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M.Hinton.
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The causes, effects and mitigation strategies relating
to coastal landslides at Highcliffe and Naish Farm on
the Dorset – Hampshire border.
A Report submitted as the examined component of the Project Module
SXG 390.
Michael Trevor Hinton
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September 2007
SXG 390
M. Hinton.
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Abstract
Coastal landsliding on the Hampshire – Dorset border is examined, particularly with
respect to its causes, potential effects and mitigation strategies. These landslides
occur on a cliff face that forms part of the Highcliffe to Milford SSSI and are
significant in that the geological interest is compromised by stabilisation works.
Degradation processes in this particular geological situation have been closely
studied and are reasonably well understood. Also described is the package of
engineering solutions dealing with the causes of landsliding which has been applied
to the western (Highcliffe) sector which is currently stable while the eastern (Naish
Farm) sector has continued to degrade providing geologists, stratigraphers and
palaeontologists with fresh exposures to study. The effects of continued cliff
degradation on the Highcliffe sector could have been disastrous for cliff top residents
but the treatment applied to the coastal slope here has concealed the geology. The
non-treatment, of the Naish Farm site has resulted in coast protection problems for
Christchurch Borough Council, the authority administering the Highcliffe sector,
where a terminal problem manifested itself during the 1980s. Because the Highcliffe
sector was substantially stabilised by 1985 the research base predates this date.
Currently some of the relevant material has been collected together and is included
in a website originating from the School of Ocean and Earth Sciences of
Southampton University. To place this study in a more modern, broader context
information gleaned from The DEFRA/EA Soft Cliffs, Prediction of Recession Rates
and Erosion Control Techniques publication is considered and the particular issues
involving marine erosion, weathering and the passage of ground water are referred
to. Conflicting outlooks relating to stabilisation and potential solutions applicable to
the Naish Farm segment are examined in conclusion.
281 words
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The causes, effects and mitigation strategies relating to coastal
landslides at Highcliffe and Naish Farm on the Dorset – Hampshire
border.
Table of contents
Chapter/subject
Page number
Title page
1
Abstract
2
Table of contents
3
List of figures
4
1.
5
Introduction
1.1
Scope of work
5
1.2
Objectives
5
1.3
Methodology
6
2.
Literature review
3.
The geology of the coastal cliff at Highcliffe, Dorset
6
10
and Naish farm, Hampshire
4.
3.1
The geological setting
10
3.2
SSSI status
10
The causes of landsliding at the two sites.
11
4.1
General overview of causes at the two sites
11
4.2
Analysis of landsliding processes at the two
12
Effects of sea level rise at the two sites
14
sites
4.3
5.
The effects of landsliding and mitigation measures
14
suitable for application to the geology of both sites
5.1
Potential effects at Highcliffe
14
5.2
Engineering solutions
15
6.
Conclusions
17
7.
References
20
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List of figures
Figure 1
Cliff sections and aerial photograph from the
Ian West Geological website.
Page
18
Figure 2
Cliff photographs, ( 2007) Mike Hinton,
Christchurch Borough Council.
Page
19
Figure 3
Cross section of engineering strategies at
Highcliffe reproduced from Tyhurst (1993)
Page
19a
Figure 4
Plan of Naish Farm cliff reproduced from
Halcrow Maritime (1999)
Page
19b
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1. Introduction
1.1 Scope of work
In a geological study of the Barton Beds Burton (1933) mentions ‘sudden landslips’
which cause a loss of clifftop land of ‘at least a foot a year’. Although landslides on
the part of the coast in question cannot be said to measure up to the dramatic slips to
be seen in mountainous parts of the world they nevertheless are a ‘geohazard’ in
their own right. Although there is no known recorded loss of life or physical injury
resulting from landslides here baring fossil hunters becoming stuck in the clay,
landowners have been greatly concerned by the insidious erosion of their property.
The development of Highcliffe village as a retirement area saw extensive
development of land, in some cases precariously and some would say unwisely close
to the cliff top.
