sites................................... 47
Table 4.2
Dike crest height based on longitudinal profile measurements in 1997 and
2007. .................................................................................... 54
Table 4.3
Projected sea level rise for Ballum-Koldby based on IPCC A2 scenario....... 56
Table 4.4
Loss of land due to permanent flooding for scenarios I and II. ................ 57
Table 4.5
Change in return period for dike overflow considering scenarios I and II .... 58
Table 4.6
Measurement accuracy for different time periods............................... 60
Table 4.7
Measured parameters at three gauging stations. ................................ 61
Table 4.8
Coastal retreat of coastline and profile sections [m/yr] in the period 19772006. .................................................................................... 67
Table 4.9
Coastal retreats of coastline and profile sections for the time period 19771996 ..................................................................................... 67
Table 4.10
Autonomous coastal retreat of the inner profile for the extended time period
(1957-1962 and 1977-1996). ......................................................... 68
Table 4.11
Coastal retreat in survey line 5510 due to the storm surge 8/9th January
2005. .................................................................................... 70
Table 4.12
Projected sea level rise for Thorsminde based on IPCC A2 scenario. ......... 71
Table 4.13
Assumed increase of the significant wave height Hs at Thorsminde........... 71
Table 4.14
Assumed autonomous coastal retreat values for the impact assessment. ... 71
Table 4.15
Loss of land due to permanent flooding for scenarios I and II. ................ 72
Table 4.16
Average coastal retreat of the inner profiles at Houvig assuming constant and
accelerated (autonomous) coastal retreat. ....................................... 72
Table 4.17
Width of the first dune row at an elevation of +5 m DVR90 in 2025 and 2050.
........................................................................................... 73
Table 4.18
Width of the first dune row after an assumed occurrence of the reference
storm surge in 2050. .................................................................. 74
Table 4.19
Calculated erosion and deposition at Løgstør, 1968-2005. ..................... 80
Table 4.20
Return periods and water levels in years (MT) in 2050 for climate change
scenarios................................................................................ 84
Table 4.21
50 and 100 yr. return water levels and areas liable to flooding in the infested
areas of Løgstør by 2050. ............................................................ 84
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P114
Table 4.22
The projected averaged total coastal erosion until 2050 at Løgstør based on
the current rate and contributions from permanent flooding and additional
erosion due to sea level rise......................................................... 86
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P115
1
1
3
0
2
0
0
0
2
2
0
1
1
0
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
5-6 December
The east coast of Funen
Mariager and Randers Fjords, parts of coasts of Lolland & Falster
Skagen
30-31 October
21 February
Danish waters
The west coast of Jutland
3-4 December
29-30 January
Roskilde Fjord
4-7 February
Randers Fjord
16-17 November
Inner Danish Waters
21–22 February
Inner Danish Waters
Ringkøbing Fjord
25 January
3-4 November
The Lim Fjord (Løgstør-area)
The east coast of Sealand towards the Sound and the Baltic Sea
The west coast of Jutland - Blåvands Huk and the Wadden Sea
Area acknowledged
13–25 January
17 January
9 January
Date
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
Acc.
Year
-
11(5)
33 (18)
-
6 (-)
109 (-)
305 (-)
10 (8)
-
-
-
41 (28)
558 (c. 380)
-
994 (643)
9 (8)
17 (9)
30 (-)
42 (-)
No. of claims/(claims
acknowledged)
-
1.7 (3.8)
2.7 (4.9)
-
29,7 (-)
3.6 (-)
13.4 (-)
1.4 (1.8)
-
-
-
5.6 (8.6)
5.3 (7.9)
-
5.8 (8.9)
2.6 (3.0)
1.6 (3.0)
2.4 (-)
4.9 (-)
Avg. pay-out
(t€)
-
18.9
88.5
-
178,3
387
4080
14.2
-
-
-
240
2980
-
5700
23.8
26.7
73.4
206.9
Total payout
(t€)
Occurrences of storm related coastal flooding events acknowledged by the Danish Storm Council 1991- August 2008, the areas affected, number of insurance claims,
approximate averaged and total pay-outs. Pay-outs are converted to Euro (€) in 2007-values. Compiled from DSC (1992; 1994; 1997; 2000; 2001a,b; 2002; 2003; 2004;
2005; 2006; 2007).
