naish beach - Leeson House

NAISH BEACH
BARTON ON SEA
COAST PROTECTION SCHEME
ENGINEERS REPORT
CONTENTS
1
EXECUTIVE SUMMARY
2
BACKGROUND
2.1 Christchurch Bay
2.2 Naish Cliffs
2.3 Barton on Sea
3
THE PROBLEM
3.1 Factors affecting erosion
3.1.1 Updrift developments within the coastal cell
3.1.2 Stability of the cliffs
3.1.3 Stability of the beaches
3.1.4 Prediction of erosion rates
3.2 Effects of the without project option
3.2.1 Geological sites
3.2.2 Naish holiday village
3.2.3 Cliff top properties
3.2.4 Recreational use of the cliffs and beaches
3.2.5 Roads
3.2.6 Amenity land
4
ALTERNATIVE SCHEMES
4.1 Design criteria
4.1.1 Ground water induced erosion
4.2 Do nothing
4.3 Managed setback
4.4 Do minimum
4.5 Low level of investment – beach recharge by beneficial use of dredged materials
4.6 Beach recharge using commercial sources
4.7 Extension of the Rock Revetment from Chewton Bunny to Barton on Sea
4.8 Rock or wooden groynes
4.9 Offshore breakwaters
4.10
5
Perched beach and Shingle Renourishment
COSTS OF ALTERNATIVE SCHEMES
5.1 Price Base
5.2 Research and monitoring
5.2.1 Summary of the research programme
5.2.2 Research costs
5.3 Planning and design
5.3.1 Summary of the planning and Design Programme
5.3.2 Design costs
5.4 Engineering works
5.4.1 Calculating costs
5.4.2 Maintain to a declining standard
5.4.3 Rock revetment
5.4.4 Shingle recharge
5.4.5 Perched beach
5.4.6 Offshore breakwaters
5.5 Summary
5.6 Royalties
5.7 Contributions
6
BENEFITS OF ALTERNATIVE SCHEMES
6.1 Assumptions
6.2 Assessment of economic benefits
6.3 Naish holiday village
6.4 Residential property
6.5 Open space and development land
6.6 Recreation
6.7 Disruption of services
6.8 Geological site
6.8.1 Geological uniqueness of the site
6.8.2 Teaching value
6.8.3 Use of the site by geologists
6.8.4 Nature of research
6.9 Down drift beaches
6.10
Engineering advancement
6.11
With scheme damages
7
CHOICE OF SCHEME – BENEFIT COST EVALUATION
7.1 Costs and benefits
7.2 Benefit cost ratios
7.3 Selection of the preferred scheme
7.4 Sensitivity analysis
7.5 Conclusions
8 PROPOSED SCHEME
8.1 Scheme outline
8.1.1
Shingle renourishment
8.2 Design
8.2.1 Research and monitoring
8.2.2 Design conditions
8.2.3 Hydraulic model studies
8.2.4 Performance of alternative beach recharge designs
8.2.5 Environmental considerations
8.2.6 Materials for renourishment
9
BEACH MANAGEMENT PROGRAMME
9.1 Threshold levels
9.2 Settlement and shingle loss
9.3 Routine maintenance requirements
9.4 Monitoring programme
9.4.1 Onshore surveys
9.4.2 Offshore surveys
9.4.3 Dynamic groynes
9.4.4 Database
10 IMPACTS OF THE PROPOSALS
10.1
Influence of the proposals on coastal processes
10.2
Environmental impact assessment
10.3
Integration with the shoreline management plan
11 CONTRACT DOCUMENTS
11.1
Contractual arrangements
12 REFERENCES
FIGURES
1 LOCATION PLAN – CHRISTCHURCH BAY
2 NAISH CLIFFS
3 BARTON ON SEA
4 HISTORICAL RECESSION RATES
5 WITHOUT SCHEME PREDICTED EROSION LINES
6 WITH SCHEME PREDICTED EROSION LINES
7 EXPOSURE OF GEOLOGICAL RESOURCE
8 LAND USE NAISH CLIFFS
9 COMPARATIVE WITH AND WITHOUT SCHEME EROSION
10 SEDIMENT GRADINGS
11 TYPICAL BEACH PROFILES
12 SCHEME OUTLINE
APPENDICES
APPENDIX I RESEARCH AND MONITORING PROGRAMME
APPENDIX II HYDRAULIC MODEL STUDIES
APPENDIX III BENEFIT COST EVALUATIONS
APPENDIX IV CAPITAL ASSETS AT RISK
1
EXECUTIVE SUMMARY
Naish and Barton beaches have historically provided protection against wave erosion to
the toe of the soft sand and clay cliffs. They also provide dynamic toe weighting which is
crucial to the overall cliff stability. The beach volume has declined in volume over a
number of years, due largely to the reduced shingle supply from the west. This is the
result of starvation caused by coast protection works west of the administrative
boundary, but within the same coastal cell. The declining volume of the beaches has
resulted in increased rates of erosion at the toe of the Naish Cliffs, reduced toe
weighting to the vulnerable Barton Cliffs and erosion throughout Christchurch Bay as far
east as Hurst Spit (Figure 1). The beaches at Milford and Hordle Cliff to the east are
becoming smaller and the concrete seawalls are showing considerable signs of distress,
due to scour which has resulted from the lack of an appropriate supply of beach material.
This trend will continue without appropriate remedial works and will result in extensive
damage to property, land and existing coastal defences.
The proposals presented in this report will provide a low cost supply of beach material to
the starved beaches to the east, will provide dynamic toe weighting to the Barton cliffs
and will provide direct protection from wave attack to the toe of the Naish cliffs.
The proposals will reduce the rate of erosion at an internationally important geological
site, which lies within the study area and which is a rapidly diminishing finite resource.
The site is described by English Nature as one of the most important geological sites in
Europe.
The assets to be protected include residential property, holiday homes, an important
recreational area and, most importantly, will extend the life of a site of international
conservation value.
An innovative short term coast protection scheme, comprising shingle renourishment
and dynamic shingle groyne structures has been designed to slow the rate of cliff
recession, whilst still allowing the cliffs to erode. A comprehensive monitoring
programme is planned to follow the main phase of works.
The works will extend from Chewton Bunny to the eastern end of the defended section
at Barton on Sea, a distance of approximately 3km (see Figures 2-3), but the scheme
will eventually link with a beach management plan for the whole of eastern Christchurch
Bay, which is expected to evolve from the in progress shoreline management plan study.
The works will essentially be divided into two types of construction:
(a) Shingle renourishment
(b) Dynamic shingle groynes
Beach recharge is the only option that would be considered by English Nature as an
acceptable solution at this site.
Associated British Ports (ABP) have recently sought approvals to dispose of materials
dredged from the approach channel to the port of Southampton. The MEPD have
awarded a FEPA licence for the disposal of 6 million of the 7 million cubic metres at the
Nab dumping ground but have been instructed to find beneficial use for the remaining 1
million cubic metres of material, of which much is sand and gravel of variable quality.
ABP have offered to supply the material for use in beach recharge projects to local
authorities at no cost and have agreed that the costs of dredging and transport of the
material will be borne by their contract. The project is scheduled to commence in June
1996 and with a contact period of 60 weeks.
This scheme will make beneficial use of sands and gravels from the dredging of the
approach channel to Southampton Water. This source of supply offers a rare
commercial advantage over the supply of materials from other licenced commercial
areas. It is anticipated that the single will be brought to the site by sea and will be
discharged by pumping onto the beach. It will then be redistributed by mechanical plant
on the land.
A draft legal agreement has been drawn up between ABP and the NFDC, to enable
beneficial use to be made of the dredged sand and gravel, which will become operative
only in the event that MAFF approval is given to this scheme. This will form the Contract
for the works.
Costs will comprise mobilization charges for the temporary works and the time spent in
landing and redistributing material. Time related costs of dredging and transport
between the dredging site and the beach recharge site will be paid by ABP, as part of
the dredging operation. This represents a significant time and plant related cost saving
on normal beach recharge schemes. The scheme costs could be significantly higher
therefore, if beach recharge materials are provided from other sources.
It is proposed however, that this scheme be approved only on the basis of the proposed
supply of beach material from Southampton Water and that other commercial supplies
should not be considered. The estimated works cost of the preferred option is £786,000.
Construction of the scheme by using an alternative supply of equivalent material from
commercial licensed dredging areas could cost £4.2 million.
The average benefit cost ratio for the preferred option, using the Southampton Water as
a source, is 1.63 whilst the average benefit cost ratio for the same scheme but using
commercial licensed sources is 0.37.
The average net present value for the beneficial use beach recharge from the
Southampton Water source is £623,591 and is -£2,708,869 for an equivalent commercial
licensed area alternative.
2
BACKGROUND
2.1 Christchurch Bay
Christchurch Bay is located on the south coast of England, facing extensive fetches
across open sea to the south and south west (Figure 1). The bay is backed by a
sequence of soft cliffs which have a simple geological structure, comprising clays, sands
and gravels. The geological strata dip gently from west to east. These deposits provide
much of the sediment budget for the bay, together with sediments derived from beaches
to the west. The shingle beaches are formed largely of material eroded from the plateau
gravel cliffs within Christchurch Bay, which consist of sub angular flint pebbles in
matrices of coarse sand. They were deposited during the glacial fluctuations of the
Pleistocene period and are easily eroded.
The predominance of winds from the south west results in a net longshore transport of
sediment from west to east. Hurst Spit and the Shingles Banks, which lie at the eastern
end of Christchurch Bay, form major sinks for shingle derived from erosion of the plateau
gravels of the bay. The development of Hurst Spit is linked to the development of
Christchurch Bay where, over the last 300 years, considerable human interference with
the natural processes has taken place in the form of mining exploitation and the
construction of coast protection works. Between 1848 and 1870 ironstone mining was
carried out on Hengistbury Head and the removal of ironstone boulders from the beach
resulted in the rapid transport of shingle around the Head, creating Mudeford Spit.
Coast protection works began in 1840 with a groyne scheme at Highcliffe. In 1938 the
long groyne at Hengistbury Head was built, almost stopping the movement of shingle
from Poole Bay into Christchurch Bay and increasing rates of erosion between
Hengistbury Head and Barton on Sea. This prompted a series of protection works at
Mudeford, Highcliffe, Barton and Milford, beginning in 1944. The most extensive works
constructed between 1964 and 1969, at Highcliffe and Barton enormously reduced the
eastward movement of shingle past Barton on Sea. Reinforcement of the Becton Bunny
sewage outfall in 1970 created another major obstacle to shingle movement.
These activities have significantly reduced the volume of sand and gravel now eroding
from the soft cliffs to form beaches, and disrupted the natural eastward movement of this
material around Christchurch Bay and onto Hurst Spit.
It is estimated that the main body of beaches in Christchurch Bay is declining in volume
by approximately 10,000 – 20,000 m3 per year. Introduction of material in the starved
zone between Hurst Castle and Milford on Sea, by the Hurst Spit stabilization scheme,
will provide a local supply of beach material within this area, but the beaches between
Naish cliffs and Milford on Sea will continue to have a shortfall of material due to the
enhanced rate of erosion and low supply of beach feed from the west. The littoral
transport rate has been estimated at 15,000 – 20,000m3 per year. Much of the material
is lost from the system at Hurst Point, and is taken offshore in the fast ebb currents and
is deposited on the Shingles Banks. The material remaining in the shoreline system is
transported around Hurst Castle and is deposited on the active recurve known as North
Point.
2.2 Naish Cliffs
The cliffs (Figure 2) comprise the Bartonian series of the Eocene clays aged at
approximately 54 million years. The clays are capped with Pleistocene gravels. The
Barton clays are well researched and the classification of the zones is universally used
as a descriptive basis. The apparent dip is from west to east at an angle of about 2
degrees and although the strike is virtually parallel with the coast there is also an
apparent dip inland. The zonal stratigraphy has been fixed by academic studies (Barton,
1985, 84, 73). The exposure of the A2 and A3 zones outcrop uniquely at this location
and this outcrop is a rapidly diminishing asset.
Whilst the geology is relatively simple the failure mechanisms are very complex,
comprising a combination of bench sliding and rotational slips which are driven primarily
by ground water processes, in combination with local geological variations. These
processes are further compounded by the interaction of wave action with the toe of the
soft cliffs. This results in rapid erosion of the cliff toe and increased instability in the cliff
system due to oversteepening of the lower slopes and reduced toe weighting. The cliff
system cannot therefore achieve its natural equilibrium slope, due to the constant
removal of the toe material.
2.3 Barton on Sea
Barton on Sea (Figure 3) comprises a similar geological sequence to the Naish cliffs, but
the dip results if exposure of younger beds at sea level than in the Naish cliffs. The site
has a long history of cliff instability and coast protection works. There is at present an
extensive drainage system in place together with a rock revetment and groyned beach.
The revetment is performing extremely well but the drainage system is becoming old and
recent studies have identified additional failure mechanisms to those which the drainage
system is designed to deal with. The beach has become very denuded due to the lack
of supply of material from the west and the poor hydrodynamic performance of the
existing groyne system.