1.2 Objectives
The objectives of this report are listed below:-
•
Describe the geology of the coastal cliff at Highcliffe, Dorset and Naish Farm,
Hampshire.
•
Determine the probable physical processes that have caused landsliding at
the two sites.
•
Examine and describe the effects of landsliding at the two sites.
•
Describe and evaluate the measures taken to mitigate the effects of
landsliding at Highgcliffe.
•
Describe the concept of the ‘compromise solution’ as a mitigation measure
which may be suitable for application at Naish Farm.
Any attempts at mitigation of this hazard must be based on understanding the causes
and whilst coastal landslides mainly result from erosion of the cliff toe by the sea
other, sometimes complex, factors have a major part to play. This report firstly
examines the geology of the coastal cliff. An understanding of this is essential as the
ground conditions underpin the causes of landslides. The site is an SSSI because of
the geological interest and this has led, in the past, to differences of opinion as to
how and why, or even if, mitigation measures should be applied.
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1.3 Methodology
Much of the literature studied for this report has been described as ‘parochial’, which
is inevitable due to the localised nature of the geohazard.
There are general
principles which can be applied to the study and prevention of coastal landslides but
inevitably local research is required before any attempts are made to understand the
causes of landslides at a specific site. Mitigation inevitably is closely linked to the
causes meaning that engineering schemes are also site specific and localised. As a
result of this fact many of the references used in this report are not peer reviewed
studies but are commentaries on a local situation by local authority engineering staff
or their consultants.
Because many of the references were extracted from local authority records they are
unlikely to be readily available to the casual enquirer, there being no search system
in place to cope with public enquiries.
The writer has taken advantage of his
employment situation to search records, and acknowledges Christchurch Borough
Council Engineering Services Section, specifically Peter Barker, Head of Operations,
now retired, for his permission and encouragement to use information relevant to
SXG390.
(502 words)
2. Literature review
Because the subject of this study is provincial and localised the literature also tends
to be provincial and localised.
Of the 17 references 8 were gleaned from the
Engineering Services archives of Christchurch Borough Council and of these, 7, i.e.
Barton (1973), Burden (1991), Burton (1933), Halcrow Maritime (1999), Mockridge
(1982), Tyhurst (Undated) and Tyhurst (1993) are somewhat dated technical reports
or peer reviewed papers relating to geology, landslide research or stabilisation
projects on the coastal cliff at Highcliffe on Sea at the extreme eastern boundary of
this local authorities area of jurisdiction. Bristow et al. (1991), Hooke (1998) and
Melville and Freshney (1982) are textbooks, either specific geological publications, or
in the case of Hooke (1998), a textbook, a substantial amount of which relates to a
case study involving conservation issues raised by the Highcliffe – Naish Farm
coastal degradation situation. McInnes (2003) is a publication by the Standing
Committee On Problems Associated with the Coastline, a group of interested
agencies covering the central south coast. There are no specific references in this
publication to the Highcliffe – Naish Farm site although coastal landsliding issues on
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the Isle of Wight figure significantly. The publication is only referred to here for
figures given for sea level rise for the central south coast as a result of climate
change.
Internet searching revealed 5 sites with either a specific or generalised relevance to
this report. Barton and Coles (1984) and Barton et al. (2006) deal with further
research into Barton Clay cliff degradation. Lee (2002) is a fairly recent DEFRA/EA
publication on the management of soft cliffs and the Maine Geological Survey,
Coastal Landslide Hazards (2005) is an interesting highly illustrated website devoted
to soft cliff landsliding in Maine, USA, used here for its high standard of presentation
and educational value. There is no attempt made here to compare or align the
information given to the Highcliffe – Naish Farm Site, it is used simply for its
generalisations and visual treatment of soft cliff landsliding. West (ongoing), is
primarily devoted to cataloguing local coastal geology and is constantly updated,
many studies are listed including some used in this report and the history of coastal
landsliding in Christchurch Bay is documented. It is intended for general interest use
although primarily focussed on students at all levels up to degree standard.