Annex A
P116
1
4
1
20
2006
2007
2008
Total
Hanstholm (west coast of Jutland) and the western parts of the Lim F.
Southern Kattegat area and the Sound
18-19 March
9-10 November
Hanstholm (west coast of Jutland), the western part of the Lim Fjord, and parts of
the inner Danish waters
The Sound between Helsingør and Køge
19 January
1-2 March
The Lim Fjord and around Aarhus on the east coast of Jutland
Inner Danish Waters
The west coast of Jutland, the Lim Fjord, the east coast of northern Jutland and
Læsø
12-14 January
1-2 November
8-9 January
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
1
2005
c. 30
80
1
40
96
3960
413 (359)
?
?
*incl. above
*incl. above
*incl. above
14.3 (-)*
9.5 (10.9)
?
?
*incl. above
*incl. above
*incl. above
58667*
3918
P117
Annex B
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P118
Ministry of Transport, Public Works and
Water Management VenW [NL]
National Institute for Coastal and Marine Management
Rijkswaterstaat RIKZ (project management)
PO Box 20907
2500 EX Den Haag
The Netherlands
Contact: Niels Roode
E-mail: [email protected]
Tel: +31 70 311 4368
Fax: +31 70 311 4600
Road and Hydraulic Engineering Division
Rijkswaterstaat DWW
PO Box 5044
2600 GA Delft
The Netherlands
Contact: Wout Snijders
E-mail: [email protected]
Tel: +31 15 251 8428
Fax: +31 15 251 8555
Ministry of the Interior of the Land Schleswig-Holstein [D]
Emergency Planning & Disaster Management Division
PO Box 7125
D-24171 Kiel
Germany
Contact: Matthias Hamann
E-mail: [email protected]
Tel: +49 431988 3470
Fax: +49 431988 3480
Schleswig-Holstein State Ministry for Agriculture,
Environment
and Rural Areas [D]
Coastal Defence Division
PO Box 7125
D-24171 Kiel
Germany
Contact: Jacobus Hofstede
E-mail: [email protected]
Tel: +49 431 988 4984
Fax: +49 431 988 66 4984
Danish Coastal Authority [DK]
Højbovej 1
PO Box 100
DK-7620 Lemvig
Denmark
Contact: Thorsten Piontkowitz
E-mail: [email protected]
Tel: +45 99 6363 63
Fax: +45 99 6363 99
Flemish Ministry of Transport and Public Works [B]
Agency for Maritime and Coastal Services
Coastal Division
Vrijhavenstraat 3
B-8400 Oostende
Belgium
Contact: Tina Mertens
E-mail: [email protected]
Tel: +32 59 55 42 49
Fax: +32 59 50 70 37
Flanders Hydraulics Research
Berchemlei 115
B-2140 Antwerp
Belgium
Contact: Toon Verwaest
Email: [email protected]
Tel: +32 3 224 61 87
Fax: +32 3 224 60 36
Environment Agency [UK]
National Flood Risk Management Policy Team
79 Thorpe Road,
Norwich NR1 1EW
United Kingdom
Contact: Rodney Hicks
E-mail: [email protected]
Tel: +44 1473 706 521 (office)
Tel: +44 7900 678460 (mobile)
Lower Saxony Water Management, Coastal Defence and
Nature Conservation Agency / NLWKN [D]
Division Norden – Norderney
Jahnstraße 1
DE-26506 Norden
Germany
Contact: Holger Blum
E-mail: [email protected]
Tel: +49 4931 947 158
Fax: +49 4931 947 125
CONSEQUENCES OF CLIMATE CHANGE ALONG THE DANISH COASTS
P119