3
THE PROBLEM
The problem may be conveniently divided into three discrete elements
a) Cliff recession on the unprotected cliffs at Naish beach
b) Stability of the protected cliffs at Barton on Sea
c) Diminishing beach volume within eastern Christchurch Bay
The unprotected section of the cliffs at Naish is retreating at an average rate of
approximately 2.4 m per year (Figure 4), and this rate is increasing. This is resulting in
rapid destruction of a geological exposure, which is of international environmental
importance. The amenity value of the site has fallen already due to the limited access to
the foreshore, much of which can only be accessed over low tide periods. Losses of
holiday homes are occurring and whilst these have been re – sited previously, the site is
now becoming too small to relocate the at risk holiday homes. Residential property is
also vulnerable and the loss of high value properties are expected within the next 20
years.
The protected cliffs at Barton on Sea are retreating in a less predictable manner, this is
due to the marginal factor of safety offered by the current protection works. A number of
cliff top properties are vulnerable, if large scale cliff slips, such as those which have
occurred during the past few years continue.
The beaches of Christchurch Bay are collectively eroding at a rate of 10,000 – 20,000
m3 per year due to the diminished supply of material from the west. This has resulted in
undermining of concrete seawalls at Hordle Cliff and failure of sections of seawall due to
the lack of protection. Cliff recession rates have also increased due to the low supply of
beach material.
3.1 Factors affecting erosion
3.1.1
Updrift developments within the coastal cell
Sine the late eighteenth century, human activity, connected with mineral exploitation,
and with the protection of the developing settlements from erosion by the sea, has
drastically changed the natural coastal processes around Poole and Christchurch Bays.
In particular, the construction of coast protection and flood defence structures over the
last 70 years has stopped the erosion of sand and gravel from the soft cliffs along much
of this stretch of coastline. Consequently, the volume of shingle moving in the littoral
drift has declined and, as a result, the beaches have decreased in size; a process which
has accelerated markedly since the 1940’s when large scale groyne construction began
at Bournemouth and Christchurch.
The recent construction of the Highcliffe groynes scheme and Chewton Bunny rock
bastion coast protection scheme has slowed the rate of sediment supply onto the Naish
beach and Barton on Sea frontage. Coastal monitoring has identified a change in the
pattern of erosion on the Naish beach frontage (Figure 11) since the construction of the
terminal angled strongpoint. This has resulted in accumulation of material in the lee of
the angled strongpoint and focusing of erosion further to the east, at the toe of the Naish
cliffs, where beach levels are eroding more rapidly. The strategic coastal monitoring
programme, which was set up in 1989 has been unable to identify trends arising from the
groyne scheme as these were constructed prior to the commencement of this
programme. There is however, no doubt that the rates of erosion of the cliffs have
accelerated since the introduction of the coast protection works west of the
administrative boundary. Aerial photographs confirm these trends.
3.1.2
Stability of the cliffs
The main problems affecting the stability of the cliffs are:
(i)
(ii)
Sediment transport – more shingle is being lost off the beaches than is being
supplied from the Christchurch Bay beaches to the west
Ground water and geological processes
Erosion of the cliffs results from interaction of a complex system comprising three
principle components which make up the cliff unit: these are the cliff toe, the cliff slope
and the cliff top.
The cliff toe is affected primarily by the supply of debris material from the ground water
induced cliff processes, the supply of beach material in the longshore transport and the
wave attack at the cliff toe.
The cliff section exhibits the classic bench sliding failure mechanisms and the active
aquifers relating to the A3 and D horizons are clearly visible within the coastal section.
Geological and groundwater processes predominate within the cliff slope and cliff top
areas.
3.1.3
Stability of the beaches
Beaches have been monitored since 1988 and the analysis shows a constant trend of
falling beach levels and landwards recession during that period. The beach immediately
adjacent to the Chewton Bunny coast protection works actually increased in volume
locally, due to the diffraction effects of the rock bastion, but immediately downdrift the
beach has eroded more rapidly since the installation of this structure. The beach has
reached a situation where scour and erosion are likely to continue unless some artificial
introduction of sediment occurs.
3.1.4
Prediction of erosion rates
Historical rates of cliff recession have been determined by analysis of aerial
photography, Ordnance Survey mapping, large scale mapping based on aerial
photography (1982, 1987 and 1994) and beach profile records. Figure 4 shows the
historical rates of erosion of the Naish Cliffs. This indicates a recent acceleration in
erosion rates. Historical erosion rates of 2.4 m per year occur over much of the section.
Analysis of aerial photography and academic research (Barton 1985, 84, 73) indicates
that the cliff erosion does not occur at an even rate on a year by year basis, but that
periods as long as 2 years can occur without perceivable erosion of the whole of the cliff
top, along the length of the vulnerable frontage. Cliff falls of 2-8 metres width generally
occur in a single erosion event. Erosion of the cliff top averaged over a period of 5-10
years has resulted in the loss of in excess of 2.4 metres per year.
The foot of the cliffs shows a similar rate of erosion and it is this factor which results in
maintenance of a continually oversteep and unstable cliff section. The eroding cliff toe
provides no benefits to the stability of the cliff section, as any toe weighting released by
groundwater induced slippage is rapidly removed by wave action. The rate of cliff toe
erosion keeps pace therefore with the cliff top.
A sensitivity analysis has been carried out on the rates of erosion. It seems unlikely that
the erosion rates will slow during the course of the analysis period and a base erosion
rate of 2.4 m per year is a sensible low estimate of rates. Whilst short term erosion rates
vary through the frontage, the pattern of change is not predictable and an average
erosion figure has been taken for the entire section. The local spatial variations will
balance out during the course of time. Despite the gradually changing geology, which
results from the very shallow inland dip of the strata, the groundwater geological
processes are unlikely to change during the analysis period and erosion rates have been
assumed to remain constant in this respect. As the equilibrium slope for the geology
cannot be achieved due to the constant pace of wave induced toe erosion with the
groundwater induced erosion, the effects of groundwater and geological interaction are
assumed to remain constant.
Recent studies commissioned by MAFF (Ball et al 1991) to examine the economic
effects of sea level rise have suggested that the effects of sea level rise on the rates of
erosion of soft cliffs can be very dramatic. The apparent increase in erosion rates seems
to reflect these trends at this site. An assumed rate of relative sea level rise of 5mm per
year, taking into account the combined effects of sea level rise and isostatic
readjustment have been made for this site, based on recent MAFF publications (MAFF
1994) and academic research (Bray 1994). The theoretical methods have been applied
to this site to examine the effects of a scenario which combines the effects of sea level
rise with longshore transport and cross shore profile adjustment, due to wave induced
erosion. The effects of sea level rise will theoretically increase the rate of erosion during
a 30 year period. Assuming a relative sea level rise of 5mm per year the rate of erosion
will increase from the recent historical rate of 2.4 metres per year to 3m per year during
a 20 year scheme life. This analysis is perhaps something of an oversimplification of the
processes however and does not address fully the effects of ground water and
geologically induced processes. Nevertheless it does seem reasonable to expect
accelerated erosion at the cliff toe with erosion rates in excess of the current rates. This
increase in the predicted erosion rates is in keeping with recent trends.
The effects of beach starvation resulting from coast protection works to the west will not
enhance the performance of the beach cliff interaction but this is difficult to predict and
no further allowance has been made for the effects of locally falling beach levels on cliff
toe erosion.
No further factor of safety has been included within the estimates of future erosion
trends, although the assumptions for the timing of losses of residential property and
holiday homes provide a further safety margin.
Erosion rates based on the historical erosion rates (2.4 m per year) and increased to
include the effects of relative sea level rise and beach lowering (to 3m per year) have
been used as the basis for the assessment of losses. Erosion contours showing the
predicted without scheme losses for the next 30 years are shown in Figure 5 at five year
intervals.
3.2 Effects of the without project option
If the stability problems are ignored, and no further maintenance nor any form of long
term stabilization undertaken, (the “do nothing”option) recession will occur at the rates
shown in Figure 5. This will have the following effects.
3.2.1
Geological sites
The unique exposure of stratotype Bartonian is a vulnerable asset which is declining in
size. Whilst it has been impossible to assign a monetary value, its preservation is seen
as the most significant factor at this site.
The site lies within the Barton on Sea Site of Special Scientific Interest. The
international type sequence for the Bartonian Eocene beds are exposed at the western
end of the site. The A2 and A3 horizons outcrop uniquely at this site. The geological
sequence dips at an angle of 2 degrees from west to east along the coast, and
exposures which are seen in the cliffs at Naish disappear underground at Barton. The
strike line is not however parallel, but is at a slight angle, to the coast. This results in a
dip at a very shallow angle to landwards. The river cut chine of Chewton Glen veers to
the north east as it works its way inland, immediately adjacent to these exposures.
The A2 zone of the Bartonian series is 9.2 m thick at this location and is visible above
the beach only east of Chewton Glen for a distance of about 220 metres. The horizon
disappears below the beach at a level of about 2.5m ODN. The elevation of the zone at
this location is such that no more than half of the thickness is visible above the beach
and the dip of the strata takes this bed below sea level some way to the east of Chewton
Glen. A sketch of the resource which is clearly finite is shown in Figure 7. The finite
resource was calculated to have a volume of 91,665m3 at the Chewton Bunny outfall
public enquiry in 1991 (Tyehurst 1991). The resource has since diminished further and
the rates of erosion are apparently increasing. 37% of the resource had been lost
between its notification in 1953 and 1991.
A summary of the principal statistics describing the size and evolution of the geological
reserve is shown below ( after Tyehurst 1991).
1953
Length of A2 zone
Max height of
Visible exposure
Elevation area
North/south extent
Of A2
Volume of A2
Resource
263m
5.26m
1991
2011 Loss 1953(predicted) 1991
225m 201m
14%
4.5m 4.00m
14%
692m2 506m2 402m2
635m2 543m2 483m2
27%
14%
146,454m3 91665m3 64722m3 37%
Predicted loss
1953-2011
24%
24%
42%
24%
56%
(based upon a datum level of 2.5m ODN)
The exposure will therefore eventually be lost if erosion continues at the present rate.
Some erosion is needed to ensure that the exposure continues to be kept clean, but
there is a need to reduce the rate of erosion to extend the life of this important finite
environmental resource.
Without a scheme erosion of the geological resource will continue at rates up to 3m per
year. This would result in a 20% reduction of the current area of exposure within a
period of just 15 years.
3.2.2
Naish Holiday Village
The Naish holiday village lies at the western end of the study area and comprises a
variety of types of holiday homes, including permanent brick structures, serviced wooden
chalets on concrete bases, mobile caravans, car parks and service roads. The
properties are used all year round. An electricity sub station also lies within the site and
this is at risk. Losses of the holiday homes and associated services occur on a regular
basis and without the project152 serviced buildings, 17 permanent serviced caravan
sites and two permanent buildings would be lost to cliff recession if no action is taken, in
addition to service roads and tarmac car parking area. Predicted losses without project
are identified on Figure 5.
3.2.3
Cliff top properties
A number of large residential cliff top properties including large houses and a block of
flats, lie within the at risk zone at the eastern end of the study area. If erosion continues
at the current rate many of these properties will be lost by year 15 of the analysis.
Predicted losses without project are identified on Figure 5. A total of 16 properties are
vulnerable within a 30 year period.
3.2.4
Recreational use of the cliffs and beaches
The beaches at Naish cliffs are already denuded and most of the indigenous shingle has
been lost from the system. The western end of the site is used as an amenity beach but
the beach is now to narrow to be used along its full length. Access along the toe of the
cliffs between Barton on Sea and Naish beach is restricted to low water periods and safe
passage from one end of the beach to the other is restricted to only about 50% of the
tidal cycle. As the cliffs and beach continue to erode, the accessible areas will diminish
further preventing recreational walking, bathing and access through the site.
3.2.5
Roads
Roads providing access to the residential property will be affected by erosion rates and
these will be lost if no action is taken, resulting in loss of access to the residential
properties. No allowance has been made for the costs of either relocating the road
(which is a dead end) serving the properties within the benefit cost analysis as the costs
of this are dealt with by other legislation.
3.2.6
Amenity land
The cliff top areas are public open space to the east of the site and lie within the
development land of the holiday village to the west of the site. The combined los of land
will occur at a rate of 1.1 acres per year without project.
4
ALTERNATIVE SCHEMES
4.1 Design Criteria
Probabilistic risk assessment procedures have been applied to a range of management
scenarios. These have provided the basis of assessment for potential beach and cliff
management solutions, together with statistical analysis of joint probabilities of waves
and water levels, a wide range of beach geometries, and allowing for a range of levels of
investment.
The engineering schemes, which provide alternatives to beach
management are far easier to assess in terms of stability, and preliminary designs for
these have been based upon the extreme conditions derived from the wave climate and
water level studies.
The alternative management strategies have been assessed with consideration of the
following criteria:
a)
b)
c)
d)
ability to solve the potential problems identified in Section 3.2
acceptable environmental impact
technical soundness
reasonable cost
The purpose of the scheme is perhaps slightly unusual in that the aim is to slow the rate
of recession rather than to halt recession completely. It also provides the opportunity for
review of an innovative scheme designed to protect an environmentally sensitive site.
This strategy will prolong the life of the assets and will maintain the environmental value
of the site. Any scheme which sought to halt the rate of erosion, or reduce the extent of
geological exposure would be strongly resisted by English Nature. The findings of a
public enquiry held to determine the outcome of a scheme on the adjoining section of
coast at Chewton Bunny found in favour of English Nature and any similar proposals
would be strongly resisted. Cliff regrading or surface drainage systems would not be
acceptable.