Coastal landslides are not new but have occurred ever since the sea lapped a raised
shoreline. To classify as a geohazard a coastal landslide must be perceived as a
threat to human interests and in the case of the Highcliffe – Naish Farm site the
threat is to residential property, at least as far as the Highcliffe sector is concerned.
At Naish Farm the geohazard is low value land loss combined with ‘knock on’ effects
further along the coastline. To mitigate the effects of landsliding the causes must be
removed and whilst the causes of coastal landslides are broadly similar to those
demonstrated in Maine Geological Survey, Coastal Landslide Hazards (2005) the
situation at Highcliffe – Naish Farm is modified by the geological situation peculiar to
the site. Although the main theme of this reference is the likely acceleration in the
rate of landsliding as a result of sea level rise caused by global warming on the
Maine (USA) coast the actual mechanics of cliff landsliding are clearly represented.
The processes of toe erosion, rotational slumping, the cyclic nature of landslides and
the tell tale or detective signs of subtle ground movements are clearly illustrated. Lee
(2002) explains that each individual site is unique because the causes are entirely
dependent on local conditions and that whilst marine erosion of the cliff toe promotes
slope instability other factors such as site geology and ground water behaviour play a
significant part in landsliding characteristics.
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Published geological studies relating to Highcliffe – Naish Farm go back at least to
the mid-nineteenth century.
The primary attraction being the wealth of superbly
preserved fossil material to be found in the cliffs. Bristow et al. (1991) and Melville
and Freshney (1982) deal with the geological situation in relation to the broader
context of the Hampshire Basin and more specifically in relation to the more localised
situation of the context of the Bournemouth area. The Barton Clays are placed within
the division of the Barton Group which includes the underlying Boscombe Sand and
the overlying Chama Sand and Becton Sand. The British Geological Survey groups
these previously separate entities together because they are all truly marine
sediments. Although dealing primarily with the fossil content of the clays Burton
(1933) is ubiquitously mentioned by subsequent commentators because of the
lettering system applied to the different horizons within the Barton Beds by the
author. This lettering system established more than 70 years ago has persisted to
the present day and is used by all commentators no matter what their interest.
Broadly speaking the individual horizons become increasingly sandy the higher up
the sequence. Hooker (1976) in Daley in Hooke (1998), examines the geological
column from a sequence stratigraphy point of view identifying a marine transgression
and regression in the beds exposed in the Highcliffe – Naish Farm cliff.
Whilst coastal landsliding causes are broadly dealt with by Lee (2002) and Maine
Geological Survey, Coastal Landslide Hazards (2005), Barton (1973) followed by
Barton and Coles (1984) and Barton et al. (2006), investigated the causes at the
Highcliffe – Naish Farm site specifically. Research since 1973 seems to indicate that
the causes of landsliding at this site are reasonably well understood although Burden
(1991) comments on the lack of up to date research at that time. All of the
stabilisation techniques employed at Highcliffe were in place by the mid 1980s and
because of the success of these schemes it is implied that enough was known then
by engineers working on schemes to effect stabilisation. There is always room for
further research and the threat of accelerated rates of coastal erosion resulting from
sea level rise may well stimulate this.
Mitigation of landslides as a prevention of the possible effects is broadly dealt with by
Lee (2002).
Specifically, the Highcliffe - Naish Farm site is covered by Mockridge
(1982) and Tyhurst (1993) where engineering measures intended to tackle the two
root causes of coastal slope degradation are described. Mockridge (1982) states that
the design for stabilisation at Highcliffe originated with Halcrow. Prevention of cliff
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toe erosion and drainage of ground water being the two lowest common
denominators requiring attention.
Barton in Hooke (1998) deals with the thorny issue of stabilisation on cliffs with an
Earth Science conservation value by suggesting ‘compromise solutions’ which will
slow although not altogether prevent long term landsliding. The construction of
offshore reefs or artificial beach nourishment may be a sufficient middle-of-the-road
compromise to balance the needs of those who value the fresh exposures that
landsliding reveals against those who object to the loss of land. This may well be the
way forward for the Naish Farm site. It is generally acknowledged that permission
would never be granted today for the type of stabilisation measures that have been
employed at Highcliffe in the past where the geology has mostly been blanketed
although some would argue that blanketed geology is better than geology that is
wasted by the sea. Barton in Hooke (1998) mentions the frequently quoted naïve
argument, ‘If it’s such a valuable resource why do you campaign for it to be left alone
so that the sea washes it away?’
(1233 words)
3. The geology of the coastal cliff at Highcliffe, Dorset and
Naish Farm, Hampshire
3.1 The geological setting
All geological literature relating to local sites describes a long history of interest in the
south facing coastal cliff fronting Christchurch Bay (Hooke, 1998). Geological maps
show the shallow syncline of the Hampshire Basin, the chalk bowl, to be filled with
Tertiary sediments (Melville and Freshney, 1982).
An absence of inland sites
suitable for study, such as quarries, means that exposures on the coastal cliff
assume a major importance. Sediments deposited on the western side of the basin
follow the shallow west to east dip of about 1 degree meaning that whilst long
continuous exposures of the same horizon are revealed there is a steady change
with horizontal distance and exposures become progressively younger from west to
east. The 30 – 35 m high coastal cliff at Highcliffe and Naish Farm exposes clay and
sandy clay marine sediments of late middle to early late Eocene age. Additionally 5
– 6 m of Pleistocene sandy gravels unconformably cap the Eocene sediment. The
British Geological Survey (Bristow et al. 1991), describes these as River Terrace
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Deposits related to an ancient river system that once occupied the Solent area during
periods of low sea level.
Amongst pioneering commentators the work of Burton (1933) stands out as important
here as it was he who assigned the lettering system to the individual horizons of the
Barton Beds and his system is followed by all subsequent commentators. Burtons
interest was in the superbly preserved fossils to be found in the matrix of the sandy
clay and sand sediments and consequently his ‘faunal horizons’ mark changes in the
fossil record in preference to lithological changes. Burton listed horizons A1, A2, A3,
B, C, D, E, F, G, H, I, J, K, and L with horizon A0 being added by a later
commentator (Figure 1) . Our interest only concerns horizons up to about F as the
remainder lie further to the east outside the sites in question. When Burton carried
out his study the whole of the cliff face was available to him as there were no
attempts at stabilisation at that time. Interestingly Burton mentions ‘sudden landslips’
which he estimates to result in a cliff top recession rate of ‘certainly more than a foot
during a year’.
More recent commentators in Hooke (1998) recognised a series of marine
transgressions and regressions in the sequence. Daley quotes Hooker (1976) in
describing the Barton sediments as representing four transgressive – regressive
cycles. The lower beds in the Highcliffe – Naish Farm section are described as Cycle
1 (Beds A0 to A3), commencing with a pebble bed representing a transgressive lag.
Bed A3 with its abraded fossils represents reworking due to regressive phase
shallowing. Cycle 2 (Beds B – I) commences with a sharp burrowed junction with
rolled shells probably sourced from Bed A3. The glauconitic silty clays at the base of
this cycle represent a transgressive phase.
3.2 SSSI status
The whole of the cliff face fronting Christchurch Bay which includes the 3 km
combined Highcliffe and Naish Farm sites is a Site of Special Scientific Interest first
notified in 1953 (Halcrow Maritime, 1999), Under Section 28 of the Wildlife and
Countryside Act (1981) the Site was notified in March 1991. The cliff face exposes
the only complete late - middle to early - late Eocene sequence in the world and is
the Type Section or International Stratotype for The Bartonian stage of the Eocene,
(Hooke, 1998), the age of which ranges from 41.3 Ma to 37.0 Ma. Consequently the
natural, gently sloping to the east, rock exposures revealed on this stretch of
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coastline are highly valued by geologists, stratigraphers and palaeontologists.