Alternative schemes which could not provide technically sound solutions have not been
fully costed and included within cost benefit analyses. Those which are environmentally
unacceptable but which might be fully or partially functional have been included in the
economic analysis.
It is desirable to reduce the rate of erosion along the frontage to less than 0.5m per year,
but it is unlikely that any of the proposed schemes would provide that degree of
protection, in view of the complex combination of groundwater, geology and wave
induced erosion processes. If the erosion rate is reduced to much less than 0.3-0.5m
per year there is a risk that the continued erosion of the geological exposure will be
insufficient to maintain clean exposure of the type sequence.
The scheme has to address two separate processes which must be considered together,
but which may be treated by quite separate schemes.
a) ground water induced cliff erosion
b) wave induced cliff and beach erosion
4.1.1
Ground water induced erosion
The effects of ground water induced bench slides within the cliffs has resulted in
recession of the cliff top as the cliffs seek to achieve a naturally stable slope. The
erosion of the toe of the slopes results in maintenance of the over steep cliff profile and
hence the cliffs are unable to reach a stable equilibrium slope angle without an
appropriate degree of support at the cliff toe.
Treatment of the ground water induced bench sliding of the cliffs to achieve stability
could include various combinations of grading, cliff drainage and toe weighting. Grading
of the cliffs would provide a totally unacceptable solution, as the important exposures of
geological strata would be lost. Such an approach would not be in keeping with the
NFDC policies which supports the need to preserve SSSIs. The recent difficulties
experienced by Christchurch Borough Council at Chewton Bunny and the results of the
public enquiry on the proposed scheme suggest that this is not the best approach to this
particular problem. A careful staged approach to the treatment of this site, in close
liaison with English Nature, is the best way of ensuring preservation of the geological site
in parallel with acceptable erosion rates.
The alternative treatments must be dealt with separately, but considered together, for the
wave induced and ground water induced processes.
The optimum scheme, from a performance point of view, would allow 0.3-0.5m per year
of erosion, thus maintaining clean exposure of the geological sequence whilst preserving
the cliff top assets for the life of the scheme. It is accepted that a scheme that would
eventually halt the erosion would need to include a combination of toe protection,
drainage and grading of the cliffs. A scheme which could reduce erosion rates to less
than 0.5m per year would need to include a combination of drainage and toe protection,
but regrading could be avoided. The aquifer source of the A3 horizon, which accounts
for a considerable amount of cliff slip activity is some way inland from the coast and
interception of the water further inland will help to reduce the pore water pressure in the
most suspect area. There is also a need to intercept A3 and D aquifers well back from
the face of the scarps.
The technology to provide the drainage element of such a scheme is at present
insufficiently developed to allow a large scale scheme, that would be environmentally
acceptable, to be constructed with sufficient confidence in the performance of the design
to justify its use. Conventional drainage systems, using excavations to install the
drainage works would be unacceptable at this site. A public enquiry in connection with
works at Chewton Bunny found in favour of the conservation agencies when an
excavated drainage system was proposed. A drainage scheme is not proposed at this
stage due to the limited confidence in the unproven technology.
Recent trials of moled drainage systems at Barton on Sea are demonstrating positive
results. Such drainage systems can be installed without the need for excavation into the
cliffs. The system has excellent potential when no disturbance on the surface is
desirable, as it can be installed without disturbance to the geology and without visible
surface effects. Installation costs of such drainage works are relatively cheap, by
comparison with alternative drainage installation methods. Such a drainage scheme
could reduce the rate of cliff erosion, but validation of the methodology needs to be
confirmed. Detailed assessment of the performance of the trial sites prior to a major
installation of a previously unproven technique is recommended. This will take at least a
further 3 years of monitoring prior to gaining sufficient data to provide statistically valid
confidence in the technique. This method is the only drainage method that is likely to be
acceptable to the conservation agencies with geological interests.
Experimental works at Barton on Sea and Naish cliffs, using moled drainage schemes,
will be monitored and analysed during the next few years. A 3 year research scheme
has recently been proposed to the EPSRC, to be funded jointly with NFDC, SCOPAC
and AE Bartholomew/Civils UK (specialist contractors), to analyse the performance of
such drainage systems. The research will be carried out by the University of
Southampton, Department of Civil Engineering. This research is supported by English
Nature who view this innovative method as a potentially acceptable method of cliff
stabilization works at environmentally sensitive sites. If the trial system demonstrates
satisfactory performance such a solution could be appropriate on the Naish cliffs
frontage, but this could not realistically be a serious option unless the strategic research
demonstrates positive performance. A critical review of the research results will be
carried out and a longer term stabilization scheme which will include cliff drainage
elements will be considered, if appropriate, in about 4-5 years time.
It is not proposed at this stage to carry out the drainage element of works, but this will be
considered in the future. The proposed scheme will reduce the rate of cliff recession to
about 1.2m per year but this rate is till high and in the medium term it may be appropriate
to carry out a second phase of works which will reduce the erosion rates further. Ideally
erosion rates should be reduced to less than 0.5m per year and this could potentially be
achieved using a moled drainage system. The detailed costs of this scheme have not
been included within the benefit cost analysis.
The preliminary costs of a moled drainage system could be based around the
performance of the drainage system to the east. A series of arrays of 5 drainage pipes
at intervals of 50m with each pipe having a length of approximately 130 metres would
cost approximately £700,000. The costs of this possible second phase have not been
included within the benefit cost analysis.
4.2 Do nothing
Doing nothing will cause the potential problems identified in section 3.2 and accepts that
the cliffs will continue to erode at the projected rate. The damage suffered in monetary
terms is used as the baseline in the economic analysis.
4.3 Managed setback
This alternative, which accepts that the site is abandoned, cannot satisfy the relevant
criteria used by the MAFF in their definitions of an acceptable location for managed
setback. Destruction of an internationally important environmental site would result, as
opposed to the development of an environmentally beneficial habitat. Managed setback
would not increase the intertidal habitat and an important SSSI would be lost.
The introduction of managed setback would certainly not create a more stable and
sustainable coastal defence, as recently constructed coast protection works to the west
have already been outflanked and needed additional works. There is clear indication
that the works at Barton on Sea are in danger of being outflanked as well.
In summary, none of these effects are of environmental benefit and most are positively
harmful. This is an environmentally unacceptable alternative and does not solve any of
the potential problems. Managed setback is not therefore a viable option at this site and
has not been carried forward to the economic assessment.
4.4 Do minimum
The “do minimum” option is based on maintaining the existing beaches without
additional capital works, and thus maintaining to a declining standard.
Until recently the management policy at Naish Beach has been to allow the cliffs to
continue to recede. The loss of volume from the beach is about 4000m3 per year,
hence the declining standard of protection offered. The declining standard of protection
is also due in part to the reduced supply of sediment transport resulting from high losses
from the system due to the coast protection works at Highcliffe. Maintenance to a
declining standard is therefore an inefficient method of maintenance and will result in
economic, recreational and conservation losses.
Maintenance to a declining standard could be achieved by maintaining the current beach
levels by annual importation of the net loss of beach materials from the unprotected cliff
frontage. This will not offer better protection but should prevent the erosion rates from
increasing significantly. This will result in aversion of some of the potential losses, but
only on a short term basis. The current recreational value of the site will be retained.
Maintenance is an expensive option as the unit cost of beach recharge and spreading
will be very high. Materials will have to be delivered to the site by road as the
mobilization costs of a dredger are not economic for such small quantities.
This scheme does not meet the technical requirements required to protect against the
problems discussed in section 3.2. The environmental impact of this option is not
satisfactory since the geological succession will continue to erode at the same rate. This
option has been carried forward for financial assessment in the benefit cost analysis
section.
4.5 Low level of investment – beach recharge by beneficial use of dredged
materials
Associated British Ports Southampton (ABP) has prepared proposals for a large scale
dredging project, to deepen the approach channel to the port. The soils investigations
have identified approximately 1 million cubic metres of sands and gravels from the
dredging arisings. ABP have been instructed by MAFF to find beneficial use for the
sands and gravels and the FEPA licence for disposal of the dredging arisings is
conditional upon ABP seeking to find beneficial uses for this material. Examination of
the soils investigations has revealed that much of this material is potentially suitable for
use in beach recharge operations. ABP have indicated that they would be prepared to
bear the costs of dredging this material and transporting the dredged material to the
Naish beach, Barton on Sea site, if the costs of landing and spreading the material are
borne by NFDC.
A medium scale beach recharge scheme has been designed on the basis of the
potential grading of materials which might be obtained from Southampton Water, making
beneficial use of the dredged sands and gravels.
The scheme comprises open beach recharge and the construction of innovative dynamic
shingle groynes. This option potentially lengthens the effective life of the scheme over
an open beach solution but at virtually no extra cost per unit volume. The open beach
recharge is a well proven method of providing limited life dynamic protection to toe
erosion and the addition of dynamic shingle groynes is a new experimental concept
which has recently been the subject of MAFF funded strategic research (HR 1995a).
This option has been tested extensively in 3D wave basin studies, which have indicated
a significant improvement in the performance of this scheme over a conventional open
beach solution (HR 1995a). The model tests have indicated that the groynes should be
able to withstand fairly severe and extreme storm activity without degrading fully. The
model tests are the first of their kind on dynamic groynes however, and the results
should be viewed with caution. Some questions remain over the life of the shingle
groynes, but there is clear evidence that they provide improved hydraulic performance
and offer an improved standard of protection, even if their life is shorter than the model
tests have indicated. As the groynes are destroyed by storm wave action, they will be
integrated into the beach and the worst scenario is that the system will perform with the
same effectiveness as an open beach. It seems appropriate therefore that the
experimental aspect of the scheme should be considered in the same effective life
context as an open beach scheme, although there is some considerable potential for
improved performance. Any improvement on performance resulting from the dynamic
structures is therefore a bonus which is not reflected in the benefit cost analysis.
The low cost scheme life is potentially relatively short, as losses could be expected at a
rate of 8000m3 per year in line with the current estimated potential rates of loss from the
beach system.
The potential source of beach material contains a fairly high proportion of sand and silt
size material which might be expected to be lost in suspension during the first 2-3 years.
An allowance of 50,000m3 has been made for the initial losses over a 3 year period
following construction. This is approximately double the normal beach losses from this
system. A scheme comprising 325,000m3 of beach recharge material could therefore
be expected to lose approximately 128,000m3 within a scheme life of 20 years, in
addition to the initial losses. This will result in approximately 150,000m3 of the beach
remaining after 20 years. It will still provide a reasonable level of protection at that time,
although the frequency of overtopping will have begun to increase again. No interim
recharge or maintenance should be necessary during this period, although careful
monitoring of the experimental aspects of the scheme will provide valuable support to
the development of dynamic groyne solutions, which may provide effective solutions
elsewhere and which may form the basis of longer term solutions at this site.
The design concept has met with approval from English Nature, who have provided
positive support for this design approach, but have offered suggestions which will further
enhance the preservation of the site. In the light of the recent difficulties experienced by
the Christchurch Borough Council at the western end of Naish beach, this support is very
siginificant at such an environmentally important site.
The degree of protection offered by the scheme will decrease, but not stop the present
rate of cliff recession. It is expected that the erosion rate will fall from the predicted 3m
per year to 1.5m per year , during the course of the scheme life, taking into account the
large component of ground water driven erosion which will continue. The main benefit of
the scheme is to provide toe protection and support to the cliffs. This will reduce the rate
of over steepening and therefore slow the rate of erosion due to the effects of ground
water induced slumping in conjunction with the cliff slope.
The scheme life is conservatively estimated at 20 years during which no intermediate
recharges would be required to maintain the required level of protection. During this
period the initial placed volume of 325,000m3 is expected to reduce in quantity due to a
combination of longshore transport, storm action and loss of materials during initial
adjustment of the scheme (15% in first three years). The following rates of loss are
expected:
Initial losses
Annual losses storm/longshore
Material remaining after 20 years
50,000m3 first 3 years
8,000m3 per year (years 4-20)
150,000m3
The life of the scheme could reasonably be expected to extend beyond this period due
to the introduction of dynamic groynes, which have the potential to slow longshore
transport and to provide increased energy dissipation. The potential problem with these
structures is that they have a life limited by the severity and sequence of wave
conditions. Although the model studies have indicated that the dynamic groynes
provided better performance than an open beach with 30% greater beach volume a
more conservative assessment of their performance has been made, in line with the
expected performance of an open beach.
This option has been carried forward to the economic assessment.
4.6 Beach recharge using commercial sources
A similar scheme to the option discussed above in section 4.5 could be achieved using
commercial sources of material offering the same performance, but this option would be
significantly more expensive to achieve.
The costs of this approach are carried forward to the benefit cost analysis for
comparative purposes.
4.7 Extension of the Rock Revetment from Chewton Bunny to Barton on Sea
The rock revetment at Barton on Sea is very successful in resisting wave attack and
extension of this could be a practical and functional solution to the problem on the Naish
beach frontage.
However, it would:
(i)
Reduce the sediment supply to the existing shingle beach
(ii)
Destroy the exposure of an internationally important geological site
(iii)
And (iv)
Be at risk from undermining of the toe of the revetment due to the soft
substrate
Suffer reduced amenity value due to limited access and poor aesthetics.