Additionally the cliff east of Chewton Bunny forms fairly easily accessible, excellent,
barely vegetated examples of degrading soil slopes of interest to engineering
geologists. Burden (1991) mentions their value as a teaching aid visited by
generations of students :- ‘The site has been visited by student undergraduate
parties for many years and has played a vital role in the training of geologists, on
whom the economy depends to find coal, oil, gas, water and building materials’.
Also,’……… regularly used by established academics and research students from
both Britain and overseas’.
It is against this geological background that the causes, effects and mitigation
strategies relating to coastal landslides at Highcliffe and Naish Farm are considered.
Differing outlooks, especially in recent years, have resulted in clashes of interest and
a ‘Property vs. fossils’ debate has ensued which resulted in a public enquiry (Burden,
1991), called to examine Christchurch Borough Councils proposal to extend its coast
protection works into the Naish Farm sector.
(762 words)
4. The causes of landsliding at the two sites
4.1 General overview of causes
Before a boundary change in 1974 split this part of the coast between Hampshire
and Dorset both the adjacent Highcliffe and Naish Farm sites were in the county of
Hampshire.
Also, it should be pointed out, that at this time only preliminary
stabilisation works had been carried out at the eastern end of the Highcliffe site
(Mockridge, 1982). Bartons (1973) pioneering study therefore examined almost the
complete combined lengths of the two adjacent sites, almost 3 km, which is made up
of the gently easterly dipping Barton strata from horizon A0, at beach level in the
west to horizon F, at cliff top level in the east. About 5 m of Pleistocene gravels, the
Plateau Gravels, overlie the clay strata combining to give a cliff top height of 30 – 35
m.
The factors involved in coastal landsliding are listed as (Lee, 1982) :-
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External factors:•
Undermining of the cliff by wave action
•
Oversteepening of the cliff by wave action
•
Unloading of the cliff by removal of debris from the toe
•
Lowering of beach levels
Internal factors:•
Weathering
•
Stress relief and swelling
•
Strain softening
•
Groundwater level changes
•
Shrinkage
4.2 Analysis of landsliding processes at the two sites
Barton concluded that it was the geological make up of the site which caused the cliff
to assume a terraced profile, (Barton, 1973), and calculated that the overall gradient
of the cliff would be stable at 14 degrees (Barton and Coles, 1984). Because of the
rapid rate of degradation and minimal vegetation cover he concluded that this was an
ideal site on which to study soil slope failure mechanisms. The processes which
resulted in landsliding along the complete combined length of both sites were
examined and five were listed as combining together to cause the transport of cliff
top and mid cliff material from the top of the Plateau Gravel progressively down to
beach level, a maximum descent of 30 – 35 m. The primary cause of land sliding is
listed as marine erosion of the cliff toe, the process resulting mainly from winter
storm conditions which overwash the base of the cliff.
Weathering and the percolation of groundwater which issues from the gravel – clay
interface also contributes substantially to cliff sliding (Lee, 2002). Partly attributed to
weathering and partly to the saturation of the clays by groundwater are the erosive
effects of streams and mudflows. Weathering is the main cause of scarp spalling,
the collapse of the often near vertical gravel face at the cliff top resulting from the
saturation of the gravels during storm conditions when they are subject to driving rain
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General instability of the cliff face results in longitudinal
portions of the cliff top edge, up to 5 m in width, breaking away and slumping down
the face in a classic rotational fashion (Lee, 2002 and Maine Geological Survey,
Coastal Landslide Hazards, 2005), putting horizontal pressure on accumulated
rubble lying on the bench below. This process is named scarp slumping.
The seaward directed pressure resulting from scarp slumping when combined with
water lubrication of certain interfaces in the strata causes the all important process of
bench sliding to which Barton attributed 93% of the mass transport process (Barton
and Coles, 1984). This occurs at the bases of the A3, D and F horizons (Figure 1).
All slip planes lie close to septarian nodule horizons, a fact which may or may not be
significant.
Barton (1973) concluded that the factors underlying the sliding tendency of certain
horizons may stem from separation which had occurred during tectonic geological
folding processes or from sand flushing from sand rich horizons allowing softening of
the underlying clay. He suggested that this matter should be the subject of further
research.
Barton, Hillier and Watson (2006) concluded that the D zone was the dominant slip
surface and identified a 7 mm thick dark chocolate brown clay layer at the base of the
zone which had a slightly lower shear strength than clay from its parent zone. This
thin clay layer was subjected to modern analytical techniques.
X-ray powder
diffraction mineralogical analysis, X-ray fluorescence chemical analysis, scanning
electron microscope study of the microfabric and ring shear tests were applied to the
clay from this layer. The authors found the layer to be slightly more clay rich and to
have clay particles realigned relative to the direction of shear.
The authors
concluded that lateral rebound responses to marine erosion of the cliff toe initiated
progressive failure leading to compound landsliding of the clay slopes.
Rates of cliff top recession or land loss are dealt with in Mockridge (1982) and Barton
and Coles (1984). The figure given for Highcliffe before the site was stabilised is 0.7
m/yr with an extreme figure of 1.7 m/yr given for a specific locality on the Naish Farm
section.
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4.3 Effects of sea level rise at the two sites
At this point it may be appropriate to consider the future effects of sea level rise
forecast to accompany climate change on both the unstabilised Naish Farm site and
the stabilised Highcliffe site. Figures published in McInnes (2003) for this part of the
coast forecast a rise of 0.8 m by 2080 and an increase in periodic rainfall intensity.
The beach gradient fronting Naish Farm is quoted as 1:60 (Halcrow Maritime, 1999)
meaning that for every unit rise in sea level a x 60 incursion can be expected. This
would result in a 0.8m x 60 = 48m retreat of the cliff toe by 2080. Also more intense
periodic rainfall would play its part in softening the cliff face and accelerating
degradation processes caused by the seepage of ground water.
The securely
buttressed Highcliffe site is forecast to suffer steeper more stony beaches requiring
more frequent maintenance and artificial nourishment. The beaches are forecast to
lose their current attractive summer amenity value due to the probability of having to
be bolstered with coarse stone not conducive to beach leisure pursuits (Tyhurst,
2000)
(984 words)
5. The effects of landsliding and mitigation measures
suitable for application to the geology of both sites
5.1 Potential effects at Highcliffe
A walk along the undercliff at Highcliffe will quickly reveal to even the most casual
observer why the local authority was under pressure from some of its more
prosperous Council Tax payers to stabilise the coastal slope fronting the village of
Highcliffe on Sea.
Amongst other prestigious properties, blocks of flats such as
Hambledon Court, Cheynies Court, Marlow Court, Fulmar Court,
Beacon Court,
Harrington Court, Wharncliffe Court, Marina Court and Palma Court stand sometimes
within 30 m of the cliff edge. These choice residences overlook the south facing
Christchurch Bay with a view of the Isle of Wight and The Needles. Assuming a
conservative cliff top recession rate of 0.5 m/yr little mathematical expertise is
needed to appreciate that owners of these properties harboured some concern about
the medium term future of their tenure!
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5.2 Engineering solutions
Soft cliff stabilisation is dependant on the implementation of measures to overcome
the following five factors (Lee, 2002)
•
Reducing pore water pressure in slopes through surface and sub surface
drainage.
•
Reducing de-stabilising forces by re profiling cliff or landslides.
•
Increasing stabilising forces by adding weight to the toe.
•
Supporting unstable areas by construction of retaining structures.
•
Preventing the erosion of exposed slopes and cliffs.
Implementing these stabilisation measures contrasts quite well with Bartons five
degradation processes (Barton, 1973) with cliff toe protection and surface and sub
surface drainage measures emerging as the principal factors in achieving cliff
stability. The detail of the mitigation measures applied to the Highcliffe cliff is set out
in Mockridge (1982) and Tyhurst (1993).
By 1971 a stout timber revetment and 11 sheet piled groynes stretched the length of
the cliff between Highcliffe Castle and Chewton Bunny.