This type of structure, whilst acceptable at Barton on Sea where much of the geological
interest has already been lost, would strongly be resisted by English Nature at Naish
beach due to the loss of the exposure of the international stratotype sequence of the
Eocene. Although this scheme could provide a functional part solution to the problem at
a high level of investment, there are some technical doubts about its performance, and
its environmental impact would be unacceptable and its extension would be strongly
resisted by the local and national conservation agencies. It has however been carried
forward to the economic assessment for further analysis.
4.8 Rock or Wooden Groynes
Whilst groynes may reduce the rate of longshore transport along the beach, they have a
very high risk of becoming outflanked due to the cliff erosion process. They would
therefore be inappropriate and non functional at the western end of the site. This option
has been discarded on technical grounds.
4.9 Offshore Breakwaters
Offshore breakwaters provide effective protection to beaches at many sites and could be
designed to be effective offshore from Naish beach. This solution was originally the
most favoured environmental approach to solving the problem along the sensitive beach
frontage of the Naish cliffs. Since the costs of detached rock breakwaters are virtually
always the most expensive way of treating beach erosion and costs are approximately
proportional to the square of the height of the structure, cost aspects of the scheme were
considered at an early stage in the design process. Preliminary designs were based
upon the minimal empirical design methodology that exists for these structures. These
were considered together with the estimated maximum cost which might be allowed
within the benefit cost analysis. A series of layouts of comparable cost were tested in 3
dimensional physical model studies, to optimize the scheme effectiveness. None of the
schemes tested within the allowable costs could provide a solution (HR 1995).
Technically acceptable solutions could be achieved but these would be at considerable
expense. This option whilst environmentally attractive, would cost several orders of
magnitude more than the alternative schemes. The high cost alternative has been
carried forward to the benefit cost analysis.
4.10
Perched beach and Shingle Renourishment
A perched beach and beach renourishment could achieve both the environmental aims
and the technical aims of the scheme at the Naish beach site. This scheme has the
advantage that the energy absorption characteristics of the system will be improved
significantly. The beach will continue to dissipate wave energy in an efficient manner.
Similarly, the environment will not be changed significantly from the existing conditions
and the area will retain most of its present natural and visual characteristics. The
offshore toe of the structure presents some potential problems however and there is a
risk that scour may result. The efficiency of the system is such that little material is able
to escape and consequently there is only a limited benefit to the coast further down drift.
There is also a further requirement for additional structures to tie the beach
replenishment into the existing system, without adverse affects on the adjoining
coastline. This design will involve the high cost of using rock making this option far more
expensive than the alternatives available.
This option has been carried forward to the economic assessment.
5
COSTS OF ALTERNATIVE SCHEMES
5.1 Price Base
The defined price base for costs is May 1996.
5.2 Research and monitoring
5.2.1 Summary of the Research Programme
A strategic regional research programme was set up in 1989. The aim was to collect
and record data about the forces and processes acting on the beaches and cliffs. The
basis of the monitoring is a beach and nearshore surveying programme undertaken in
conjunction with Southampton University and aerial photography taken annually. The
surveys are carried out quarterly in order to record seasonal variations in the volume of
the beaches. A wind/wave correlation survey was carried out by Hydraulics Research
between 1987 and 1989, using a wave rider buoy located off Milford on Sea. This study
was carried out in support of the Hurst Spit stabilization scheme but its results are also
valid at this site. Sampling of the beach material was undertaken in 1992-1994, and a
wave climate study, using software purchased from Hydraulics Research, was carried
out in 1994. Investigation of the Southampton Water approach channel was based on
ground investigation surveys by Alluvial Mining Limited, provided at no cost by ABP,
using side scan sonar, grab sampling and core drilling.
5.2.2
Research Costs
The committed costs of the research programme carried out by NFDC are summarized
below:
Beach surveys
Beach sampling
New Forest District Council fees
Total
Cost (£)
14,000
3,000
7,000
£24,000
5.3 Planning and Design
5.3.1
Summary of the Planning and Design Programme
Planning and design costs in addition to the New Forest District Council fee costs
include:
(a) Physical model testing undertaken at Hydraulics Research
(b) Hydrographic survey of approaches to site
5.3.2
Design Costs
The estimated costs of the planning and design work, including projected costs of
contract preparation and tender procedures, are summarized below:
Physical model testing
Hydrographic survey
New Forest District Council fees
Total
Cost (£)
43,000
5,000
32,000
£80,000
5.4 Engineering works
5.4.1
Calculating Costs
Estimated construction costs of the “maintain to a declining standard” shingle recharge
scheme have been calculated using inland gravel pits as a potential source.
Estimated costs of the alternative shingle recharge schemes have been calculated for
two possible sources of shingle, Southampton Water and commercially operated marine
dredging areas.
Where possible recent tender prices have been used to estimate costs of the works and
costs of the rock structures reflect the lowest expected costs.
5.4.2
Maintain to a declining standard
This scheme contains the following main elements:
(i)
Annual maintenance by beach nourishment of 1350 metres of the beach, with
400m3 of suitably graded shingle
Costs of shingle are based upon supply from inland gravel pits at a rate of £18/m3
Summary of maintenance costs
(i) Shingle maintenance (annual)
(ii) Contract design and supervision
(annual)
5.4.3
£48,000
£4,800
Rock revetment
This scheme contains the following main elements:
Construction of a 1300m long rock revetment. Costs are based upon recent (1996)
tender price rate for similar works
(i)
(ii)
(iii)
(iv)
Item
Rock revetment
General Items
Mobilisation
Supervision
Cost
£1,911,500
£175,000
£383,000
£56,600
Totals
5.4.4
£2,526,100
Shingle recharge
This scheme contains the following main elements:
(i)
(ii)
Nourishment of the beach between Chewton Bunny and Barton golf course
with suitably graded shingle
Construction of shingle groynes
Cost estimates are based upon two alternative supplies for this scheme design:
Southampton Water (beneficial use) and commercial sources. The Southampton Water
costs are based upon the tender price of the scheme that ABP will be accepting for their
dredging of the approach channel.
5.4.4.1 Southampton Water
Costs for the shingle recharge of the beach are based on a dredger operating 24 hours a
day, 7 days a week for 3 months and discharging over high water periods to produce
325,000 cubic metres of shingle.
Royalty payments to the Crown Estate are shown in the estimate but have not been
included in the benefit cost evaluation.
Summary costs for Southampton Water Supply
(i) Beach recharge shingle
(ii) Mobilisation
(iii) General items
(iv) Contract supervision
Total
Royalty payments
Cost (£)
567,100
167,800
50,960
33,000
£818,860
£188,175
5.4.4.2 Commercially sourced beach recharge
Estimated costs for the shingle nourishment are based on purchasing 325,000 cubic
metres of shingle of comparable grading to the Southampton Water source and using
appropriate recent tender prices for mobilization of plant and general items.
The costs of the shingle are based upon the supply from a commercial source at a rate
of £11.50 per cubic metre. This reflects an average price based on the volatile range of
prices currently being quoted by commercial aggregate suppliers in recent tenders.
Prices are subject to considerable sensitivity and may typically vary between £9 and
£14.70 per cubic metres according to supply and demand at the time of tender. Prices
are likely to be higher than for the recently tendered Hurst Spit scheme, which provided
tender rates in the range £6.2 - £9.8 per cubic metre. Higher prices at this site reflect the
longer discharge periods needed when pumping material over long distances (800m)
and the necessary use of smaller dredgers ( due to shallow water depths). The distance
to the commercial dredging areas is further and the steaming time is approximately 50%
extra. A unit rate increase of 50% therefore is a reasonable estimate for this site.
The significantly higher costs of mobilization for this option reflect the costs of the
dredging plant which would be provided from the limited aggregate production fleet of
dredgers and is based upon tender prices for a similar source.
Summary costs for commercial offshore supply
(i) Open beach recharge
(ii) Mobilisation
(iii) General items
(iv) Contract supervision
Total
5.4.5
Cost (£)
3,737,500
330,000
50,960
33,000
£4,151,460
Perched beach
Costs for the perched beach are based upon commercial supply costs of shingle and
rock based upon recent tender prices.
Cost (£)
584,000
3,052,000
192,500
608,300
56,000
£4,492,800
(i) Beach recharge
(ii) Rock sill
(iii) General items
(iv) Mobilisation
(v) Supervision
Total
5.4.6
Offshore breakwaters
Offshore breakwaters
Offshore breakwater costs are based upon recent tender prices for rock breakwaters,
increased to allow for construction from floating plant.
(i) Rock breakwaters
(ii) General items
(iii) Mobilisation
(iv) Supervision
Total
2,686,000
175,100
383,300
56,000
3,300,400
5.5 Summary
The following table shows the estimated cash cost of implementing the various options
Declining
Standard
Invest. 24000
Design 80000
Constr.
Rock Beneficial
rev
use
Recharge
24000 24000
80000 80000
2526100 818860
Commercial
source
recharge
24000
80000
4151460
Perched
beach
Detached
breakwaters
24000
80000
4492800
24000
80000
3300400
Royalty
Totals 104000
2630100 1111035
Maintenance 52800
/year
25000
/5 years
188175
4255460
4596800
3404400
25000
/5 years
50000
/5 years
The long term monitoring and management costs are identified in cash terms in the
economic analysis in Section 7.
5.6 Royalties
Royalty payments to the Crown Estate Commissioners (CEC) for shingle obtained from
Southampton Water have been calculated at a rate of 57.9p per cubic metre. This rate
has been proposed by Crown Estates but has not yet been agreed. There is thought to
be some potential for a reduction in this rate due to the expected losses of materials
after placement. This will be pursued further and a definitive rate will be agreed prior to
commencement of the contract.
As advised in PAGN these costs are not economic costs.
5.7 Contributions
A contribution towards the cost of the scheme will come from Hampshire County Council
in accordance with Section 20 of the Coast Protection Act 1949.
6
BENEFITS OF ALTERNATIVE SCHEMES
This section examines the damage which would be caused by adoption of the do nothing
option, which is used as the base case for assessing the benefits of the other options.
Section 3 details some of the risks and indicates that cliff recession will continue to occur
at the predicted rates. Some of the economic losses are assumed to commence
therefore in year 1 of the economic analysis (see Appendix iii).
6.1 Assumptions
The physical model testing carried out in 1995 indicated that wave attack on the cliffs
would occur on a frequent basis with many events per year resulting in wave attack on
the cliffs. This proved to be accurate with toe erosion of the cliffs occurring in winter of
1995 at the predicted rate.
The erosion scenario described in figure 5 demonstrates the initial losses due to erosion
during a 30 year period. The total area of land protected by the cliffs and the distribution
of land use within this area is shown in various categories in Figure 8. The categories
used are: residential, commercial and recreational.
Indicative standards of protection can not be applied to the type of protection works
discussed, but the standard for each of the options is the 1:100 year design storm. This
is further complicated when assessing the design standard of the beach scheme as
these are dynamic schemes which are expected to change with the conditions. The
schemes have however been designed to survive a combination of those events which
might reasonably be expected during the expected lifetime of the beach recharge, based
on statistical analysis of joint probabilities of waves and water levels.
6.2 Assessment of economic benefits
The economic benefits of reducing erosion rates at Naish cliffs fall into the following
categories as follows:
(i) Environmental
(ii) Capital stock
(iii) Holiday homes
(iv) Recreation
geology (no monetary value assigned)
extended life factor costs calculated for:
Residential houses; services open space
And development land
Sub station
Car parking and access
Roads (no monetary value assigned)
relocation costs calculated
no monetary values assigned
The financial benefits quoted reflect 1996 price base.
The extended life factor formula presented in the PAGN has been applied to assess the
value of capital assets resulting from implementation of the scheme. A projected
scheme life value (s) of 20 years has been applied for the beach recharge options and
30 years for the rock structure options, in accordance with the instructions given in the
PAGN. The benefit cost analysis values are based upon these calculations.
If the extended life factor formula is used in strict compliance with the PAGN instructions,
it can give misleading results however. A discussion of the applicability of this technique
with alternatives is given in section 7.4 where sensitivity of the calculations are
discussed.
The effects of scheme construction, which will reduce but not halt the rate of erosion,
have then been predicted using revised erosion contours (Figure 6). Losses of revenue
costs from commercial assets have been reassessed for the with scheme option, with
the costs assigned to the expected year of loss with and without scheme.
The combined differences of the capital and revenue related with and without scheme
benefits have then been established to determine the damage averted and the
consequent benefit cost ratios for the alternative schemes.
There are some additional very significant unquantified benefits. A major, and in the
current state of economic science, unquantifiable without scheme loss would be a
negative effect on the geological Site of Special Scientific Interest.
The contribution to the total benefit given by the recreational losses has not been
assigned any monetary value.
6.3 Naish Holiday Village
The value of the holiday homes within the Naish holiday village is based upon the
current management strategy for the site and not upon the extended life factors which
are applied to capital values. This strategy involves an adaptive response to erosion,
the costs of which are included within the benefit cost analysis.
The cliff top holiday homes, which are used on a year round basis become vulnerable to
erosion on a regular basis. The buildings are of timber construction and can be moved.