The purpose of these
structures was to protect the cliff toe and to encourage a pebble beach to accumulate
behind the revetment. Following a Halcrow design (Mockridge, 1982) a diaphragm
wall was sunk into the clay substrate at the eastern end of the cliff some tens of
metres behind the cliff top, the purpose of which was to prevent ground water oozing
from the gravel–clay interface. This accumulated water was piped via a series of
drains directly down to beach level and was harmlessly absorbed into the foreshore.
A horizontal drain ran along the cliff about half way down the slope with the purpose
of piping any accumulated ground water from the slope face directly into the drainage
system. The lower slopes which could be drenched with sea spray during storms
were drained by means of regularly spaced rock filled ditches.
Attention given to vegetation on the slopes and terraces meant that salt tolerant
grass mixes had to be developed. The obvious benefits resulting from maintaining a
vegetation cover being that root systems hold together potentially unstable soils with
plants taking up a certain amount of moisture during their biological processes.
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Additionally, dense evergreen foliage on the upper slopes was encouraged to provide
an umbrella cover to disperse the concentrated effects of wind and rain during winter
storm conditions. Tyhursts (1993) description with accompanying simplified diagram
(Figure 3) clearly sets out the mitigation strategy of engineering works that by 1985
had been applied to the complete Highcliffe frontage and his undated study, Tyhurst
(undated) describes the attention given to the vegetation at the site with salt tolerant
grasses sown on the lower slopes and selective use of small trees and shrubbery to
protect, drain and assist in stabilising the upper slopes.
Subsequently the 11 sheet piled groynes were progressively converted to substantial
random rubble moles to deflect direct wave action and the high maintenance timber
revetment was buried beneath many thousands of tonnes of imported gravel leaving
a mixed sand and gravel beach which formed a pleasant summer recreational
amenity. By the early 1990s a total of £2.5 M had been invested in the Highcliffe
frontage (Hooke, 1998), probably amounting to around £4 M at late 1990s prices.
This amounts to about £3 k per m run. As a measure of the effectiveness of the
stabilisation techniques, apart from some minor soil creep on steep slopes there has
been no major movement on this cliff section for at least 10 years and during summer
months the stabilised coastal slope forms a pleasant vegetated south facing
recreational amenity with a view of the approaches to the Solent, The Isle of Wight
and The Needles.
Figure 4 is reproduced from Halcrow Maritime (1999) and indicates the forecast cliff
recession at Naish Farm by 2049 based on a broad estimate of a 1 m/yr recession
rate. The mitigation proposal from Halcrow, if mitigation is appropriate, is to stabilise
both ends of the site allowing the large central section to reach an equilibrium
situation.
It is unlikely that similar stabilisation techniques to those applied at
Highcliffe will ever be applied to the Naish Farm section of the cliff due to
conservation issues combined with negative cost benefit analysis results. However,
both Lee (2002) and Barton in Hooke (1998) discuss ‘ compromise solutions’ which
may be suitable for application to Earth Science Conservation Sites where it might
be appropriate to slow rather than prevent cliff degradation. The construction of
offshore reefs or nourishment of beaches are two techniques that are mentioned as
possible solutions as they do not directly interfere with exposed geology but give a
degree of protection to the cliff toe.
(892 words)
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6. Conclusions
Hopefully this report has given the reader a basic understanding of the causes,
effects and mitigation strategies that apply to landslides on this stretch of the coast of
Christchurch Bay spanning the Dorset – Hampshire border.
The large chapter
devoted to the geology of the site is felt to be prerequisite to an investigation of
causes and this is stressed by all sources. Because of the early geological interest in
the coastal cliff there was a substantial literature base prior to more recent
engineering investigations which preceded mitigation schemes.
Whether or not
coastal landslides require mitigation depends on ones point of view, whether or not
the landslide is viewed as a ‘geohazard’ and whether or not the expense of mitigation
can be justified. The techniques used at the Highcliffe segment of the site would be
eminently suitable for use at Naish Farm because of the geological similarity but the
expense combined with environmental considerations preclude this at present.