They are sited on a fully serviced concrete base and these utilities have to be relocated
as they become vulnerable. The site managers (Hoburne Ltd) currently move the
buildings when they reach approximately 8 metres from the cliff top edge. This is to
ensure that failure wedges that are typically 5-8 metres wide are unable to take the
buildings with them. This adaptive response is the most economic approach. The costs
of removal of a building total £8000 for the chalets and £5000 for the serviced caravans
(which do not require crainage or temporary roads for relocation). Additional relocation
costs are also required for reconstruction of service roads and surfaced car parking
within the holiday village. This includes the cost of reconstruction of a concrete base,
disconnection of old and installation of new services, and moving costs of the buildings.
The relocation operation involves the construction of a temporary road to enable cranes
to access the sites safely across soft ground. The costs shown in the analysis reflect the
(1996) costs of removal and do not take into account the land values of losses or the
capital value of the buildings. The holiday village managers are however faced with a
significant problem within the next five years, as they will run out of space for the
relocation of the buildings and the costs of relocation will necessarily involve the
purchase of development land or loss of the ground rent paid to the estate by the
lessees of the buildings. These losses are considered further in section 6.5.
A total of 152 buildings and 17 serviced caravans will have to be moved if the scheme is
not implemented. These are identified with the expected year of loss on Figure 5.
6.4 Residential property
The value of the vulnerable properties will effectively be lost when the cliff top reaches 8
metres from the furthest side of the road servicing the property, when the services to the
property will be lost. As the failure wedges are typically 5-8 metres wide and the timing
of failures is unpredictable, this seems a reasonable margin of safety to apply. Erosion
free market value house prices have been assessed by the District Valuer with input
from local estate agents. The extended life factors discussed in section 6.2 have been
applied to the losses of these properties. Six properties are at risk within 15 years
without project. Properties which will be vulnerable and their estimated market values
are listed in Appendix IV.
6.5 Open space and development land
Open space land losses have been included within the extended life factor assessment
of capital assets. The erosion free market value of land, provided by the district valuer is
£5000 per acre for open space land and £20,000 per acre for development land, in
accordance with local market prices.
Losses of land to the east of the Naish Estate boundary are all categorized as open
space land. Losses within the Naish Estate are considered within the same category for
the first five years of the assessment and thereafter are assigned to the development
land category. This reflects the space available within the holiday village for relocation of
predicted losses for a period of 5 years. Thereafter the Naish Estates will run out of
space for relocation of the holiday homes and will need to purchase equivalent
development land if they are to continue to relocate the buildings.
A total of 19.2 acres of development land and 11.1 acres of open space land will be lost
without project.
6.6 Recreation
Recreational use of the area can be considered under the following headings.
a) Walking the cliff top
b) Beach access
A wide strip of open land lies at the eastern end of the site and this is well used
throughout the year, particularly for exercising dogs. The New Forest Tourism Study
carried out in 1983 found an average of 80 people on the cliff top at 2-4pm on days
surveyed between July 7 and September 10, with a peak of 300. In the event of no
scheme the whole of the open space cliff to area will be lost within 25 years. There are
no registered footpaths affected within the investigation area. There is no cliff path along
the frontage of the Naish Estate although access through the frontage is freely available
at present. The undercliff area is not suitable for recreational use.
Access to the beach is poor and can only be achieved safely from either Barton on Sea
or from Highcliffe (about 1500m apart). Recreational activity is concentrated in the wider
sandy section of the beach adjacent to Chewton Bunny. The section of beach to the
east is more difficult to access, due to tidal restrictions and is much less popular with
visitors. The sandy beach overlies clay at a depth of not more than 1m and continued
erosion will result in the loss of the amenity value of the site. Clay is exposed on the
beach toe from time to time and if losses of beach material continue at the current rates
the beach will lose its amenity value. A survey by Dorset Institute of Higher Education
counted an average of 220 people on the beach within 400 metres of Chewton Bunny at
3pm on days between July 7 and September 10. The density of beach users falls
significantly to the east in the narrow beach area. The owners of the Naish holiday
village consider that the beach area is a significant attraction to its visitors and have
indicated that the beach is an economic attraction valued by their staying visitors.
The introduction of a larger wider beach would provide a mixture of benefits. The main
benefit would be that the beach could be used along its full length throughout all states
of the tide, for both interrupted walking and sitting. The nature of the innovative
proposed scheme using shingle groynes and open beach recharge is, at least in the
short term, likely to provide further recreational interest in the formation of intertidal
beach pools, beach spits and barriers during the development of the eventual open
beach profile, which will form during the evolution of the dynamic groynes. The present
beach system does not provide continuous access along the beach and this can present
safety problems. There is currently a danger of being cut off by an incoming tide and
there is significant risk of accident if walkers try to escape by clambering onto the low
cliff talus slopes, which are generally extremely wet and dangerous. This danger will be
averted with a beach recharge.
The rock revetment alternative would result in a reduction in the recreational value of the
area and a loss of recreational benefit has been assumed if this option is adopted, due
to the lower use by visitors. The current users, which include anglers, bathers and
walkers, would all have much more limited use of the site.
The maintenance to a declining standard option would provide only limited recreational
benefits, by maintaining the existing recreational standard of the beach.
The offshore breakwaters will provide improved protection and local pockets of improved
amenity value where tombolos or salients form in the sheltered lee of the structures.
A contingent valuation study does not form part of this application and no monetary
estimates have been made to determine the number of visitors to the site or their
perceived value of the site as a recreational asset.
6.7 Disruption of services
The major distribution network to be affected by the erosion of the scheme is the
electricity substation within the Naish Estates. This has been assessed using the
extended life formula. The costs of disruption to other services within the holiday village
are included within the relocation costs discussed in section 6.3. No costs for relocation
of any other distribution networks have been considered within the economic
assessment.
6.8 Geological site
The site is widely regarded as one of the most important exposure sites in the UK. The
geological exposures in the Naish cliffs are unusual in an economic context as they may
be viewed as both an exposure site and also an integrity site. The site is an integrity
site, as the geological reserve is finite and with continued rapid erosion will disappear
completely from view. The site relies however, on continued clean exposure to maintain
its importance as an exposure site. If erosion is halted completely then the clean
exposure will be lost and the site will also lose much of its geological value.
An objective assessment of the value of such a site is extremely difficult and it has not
been possible to determine a monetary value for the site. The approach adopted in this
case is to draw on the results of studies to assess the use of the site and on the proofs
of evidence from the Chewton Bunny public enquiry.
Studies in 1986 (Moss et al) were carried out to examine the use of the site in a
geological context. Data collected by English Nature in a study of their own also
supplements the results. The main questions to be asked of a geological site are:
a)
b)
c)
d)
Are there other sites providing the same geological information?
Is the site more important for teaching or research?
What use is made of the site in a geological context?
What type of research if carried out at the site and is this work mainly of
academic interest or does it have appreciable economic benefits?
The following sections examine each of these issues in turn.
6.8.1
Geological uniqueness of the site
The Hampshire Basin is the only area in the country where exposure of the Bartonian
strata can be viewed. Internationally there are a few sites where parts of the succession
can be viewed (Paris Basin, Alps and Pyrenees) but the succession is not complete at
any of these sites. Parts of the succession are poorly exposed at Alum Bay on the Isle
of Wight and this section is poorly fossiliferous. There is no comparable section
elsewhere in Europe.
6.8.2
Teaching value
The site is specified as a field site in one of the GCSE syllabus examination boards. The
NCC study identified an estimated 300 visits to the site annually involving about 5000
children, on the basis of responses from the Association of Teachers of Geology. This
figure appears realistic as it is rarely possible to spend a day on the site without meeting
a group of school students. The value of the site as a teaching site for undergraduates
is more limited with only 4 university departments using the site for teaching purposes on
a regular basis. The sites are certainly well used for school and university project work.
Regular requests for information are made to NFDC for information on the site.
6.8.3
Use of the site by geologists
The site is primarily of importance for fundamental research purposes. This has been
borne out by responses from various universities to a questionnaire (Moss et al). Of 15
universities contacted 7 have used the site for research purposes whilst fewer have used
it for teaching. The site is widely used by local geologists outside of the universities and
the site is widely acclaimed by local geologists associations.
6.8.4
Nature of research
Research interest in the site is increasing. Identification and use of fossils for
stratigraphic dating and the environmental conditions under which the organisms lived is
of particular interest at this time. The research has also actively been used in the oil
industry to improve geological understanding of oil bearing strata and the associated
geology. The site is extremely important for provision of fossils and previously
undiscovered fossils have been found in the cliffs on a regular basis, including a
research project which identified ten new fossil crabs.
6.9 Down drift beaches
Beaches down drift of the proposed scheme site will be influenced by the introduction of
any of the proposed schemes.
The open beach with shingle groynes proposal will provide positive benefits by releasing
a supply of sediment into a system which is currently starved of beach materials. The
effect of this will be to slow the rate of beach erosion to the west, as the required
sediment balance to maintain the beach volume will be provided from an area which
currently contributes little to the sediment budget. The effects of release of sediment are
difficult to value in monetary terms and the precise timescale for the benefits resulting
from additional material are not possible to quantify however.
The following effects might reasonably be expected within a period of 10 years. The
beaches immediately to the east of the protected Barton on Sea frontage are declining in
volume and cliff recession is occurring at a rapid rate. There are currently no plans to
carry out works along this frontage as economic benefits are minimal and the cliffs to
provide a limited input to the sediment budget. Further east at Hordle cliff where a low
concrete seawall fronts the cliffs, the beach has reached the state where scour is likely
to persist due to the declining beach volume. Without increased input the seawall will
become undermined and fail. This is already the case in several locations where
revenue funded maintenance works have been required to protect the wall from scour.
Revenue maintenance costs have amounted to in excess of £150,000 at Hordle seawall
from 1994-1996. Whilst the increased sediment may not provide vast quantities of
material to the Hordle beach system, it will certainly provide some benefit. Further east,
the Milford seawalls are also fronted by eroding beaches, due to a declining supply of
materials and these are now giving some considerable cause for concern. A feasibility
study to examine the performance and the need for a capital scheme within this area will
be promoted in the next year. The introduction of fresh material into this part of the
system may help to delay the need for such a scheme. Finally, the notorious Hurst Spit
area has needed a supply of beach material to maintain its integrity. The arrival of
material from the west will improve the performance of the spit and may, during the next
10-15 years, impact upon the quantity of material required for the 50 year beach
management plan which will follow the Hurst Spit Stabilisation Scheme.
It is impossible to put a time frame or monetary figure to these benefits but there is clear
evidence that the introduction of additional material to the sediment budget will provide
the potential for significant reductions in costs of coast protection works in the future.
6.10
Engineering advancement
The beach recharge with shingle groynes scheme provides the opportunity for
development of an engineering solution, which could increase the life of beach recharge
schemes without the need for the construction of hard structures. English Nature have
been impressed by the concept of dynamic beach control structures and if successful
would like to see the introduction of similar schemes at other locations where hard
defences are undesirable. The fact that English Nature are prepared to support a
scheme at a site which has a notorious record and which they have previously sought to
keep unprotected, in the light of its geological importance, suggests that they view the
scheme concept very favourably. They would welcome properly documented research
on the performance of these structures.
Recent MAFF funded strategic research studies have examined the performance of soft
groynes and have concluded that such techniques are worthy of futher investigation (HR
1995). The research is based on laboratory studies and a review of earlier research on
sand groynes in Denmark. Without a large scale field trial site however some doubts
must remain about the validity of the laboratory based research. The MAFF funded
research contractor (HR) has submitted proposals to MAFF R&D for monitoring of a
suitable full scale site and Naish beach has already been earmarked for this work, if
funding is made available, and if the scheme goes ahead. The scheme also has the
added advantage of great flexibility. If the scheme does present any unforeseen
problems after a monitoring period of 2-3 years it could easily be reverted to an open
beach scheme (if the natural processes have not already done this).
The likelihood of other sites being considered for such trials seems remote, as the cost
of beach recharges are usually extremely expensive and there might reasonably be
some concerns for scheme approval of such a high cost experimental scheme. The
proposals for this scheme will benefit from the very low unit cost of the beneficial use
beach recharge materials.
6.11
With Scheme Damages
The schemes discussed in Section 4 provide differing standards of protection and offer
different benefits and disadvantages. The losses of the benefits are identified within the
appropriate year of the benefit cost analysis tables for each scheme.
The maintain to a declining standard alternative has the advantage that recreational
benefits will be retained for longer than the do nothing scenario, but all economic losses
and much of the conservation value of the site would be lost within the same time frame
as the no project option.
The low level of investment option has the advantage that most of the recreational
benefits would be enhanced as users of the beach would be able to have access for
longer periods of time.
The rock revetment option has the advantage that the scheme would require less
frequent maintenance than other options. This option would however have a significant
adverse affect on the conservation value of the site. Recreational benefits would be
much reduced due to the nature of the structure which would restrict access to the beach
and the value of the site would also fall due to the lower aesthetic value of the site. The
geological value of the site as a SSSI would be lost completely. This option would be
unacceptable to English Nature.
Any of the alternative beach recharge schemes could satisfy all of the sea defence and
coast protection requirements, and also retain the conservation and recreational value of
the site.
The cliffs will still suffer some erosion due to hydrogeological processes and some
losses will continue with any of the proposed scheme options. This is invariably the case
with cliff stabilization works. It is unlikely that approval for the scheme would be
achieved on environmental grounds, if the scheme sought to halt erosion entirely. None
of the high value residential properties will be lost with scheme, but some of the open
space land and some of the holiday village properties will still be lost. Losses to the area
of geological exposure will continue but this is in keeping with the design concept which
seeks to maintain the geological significance of the site. Figure 9 puts a clear
perspective on the effects of with scheme erosion by showing the with scheme and
without scheme predicted erosion lines. These differences are taken into consideration
in economic terms within the benefit cost analysis.
7
CHOICE OF SCHEME – BENEFIT COST EVALUATION
7.1 Costs and Benefits
A total of 6 alternative scheme options have been costed and evaluated in the economic
assessment. Where possible each option has been evaluated using estimates based on
costs derived from recent (1996) tender prices, except for the beneficial use option which
is based upon actual tender prices. The sensitivity of these schemes to alternative
supply costs are identified in Section 7.4. The scheme types are:
(i)
(ii)
(iii)
(iv)
(v)
(vi)
Maintaining to a declining standard
Low level of investment – beneficial use beach recharge
Commercial source beach recharge
Rock revetment
Perched beach
Detached rock breakwaters
Benefit cost evaluations have been prepared for each of these options which can be
found in Appendix III. The estimated costs for all options are based on 1996 prices.
Although royalty payments will be made in addition to the tender price, these have not
been included in the cost benefit assessment for the Southampton Water source
because they are a financial transfer within Government.
7.2 Benefit Cost Ratios
A summary table of the discounted costs and benefits together with benefit cost ratios
and net present values of each alternative scheme is shown in table 7.1.
7.3 Selection of the preferred scheme
Costs and damages for the alternative schemes have been assessed by calculation of
the net present values of each option and these have been compared to NPV damages
averted (Pvda) for each option (see table 7.1). The decision rule process set out in the
PAGN has been followed to optimize the scheme selection.
Maintenance to a declining standard is discarded on the basis of functionality and
environmental cost, as the assets which the scheme seeks to protect are lost. The net
present value for this scheme is also lower than for any other option. The only
significant assets which will be preserved will be the amenity value and this has not been
considered in the economic assessment.
An acceptable scheme must accommodate the requirements of the conservation
agencies in as sympathetic a manner as possible, but without compromise to the safety
of the design. Although the rock revetment option is functional, it does not meet the
environmental criteria and would be objected to by English Nature as well as other
conservation agencies. The results of the public enquiry on the Christchurch Borough
Council Chewton Glen scheme suggest that such a scheme would not be approved.
The benefit cost analysis clearly demonstrates that this option is not financially
acceptable either, providing an average benefit cost ratio of 0.68 and this option has
therefore been discarded.
The perched beach recharge with rock sill option is environmentally acceptable but is the
most expensive of the options examined. Whilst it has a longer effective potential life
than the open beach recharge options, it still only has a benefit cost ratio of 0.4. This
option has therefore been discarded.
The offshore breakwaters option is the scheme which would be most favoured on
environmental grounds, but is an expensive option which returns a benefit cost ratio of
only 0.52. Some question marks also remain over the hydrodynamic performance of this
option, particularly in respect of the influence that the breakwaters will have on
longshore transport in the medium term. This option has therefore been discarded.
The beach recharge options have been designed to withstand a 1:100 year return period
storm, but the difference in design for a 1:50 year design or 1:500 year design is
negligible due to the very small changes in design conditions over this range of
extremes, and would result in the same scheme design. This is typical of most beach
recharge schemes.
The beach recharge options are acceptable environmentally and are functional. The
beach recharge options are identical in design but use alternative sources of material,
which have a dramatic effect on the cost of the scheme. Construction using the usual
commercial sources would provide a benefit cost ratio of only 0.37, whilst the equivalent
scheme constructed from beneficial use materials gives a benefit cost ratio of 1.63. It
could be argued that the beneficial use option is a commercial supply, but is material
available for a limited period of time at an unusually low opportunity cost, reflecting local
market forces.
Options have been ranked by maximizing the NPV of programme expenditure. The
option with the greatest average benefit cost ratios (calculated according to the PAGN
instructions) that is greater than unity, and that meets the indicative performance
standard is the beneficial use beach recharge, which has a benefit cost ratio of 1.63.
This is the preferred option. None of the other alternative options can return a benefit
cost ratio of greater than 1.
7.3 Sensitivity analysis
The sensitivity of the benefit cost ratios is examined below for the range of costs and
benefits for the preferred option. Sensitivity analysis has not been carried out for the
alternative rock structure options as these have been based upon the lowest perceivable
rates and any cost changes would only worsen the benefit cost ratios.
Significant cost variations may result from the tolerance allowed within the specification,
for shingle placement. The specified tolerance limits for placement of shingle could
result in 15% extra material being required, as a result of variations in the as built
dimensions of the structures or initial variations on site conditions. This would have the
effect of increasing the tender prices of the beach recharge by approximately £84,000.
This would reduce the benefit cost ratio to 1.50.
An overall contingency allowance of 15% has been allowed in addition to the estimate
prices, to reflect changes at the site and placement tolerances, but this has not been
included within the benefit cost analysis.
Estimates for the costs of a commercially sourced beach recharge are based upon the
average of expected costs from various licensed dredging areas.
The lowest
perceivable costs could reduce the costs of this scheme and produce a benefit cost ratio
of 0.46. This is still too low for serious consideration.
The extended life formula in its basic form given in the PAGN is not applicable to this
site, as the introduction of a scheme will not extend the life of the properties by the entire
life of the scheme. This is likely to be the case with any cliff stabilization scheme when
erosion will almost certainly continue to some extent with the scheme. The formula has
been modified to take into account the actual expected benefits of the extended life of
the land and property which the scheme will save. This is not the same as the life of the
scheme as erosion will continue, but its rate will be slowed by the introduction of the
scheme. Whilst the scheme has a finite effective life, which is expected to be 20 years
for the low level of investment beneficial recharge option, it will slow but not stop the rate
of erosion. The modified extended life factor used within the scheme(s) is 10, allowing
for a 50% reduction in the erosion rates during a scheme life of 20 years. Application of
the extended life formulae to this site in its basic form would therefore produce artificially
high scheme benefits. Spreadsheets demonstrating the sensitivity of these two
approaches are presented in the Appendix III and this reduction is also shown for
comparative purposes in Table 7.2.
Table 7.2 Benefit cost ratios using alternative extended life factors
Beneficial use recharge
10 year ELF
1.17
20 year ELF
1.63
The lower bound costs for this option give a benefit cost ratio of 1.17. The benefit cost
ratio is still significantly greater than 1 however and there are also a significant number
of unmeasured benefits.
7.4 Conclusions
The economic analysis suggests that beach recharge, making beneficial use of
sediments from the Southampton Water as a source, offers the most cost effective
option for stabilizing the beach. This is the only option that is economically viable and is
the chosen option by the decision rule process. Whilst the benefit cost ratio is relatively
low, there are a number of unmeasured benefits (discussed in detail in section 6),
including environmental improvement of an internationally important geological site,
recreation benefits, reduced maintenance costs of vulnerable beaches to the east and
the opportunity to develop an innovative coast protection technique.
A further factor which should also be considered, but which does not easily fit within the
PAGN framework, is the alternative use of the dredged materials. If during the course of
the dredging programme ABP are unable to find beneficial uses for the materials, they
will eventually be permitted to dump the dredged material at an offshore deep water
dumping ground. The material which might potentially provide millions of pounds of
benefit to beaches would then be lost completely from the coastal system.
It is proposed that conditional scheme approval is given for the beach recharge option,
on the basis that of the beneficial use of the dredging arisings from Southampton Water,
and that the alternative options should not be considered.
8
PROPOSED SCHEME
8.1 Scheme outline
The main elements of the proposed scheme are as follows.
(i)
Recharge of the Naish Beach, Barton on Sea frontage with suitably graded
shingle;
(ii)
Construction of 7 dynamic shingle groynes along the beach frontage
The proposed stabilization scheme includes the following main elements.
8.1.1
Shingle Renourishment
The beach will be renourished with 325,000 m3 of shingle material. The beach recharge
material will lie within the grading envelope given in Figure 10. It will be constructed to a
level of 3m ODN and with a constant crest width of 10m. Modifications to the cross
section profile have been allowed for at the cliff toe where exposure of the geological
succession is required by English Nature.
The heavily used amenity area at the western end of the beach will be recharged with
sand, recycled from the bed of the beach to be recharged with shingle. This will permit
the amenity area of beach to be improved and will improve the hydraulic performance of
the beach in the same area.
The basis for the design of the shingle recharge is derived from the expected grading of
material from the Southampton Water dredging site.
8.2 Design
8.2.1
Research and Monitoring
The NFDC set up a research and monitoring programme in 1988 to investigate the
causes and extent of beach and cliff erosion, and to identify possible long term solutions
for management of the Christchurch Bay frontage.
A pilot study was set up to record wave conditions (Hydraulics research 1989a, 1989b)
and beach cross sections in 1988. This study has since been expanded to include
measurement of tides, off shore surveys, and additional surveys following storm events.
Annual aerial photography and photogrammetric mapping has also provided valuable
input to the beach and cliff performance.
A sophisticated programme of studies was carried out: to evaluate the unusual beach
processes; to quantify the design conditions; and to assess the cost effectiveness of any
remedial measures. The effectiveness of the alternative schemes could not be
evaluated properly without the aid of a combination of field, mathematical and physical
hydraulic model studies. These research studies provided design conditions an allowed
the response of the existing structure and the proposals for new protection works to be
assessed scientifically under the design conditions.
A full description of the research and monitoring programme can be found in Appendix I.
8.2.2
Design Conditions
The derivation of the design conditions adopted for the scheme is discussed in detail in
Appendix 1 and these are summarized below. Various combinations of waves and tides
produce alternative design conditions with similar joint probabilities. Combinations which
lie between these two extreme combinations must also be considered for the design of
an effective beach recharge scheme. The conditions given below have a probability of
exceedence of 18% during the 20 year design life of the scheme and represent the
1:100 year joint probability return period events.
The offshore bathymetry is very regular and bed contours lie parallel with the coast.
Conditions are consistent throughout the length of the site.
Table 8.1 Design Conditions
Tidal Offshore wave conditions
Inshore wave conditions
Elevation
(mODN)
Hs Tm Direction Hs Tm Direction
1.87 4.82 7.3
240
3.0 9.1
205
0.87 6.7 8.3
240
3.2 9.9
198
8.2.3
Hydraulic Model Studies
Probability of
exceedence
during scheme life
%
18
18
Following the programme of field work, wave climate studies and a preliminary
assessment of alternative schemes, a programme of physical and mathematical
modeling was designed to carry out detailed evaluation of the proposals. An extensive
series of hydraulic model studies were carried out to test the proposed designs and to
fine tune the designs for maximum cost effectiveness. The objectives of the model
studies are set out below.
(i)
(ii)
(iii)
(iv)
(v)
(vi)
(vii)
(viii)
Identify the various combinations of wave and water level conditions that
cause damage to the cliffs
Determine the rate of loss of shingle from Naish and Barton beaches, under
storm conditions and under “morphological average” conditions
Compare the performance of proposed stabilization measures with the
existing beach
Examine the effects of structures, such as dynamic groynes and breakwaters
on shingle transport
Identify threshold beach crest levels and widths to provide alarm conditions
prior to failure of the beach
Evaluate the stability and hydraulic performance of alternative structures
(perched beach, detached breakwaters)
Identify a planned maintenance programme following beach renourishment (
if appropriate)
Identify the most cost effective and environmentally acceptable strategy for
the protection of the site
Analysis of the beach profile field data indicated that wave induced damage occurs most
frequently when storm surges occur. A range of water levels including extreme storm
surges were therefore considered. The response of the shingle beach to storms, tidal
currents, storm surges and more frequently occurring conditions were also considered.
The beach response to these processes, by short term changes to the beach cross
section profile and by changes to the plan shape layout due to longshore sediment
transport, were investigated in the modeling programme. Since it was necessary to
reproduce the response of shingle to wave action, it was necessary to ensure that the
model allowed for reproduction of these two variables without any significant scale
effects.
The beach was modeled in two segments at a scale of 1:80 to accommodate the
problems of scaling. These were linked together by mathematical modeling to produce
an overall picture of the performance of the beach. The large model scale allowed the
sediment response to waves to be reproduced with a high degree of confidence.
Similarly the large model waves allowed rock armour movement to be reproduced and
monitored accurately. It was also possible to focus in detail on important features along
the length of the beach. These included the changes in the plan shape alignment of the
beach and the effects of sediment control structures.
The test programme was broken down into the following elements:
(i)
(ii)
(iii)
Mathematical modeling of the nearshore wave climate
Physical modeling of two overlapping segments of beach at a scale of
1:80
Numerical modeling of sediment transport
(iv)
8.2.4
Interactive modeling of the results from the physical and mathematical
models
Performance of alternative beach recharge designs
A range of beach recharge options were tested, including open beach recharge and a
range of combinations of dynamic groynes with various geometries. The most effective
plan layout comprised 100 metre long shingle groynes spaced at centres of 200 metres.