Compromise solutions such as offshore reefs or beach nourishment, or allowing
erosion and degradation to occur between strong points may be a way forward for
mitigation at Naish Farm. In any case this report has attempted to show that at the
location in question causes and processes are well understood, that the perception of
effects is somewhat related to ones point of view and that successful mitigation
strategies are available should they be needed and the expense justifiable. It is
commonly understood that sea level rise resulting from climate change will
accelerate coastal landsliding worldwide and sooner or later policy makers will need
to make difficult decisions as to whether to simply allow this to occur or make partial
efforts to slow down the process rather than halt it.
(322 words)
Word total :- 4696
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Cliff section showing the Highcliffe to Milford SSSI, the Highcliffe and Naish Farm sectors
occupy the left hand third of the section.
Cliff profile showing the degradation processes of the Barton Clay cliffs.
Aerial photograph showing the Hampshire – Dorset border, the stabilised Highcliffe sector is
on the left, the unstabilised Naish Farm sector is on the right. All pictures from the Ian West
geological website.
Figure1
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Cliff photographs. (2007)
M. Hinton.
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Mike Hinton. Christchurch Borough Council.
Figure 2
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M. Hinton.
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7. References
Barton M E
1973.
The Degradation of the Barton Clay cliffs of Hampshire.
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Barton M E and Coles B J 1984. The Characteristics and Rates of the Various
Slope Degradation Processes in the Barton Clay cliffs of Hampshire.
Quarterly
Journal of Engineering Geology, London, 17 117 – 136. (Abstract in West [constantly
updated])
Barton M E, Hillier S and Watson G V R 2006. The Slip Surface in the D zone of
the Barton Clay.
Quarterly Journal of Engineering Geology and Hydrogeology.
39:44, 357 – 370, Geological Society. (website)
http://cat.inist.fr/?aModele=afficheN&cpsidt=18276293 (Accessed April 2007)
Bristow C R, Freshney E C and Penn I E 1991. Geology of the country around
Bournemouth. Memoir for geological sheet 329. British Geological survey.
Burden P R , (inspector). 1991. Report on public inquiry into a scheme of coast
protection works at Chewton Bunny. Submitted to the Minister for Agriculture
Fisheries and Food for approval under section 5 of the Coast Protection Act 1949. 24
pages.
Burton E St John 1933.
Faunal Horizons of the Barton Beds in Hampshire.
Proceedings of the Geologists Association. Vol 44 131-167
Halcrow Maritime
Plan. Volume 2
1999.
Poole and Christchurch Bays Shoreline Management
Physical and Natural Environment.
Hooke J (editor) 1998. Coastal Defence and Earth Science Conservation. The
Geological Society.
Lee E M 2002. Soft Cliffs: Prediction of Recession Rates and Erosion Control
Techniques. DEFRA/EA (website)
http://www.defra.gov.uk/environ/fcd/CliffsRepFinal2.pdf (Accessed April 2007)
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Maine Geological Survey, Coastal Landslide Hazards 2005. (website)
http://www.maine.gov/doc/nrimc/mgs/explore/hazards/landslide/facts landslide.htm
(Accessed April 2007)
McInnes R G 2003. Coastal defence, a non technical guide.
SCOPAC. Cross
Publishing.
Melville R V and Freshney E C 1982. British Regional Geology. The Hampshire
Basin and adjoining areas. Natural Environmental Research Council.
Mockridge R G 1982. Highcliffe cliffs – the maintenance of coastal slopes.
Proceedings of a conference organised by the Institution of Civil Engineers.
University of Southampton. Pages 235 – 242.
Tyhurst M F Undated. Coastal vegetation work at Highcliffe on Sea. Christchurch
Borough Council. Technical Services Division.
Tyhurst M F 1993. Cliff Stabilization Techniques at Highcliffe. A students guide.
Christchurch Borough Council. Technical Services Division.
Tyhurst M F 2000 Global warming. The Effect of Sea level Rise on Christchurch’s
Coast. Christchurch Borough Council. Technical Services Division.
West I (constantly updated) Geology of the Wessex Coast. (website)
http://www.soton.ac.uk/~imw/ (Accessed April 2007)
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