Shorter structures were tested but did not perform as efficiently. Whilst the modeling
techniques have not previously been calibrated at full scale for dynamic groynes, they
have been widely used to test shingle beaches. The 100 metre groynes provide better
protection than an equivalent open beach recharge comprising 50,000m3 more material,
for the range of test carried out. Whilst the groynes will perform best under moderate
wave conditions (Hs less than 1.5m) they deform gradually under the more extreme
conditions. The life of the scheme is affected by the slope angle of the beach at the time
of construction. If it is close to the dynamic equilibrium slope of the beach for that
material the structure will evolve more slowly. Steep side slopes will deform very
quickly. The scheme life is however dependent upon the sequence of storm events.
The evolution process helps to provide additional protection. As the groyne is attacked
by oblique wave action, the roundhead deforms to feed the downdrift beach. Energy
dissipation is maximized by diffraction on the round head and consequently waves
reaching the upper beach have lower energy. Whilst the groynes reduce the rate of
transport form within the groyne bay, slowing sediment transport, the groyne roundhead
provides a supply of material preventing downdrift scour. As the groynes evolve,
downdrift spits form from the roundheads, again providing increased protection by the
formation of low offshore barriers. These trip waves approaching the shoreline and
provide further protection until they have eventually rolled back up the beach. Short
term tidal lagoons will form in the lee of the spits. Perhaps the most important
consideration of the beach performance is the direction of movement and evolution of
the beach. The direction of movement is consistently onshore and material will not
therefore be lost from the system. At worst the beach system will perform as an open
beach and this is the form that the beach will eventually take.
Whilst tests were carried out over a range of fixed water levels, the model did not provide
full tidal control. The results of the model tests and mathematical models have been
compiled into an extensive database. This has been analysed to provide a series of
recommendations in conjunction with the geotechnical assessment and the sediment
size analysis. The data has been analysed to optimize the programme of works. This
data will form the basis of the analytical performance of the dynamic groynes, by
comparing the model and field measurements, and will be used to develop the
techniques for design of dynamic groyne systems.
The natural shingle grading is coarse, D50 = 12mm, but most of the indigenous shingle
has been lost in the littoral drift from the site. Only small pockets of shingle remain on
the upper beach. The design cross sections and sediments transport calculations,
derived from physical model testing, have been carried out on the basis of beach
recharge using the natural beach material which would normally be found at this site.
Alternative gradings could be used in the replenishment, but these will have significant
but undetermined effects on the hydraulic performance of the replenishment.
8.2.5
Environmental considerations
The beach recharge design has been developed to allow occasional washing of the cliff
toe, to provide the necessary exposure of the geological succession. A number of
modifications have been made to the design to provide optimum exposure. The beach
crest level is fixed at a lower level at the western end of the site, within the most
sensitive part of the site. The main protection will be provided by the diffraction events of
the dynamic groyne and the developing barrier and spit at its head, although the crest
width and elevation will be increased, by recycling sand from the eastern end of the site.
The geometry of the landward trunk of the groynes and the crest of the beach has also
been modified to maximize exposure of the geological succession within the western –
most 500m of the site. Elsewhere the beach profile can be constructed to the optimum
hydraulic profile.
Previous proposals to protect the site have been based upon stopping cliff toe erosion
and there has been significant resistance to these proposals from the geological
community. This scheme has significant benefits in that geologists will be allowed
continuous access to the site along the whole of its length.
The amenity value of the beach is concentrated at its western end and the design has
been modified between Chewton Bunny and the second dynamic groyne to provide a
sandy beach, by recycling of the indigenous material from the east of the site. This will
later be covered by shingle from the recharge area. This fits together well with the
geological requirements.
8.2.6
Materials for Renourishment
The alternative beach renourishment designs have been based upon the envelope of
gradings from the Southampton Water dredging site.
8.2.6.1 Proving the Resource
The following information was provided to NFDC by ABP, based on geotechnical
investigations carried out by Alluvial Mining (1994):
(i)
(ii)
(iii)
The geological setting of the area, to define the underlying geology;
The bathymetry and the distribution of surficial sediments over the bed; and
The thickness of the superficial sediment cover, based on the analysis of high
resolution sub bottom profiling and vibrocoring.
Detailed analysis was carried out on this data by NFDC. The results of this study were
very encouraging, confirming that Southampton Water has potential for production of
coarse grained sediments. The total volume of potentially recoverable sand and shingle
within the proposed channel deepening area is in excess of one million cubic metres.
The quantity required for replenishing the beach is 325,000 cubic metres.
8.2.6.2 Engineering Options for Extracting and Placing
Placement of the material in the required locations along Naish Beach, Barton on Sea
could be achieved by:
(i)
(ii)
(iii)
Self propelled hopper dredger, pumping ashore
Static dredger, discharging into barges for pumping ashore or;
Bottom dumping from a hopper and pumping ashore from a static dredger,
through a floating pipeline from a large offshore stockpile.
The low tidal range of 2.2 metres and shallow foreshore provide severe constraints on
discharge location. A 2500m3 sized self propelled hopper dredger could pump material
ashore in a two to three month continuous operation at an estimated cost of £2 per cubic
metre, excluding royalties to the Crown Estate. Operations would probably have to be
restricted to high water periods due to the draft of the dredger and the shallow sloping
foreshore. The dredger would have to anchor approximately 700-800 metres offshore
on the 5m Chart Datum Contour. At about one quarter the cost of land won or
commercial marine supplies this is obviously a very economical means of obtaining
beach recharge material of a satisfactory grading.
8.2.6.3 Licensing Requirements
The supply of beach recharge from Southampton Water is based upon a different
permissions procedure to the Government View Procedure for marine aggregates and
this has been controlled by the Department of Transport in parallel with the MEPD. The
Crown Estate as owners of the sea bed are still entitled however to a royalty payment.
Following the offshore exploration programme, a formal application was made by ABP
for a FEPA licence for the disposal of 7 million cubic metres of dredged material. The
application has been supported by an Environmental Impact Assessment carried out by
Posford Duvivier and supplementary studies by ABP research and consultancy.
NFDC and other neighbouring local authorities were approached by ABP in 1995 to
consider the potential of the materials for beneficial use. Following some extensive
investigation of the soils reports and further discussions with ABP, NFDC approached
MAFF to ask for guidance on the potential use of the material. At this stage ABP had
advised that none of the other authorities had shown any interest in the material. The
MAFF response by the Regional Engineer was to call a meeting of all of the nearby local
authorities, to discuss the potential of this material at other sites. The result of this was a
potentially more complex arrangement with a number of organizations possibly needing
a finite quantity of material.
The ABP FEPA licence application was submitted and has been awarded on the basis
that the sand and gravel fraction of material (1 million m3) is not dumped offshore and
beneficial use must be found for this material. The Naish Beach, Barton on Sea scheme
is identified as a possible disposal site in the beneficial use reports (ABP 1995). No
adverse comments have been received in respect of the extensive consultations carried
out by ABP (in respect of this disposal site). NFDC have subsequently (February 1996)
applied for a FEPA licence for the use of this material, but MAFF MEPD had given no
indication of their view on this application at the time of preparation of this report.
9
BEACH MANAGEMENT PROGRAMME
A beach management programme has been developed for this short term experimental
scheme, in order that maintenance of the nourished beach may be carried out cost
effectively. This programme has been based upon the results of both the physical and
mathematical model studies and also on the results of the field surveys.
9.1 Threshold levels
The damage threshold condition, (defined by conditions giving rise to overtopping the
crest), is consistent along the length of the beach. The alarm cross section for the
renourished beach has been defined in terms of minimum crest elevation and minimum
crest width. This alarm value is reached when the design storm, followed in quick
succession by a 1:5 year and a 1:1 year storm, would result in failure of the beach to
existing levels. The maximum run-up levels have been defined from the extensive series
of physical model tests. The maximum run-up levels recorded by measurement of the
level of the run-up berm are fairly consistent along the beach length at a level of +3.8m
ODN. This is above the design crest height, but this natural run-up level berm will form
during storms by natural profile adjustments to wave action. It is preferable to have a
lower crest level during the earlier period of renourishment, as the beach width will be
too wide to allow frequent washing of the cliff toe and maintain the geological exposure if
the crest is very high. As the beach diminishes in size, the level of the run-up berm will
increase providing a similar degree of protection. The crest width needs to be
maintained at 3m at the design crest level for the beach to maintain an effective
performance in reducing toe erosion.
9.2 Settlement and Shingle Loss
Initially, the beach will be constructed to a significantly higher profile than required by the
design conditions. This is necessary to allow for loss of cross sectional area of the
beach by washing out of the fine fractions following placement. It is not possible to
quantify the volume of shingle lost during the life of the scheme precisely, but
considerable loss of material may occur following renourishment. An estimated loss of
50,000m3 will occur during the first 3 years. Evidence of rates of settlement, by leveling
of datum poles, suggests that in excess of 0.5m settlement may be expected during the
first year following construction. Careful monitoring of the crest levels following
construction should identify those areas that will require maintenance due to settlement.
9.3 Routine Maintenance Requirements
Routine maintenance requirements are minimal on this scheme, which is designed to
provide only 20 years of life. One of its main roles is to act as a beach management
tool, providing a feed to the beaches in the east and lessening the need for the more
expensive and complex beach management programmes in place at the eastern end of
the bay (Hurst Spit).
Reshaping of the beach may be necessary, but the experimental nature of the dynamic
groynes is intended to be monitored without interference. No maintenance is envisaged
during the first 5 years, to allow the scheme to settle and to allow the new equilibrium
rate of transport to be reached. Better estimates of long term maintenance commitments
will be made with the aid of the monitoring programme.
The scheme has a design life of 20 years, during which there will be no requirements to
recycle or top up the renourishment and maintain the dynamic beach structures. Whilst
it is difficult to assess the quantities of materials and losses form the system, estimates
have been made which suggest losses of approximately 8000m3 per year after the initial
settlement period. This programme will be revised in conjunction with the results of the
planned monitoring programme.
9.4 Monitoring Programme
The design of this scheme is innovative and it is therefore essential that the beach is
monitored frequently after construction. The beach will reduce in cross sectional area
and crest level during its life and it will eventually reach an alarm condition when it will be
necessary to renourish. It is essential to make frequent comparison between the beach
profiles, design conditions and the geometric/hydraulic framework of results developed
during model testing. A monitoring programme has been planned, to follow completion
of the works programme. The programme is outlined in the following sections. Some
elements will be implemented prior to the commencement of the works to provide
baseline data. This will allow changes resulting from the works to be identified.
9.4.1
Onshore Surveys
Land surveys of the site have been established to provide a pre scheme baseline model.
Initial surveys were carried out in 1995 and continued in conjunction with the routine
coastal monitoring programme. These surveys provide a spot height coverage on a grid
of approximately 5m. Data is stored on a PC based database and analysed using
DGM3 and FASTCAD software.
It is proposed that the whole of the beach be monitored routinely on a quarterly basis for
the duration of the monitoring programme. This will be surveyed to the same format as
previous surveys in this area. The dynamic groynes present a new challenge for
monitoring and it is the evolution of the plan shape of the beach that provides the most
useful information. Aerial surveys will be used to supplement this data.
Additional post storm surveys will be required to enhance the development of the design
methodology, which has been developed from the physical model. These surveys
should be carried out in parallel with wave measurement and tidal measurements to
provide comparison with the physical model data.
Regular sediment sampling and size grading analysis will be carried out in order to
identify changes in the grade of material and the rate of loss of the smaller fraction,
thereby providing further valuable information for the purposes of maintenance. Results
of post nourishment surveys will be compared with the database results for both the
physical model and full scale monitoring results. The beach response models may then
be calibrated, allowing adjustment of the database predictions to accommodate the
effects of different beach grading.
9.4.2
Offshore surveys
The offshore bathymetry is subject to frequent change and as a result the design wave
conditions may also change from time to time. Frequent monitoring of the offshore
bathymetry will be undertaken to identify any significant changes which may affect the
incident wave conditions. The annual hydrographic surveys currently carried out by
NFDC are appropriate for identification of nearshore changes.
A datawell omni-directional waverider buoy has been installed off Milford on Sea, and is
used in conjunction with a suitably located wind recorder to monitor wave conditions. A
telemetry link to Lymington Town Hall has been established, together with a logging
station. Contingency has been made for maintenance of this equipment during the
course of the programme. This would normally entail monthly or bi monthly checks of
the equipment and inspection of the moorings, together with changes of batteries
whenever required.
Re-analysis of the wave records will be necessary from time to time to ensure that the
design conditions have not altered significantly (due to bathymetric changes). This will
provide a much better and more reliable method of establishing the near shore wave
climate on a long term basis. The results from wave records will also be used to provide
full scale calibration/validation of the model results at the site under real conditions. This
will provide validity to the empirical framework developed by model testing and a high
degree of confidence in the performance and maintenance requirements.
The recent installation of a tide gauge by NFDC in the lee of Hurst Spit will, in the long
term, provide valuable data from which more reliable surge predictions can be calculated
for the site. The profile response database may also be used in conjunction with results
of revised surge level predictions, to give an improved prediction of the required beach
cross sections required to withstand the more severe water levels of the scheme. The
design water level may change in conjunction with medium term sea level changes.
9.4.3
Dynamic groynes
The innovative dynamic groynes may perform in a number of ways, depending on the
sequencing and frequency of storm conditions.
Their response is of particular interest during the first year and the monitoring
programme during this period will be more intensive. Surveys will be carried out
following storms with a significant wave height greater than 1.5m. Interim surveys will be
carried out on a weekly basis for the first few weeks following construction.
HR Wallingford have already carried out some development work on these structures
and a more detailed programme of field monitoring is being sought from MAFF Research
and development. If the funding is forthcoming the programme will be carried out in
connection with this project. NFDC are currently considering alternative funding of such
work and will be seeking funding for an EPSRC project to monitor the project and to
develop design techniques for dynamic groynes.
9.4.4
Database
The model data set has been stored in a series of spreadsheet databases which provide
easy access to the data, enabling the beach profile performance to be compared for a
wide range of beach geometry and wave and water level conditions. The performance
of the beach can therefore be assessed by comparison of any beach profile with a given
crest level, and cross section area above any given level, and with a range of wave and
water level conditions.
This database will enable the performance of the proposed beach renourishment to be
assessed throughout its life, by comparison with the design conditions. The wave
climate can be assessed by substituting different wave conditions into the database for
comparison with given profiles. The wide range of beach profiles in the database allows
any combination of beach geometry and wave conditions to be assessed within the
range tested. By testing over a wide range of water levels, a water level response
function can be established. The effects of changing sea level, or of revisions in the
surge water level, can therefore be taken into account by substitution of other water
levels into the database.
Initially the predicted response of the renourished beach can be established by using the
database as a look up table, by reference to the desired wave water level, wave
conditions and geometry. This method is however cumbersome to use and requires
considerable background knowledge of the database structure. The extensive database
will therefore be developed into a more sophisticated and simple to use parametric
model of the beach profile response. This will allow the beach geometry to be assessed
by substitution of geometric and hydraulic variables into a numeric model of a series of
equations. The mathematical model will be developed in conjunction with field
monitoring programmes, both pre and post construction.
10 IMPACTS OF THE PROPOSALS
10.1
Influence of the Proposals on Coastal Processes
The construction of a large scale stabilization scheme including beach renourishment
and shingle groynes will clearly have an impact on the hydrodynamic processes in the
area of the works and the adjoining area. The scheme has however been designed to
minimize changes to the existing regime and in particular to introduce much needed
sediment into the system.
The proposed groynes and beach renourishment at Naish Beach will not have a
significant effect on longshore transport to the west. The main body of the proposed
renourishment will increase the potential rate of sediment transport to the east, as the
foreshore equilibrium will be altered significantly. The profile response of the beach will
not be altered under most conditions, since the foreshore slope will not be changed.
The increased crest level will however reduce the rate of beach recession, as waves will
only reach the crest during the most extreme events. Under these conditions sediment
transport rates might be expected to increase as there will be an increased supply of
sediment to the lower foreshore.
The introduction of dynamic groynes into the system provides a solution to down drift
scour as a result of starvation. The groynes feed the down drift side of the beach as
they develop.
10.2
Environmental Impact Assessment
An EA does not form part of this application. It has not been considered necessary at
this site although the cliffs form an internationally important geological site. As the
scheme has been developed with the support of and in parallel with English Nature
these issues have already been considered fully and incorporated into the design.
Extensive consultation has taken place in addition to the statutory consultees and no
objections to the scheme have been received. Comments were received by the land
owners of the holiday village and the necessary modifications to the design have been
made to maintain the amenity value of the western end of the site. The proposals have
been welcomed at the eastern end of the site where beach access and the amenity
aspects of the frontage will be considerably improved.
10.3
Integration with the Shoreline Management Plan
Although the Shoreline Management Plan (SMP) for Poole and Christchurch Bays (cell
5f) has not yet been formally drawn up, this scheme has been designed with due
consideration to the expected outcome of the plan. Key issues which might arise from
the plan have been clearly addressed with respect to sediment transport, nature
conservation and the built environment.
The scheme design draws heavily on the results of a long term coastal monitoring
programme and a clear understanding of the regional coastal processes. As the scheme
is located at the down drift end of a very heavily defended coastline there is clear
evidence of sediment starvation at the cell boundary. The scheme provides for a beach
management plan which will form part of the SMP for Poole and Christchurch Bays and
which will integrate with the SMP for adjoining subcell 5b and 5c (the Western Solent
and Southampton Water).
11 CONTRACT DOCUMENTS
11.1
Contractual Arrangements
The shingle recharge will form a small element of a large scale dredging project which is
being funded by Associated British Ports. Due to the complex nature of the activities
involving dredging plant of a very high capital value, two separate contracts linking the
dredging and disposal operations would be extremely difficult to manage without the
significant potential for claims. Delays at either end of the job due to equipment failure
or bad weather could result in expensive counter claims between the two contracts. This
would be unacceptable to ABP and would be an unattractive prospect to NFDC. The
only acceptable option appeared to be to let a single contract comprising both the
dredging and beach recharge elements, but with separate priced bills of quantities for
the beach recharge and dredging elements. ABP agreed to this approach but have
made it clear that the tender which would provide them with the economically most
attractive package would be selected, irrespective of the rating of the bill items for the
beach recharge element of the works. This approach was discussed with the NFDC
auditors. As no commitment to take the materials has been made to ABP this method
was deemed to be acceptable. ABP have agreed that the tender sums for each of the
contractors could be viewed following the award of the Contract.
The tender that ABP have selected is the second lowest of the tender prices for this
element of the works and represents extremely good value, if compared with normal
commercial supply costs. The prices have been compared with recent tender prices for
a similar beach recharge scheme. Due to the shallow water depths offshore, a longer
pipeline is needed and considerably more time is needed to execute the same quantity
of works than the recently tendered Hurst Spit scheme. As the additional time needed to
carry out the works is considerable, the scheme costs seem reassuringly low. The
saving over commercial supply is significant. The costs to be borne by ABP are
considerable. They include for mobilization of a smaller less efficient dredger than they
would prefer to use ( as water depths are restricted only at the beach recharge site).
The dredger will have to make double the number of journeys than would a larger more
economic dredger.
The Contract will be carried out as part of the Associated British Ports (Southampton)
capital dredging contract to deepen the Southampton Water approach channel. The
specifications for the works are described within the documents as provisional priced bill
items. The specification, drawings and bill of quantities have been drawn up by NFDC,
but the Engineer to the Contract will necessarily be Associated British Ports. ABP have
agreed that a representative of NFDC should be responsible for supervision of the beach
recharge and compliance of the materials with the specification and limited powers under
the contract will be vested. The ABP contract documents and drawings form part of this
report but for convenience are bound separately. The ICE 6th edition conditions of
contract are proposed.
The bill of quantities for the beach recharge is a priced provisional bill item. ABP have
included a contract clause which limits the validity period for the beach recharge bills of
quantities. This period is 16 weeks from commencement of the contract. The contract is
expected to commence by 1 June 1996. This means that scheme approval must be
awarded prior to 13 September. If no decision is made by this date ABP will order
alternative disposal of the materials.
A legal agreement has been drawn up defining the responsibilities of the organizations.
This agreement will become operable only in the event of MAFF approvals being given
to this scheme. Execution of the contract is not practically or contractually feasible in
any other way. This will form the Contract for the works between New Forest District
Council and ABP. A copy of the draft agreement forms part of this report but is bound
separately.
12 REFERENCES
1
Hydraulics Research “Christchurch Bay – Offshore Wave Climate and Extremes”
Report EX 1934 June 1989
2
Hydraulics Research “Wind-Wave Data Collection and Analysis for Milford on
Sea” Report EX 1979 September 1989
3
Nicholls RJ “The Stability of the Shingle Beaches in the Eastern half of
Christchurch Bay” PhD Thesis Dept of Civil Engineering, University of
Southampton, 1985
4
Hague RC “UK South Coast Shingle Study, Joint Probability Assessment”, HR
Resport SR315, 1992
5
HR Wallingford “Naish Beach Coast Protection Scheme” Report EX3302, 1995
6
Barton ME, 1973, The degradation of the Barton Clay Cliffs of Hampshire
Quarterly Journal of Engineering Geology Vol 6
7
Tyehurst MF, 1991 Chewton Bunny Outfall Coast Protection Works Proof of
Evidence Section C
8
MAFF 1993, Project Appraisal Guidance Notes
9
Bray MJ, Hooke JM & Carter DJ, 1994, Tidal information; Improving the
Undersanding of Relative sea level Rise on the South Coast of England
10 Ball JH, Clark MJ, Collins MB, Gao S, Ingham A, Ulph A, 1991 “The Economic
Consequences of Sea Level Rise on the Central South Coast of England” Report
to MAFF
11 Penning Rousel EC, Green CH, ThompsonPM, Coker AM, Tunstall SM, Parker
DJ, 1992, “The Economics of Coastal Management”
12 Alluvial Mining 1994, Southampton Water Marine Geophysical/Geotechnical Site
Invetigation, Coastal Geophysics report no: 1481
13 HR Wallingford “Research into “Soft” Methods of Coast Protection” Report SR
426, 1995
14 Barton ME and Coles BE, 1984, The Characteritics and rates of the various slope
degradation processes in the Barton Clay Cliffs of Hampshire
15 Barton ME and Thompson RI, 1985, Seepage Characteristics and Landsliding of
the A3 Zone of the Barton Beds
FIGURES
APPENDICES
Appendix 1: Research and Monitoring Programme
1 Beach and Hydrographic Surveys
2 Sediment Analysis
3 Wave Climate Studies
3.1 Offshore Wave Hindcasting
3.2 Wave Refraction Modelling
3.3 Wind and Wave Recording
3.4 Design Water Level
Appendix II: Hydraulic Model Studies
1 Assessment of Existing Conditions
1.1 Shingle/Sand beach
1.2 Rock Revetment
1.3 Junction of revetment and shingle beach
1.4 Design of Beach Recharge and New Structures
1.5 Proposed beach profiles
Appendix III: Benefit Cost Tables
Appendix IV: Capital Assets At Risk
APPENDIX 1: RESEARCH AND MONITORING PROGRAMME
1
Beach and Hydrographic Surveys
An extensive programme of monitoring was commenced in 1988 when regular
topographic and hydrographic surveys of Christchurch Bay began. Routine beach
surveys are now carried out quarterly and a hydrographic survey along the same survey
lines is carried out annually. A total of 27 profiles are surveyed at approximately 200300m intervals between Milford on Sea and Highcliffe, on each survey. Additional spot
height surveys have been carried out at various locations within the study area. The
data has been stored and analysed by a PC based spreadsheet package which allows
volumetric changes to be calculated and provides a means of comparing several surveys
simultaneously. Typical results from the monitoring programme are shown in Figure 11.
The beach surveys have built up a clear picture of the response of beaches since 1988
and the predominant processes have been clearly identified by the studies.
2
Sediment Analysis
Sediment sampling and size grading analysis of the beach material has been carried out
and a comparison of the recent data has been made with earlier academic studies of the
beach sediments. The recently sampled grain size distribution on which the design of
the physical model and the beach recharge have been based are shown in Figure 10.
3
Wave Climate Studies
A wave climate study has been carried out to identify extreme wave conditions within
Christchurch Bay. The studies have been broken down into the following elements:
3.1 Offshore Wave Hindcasting
Wind records from Portland have been used to calculate extreme offshore wave spectra,
for a range of directions and return periods (Hydraulics Research 1989b).
3.2 Wave Refraction Modelling
The complex bathymetry of Christchurch Bay results in varied wave conditions at the
shoreline. Wave refraction models have been run for a range of locations offshore of
Naish and Barton beaches and design wave conditions have been established from
these studies (HR, 1995).
3.3 Wind and Wave Recording
A wave rider buoy was deployed in Christchurch Bay from 1987-1989 and an
anemometer based on the shoreline. An extreme wave analysis and a wind wave
correlation have been carried out to supplement the offshore wave hindcasting studies
(Hydraulics Research, 1989a).
3.4 Design Water Levels
The design water levels used in the assessment of the effectiveness of the proposed
renourishment and the existing beach have been derived from the following sources:
(i)
(ii)
(iii)
Field measurements during recent storms:
Previous academic research on storm surges in the area
The MAFF UK South Coast Shingle Study, Joint Probability Assessment.
Recent studies by Hague (1992) have tabled joint probabilities of waves and tides for a
number of sites along the south coast of the UK. The predictions for Christchurch Bay
have been derived from tidal records at Portsmouth. This is clearly not entirely
satisfactory with respect to the distance between the sites and the interference of the Isle
of Wight, which may well affect south westerly storm surges. The data is also intended
to provide conditions at the centre of the bay. At the ends of the bay the exposure will
change and hence so may correlations. The simple relationship derived for tidal levels
from Christchurch Bay to Portsmouth seems a little tenuous. The report also notes that
extreme data prediction is based upon a limited number of extreme positive surges. The
storms of 13-17 December 1989 are considered in the report and a surge of 0.72m is
suggested for Hurst Spit with a peak water level of 1.6m ODN. This clearly conflicts with
other local measured data. The 12 year data set interestingly shows only one event with
a static water level higher than 1.4m ODN during the whole of the period. No records
show water levels above 1.6m ODN. This compares with the local records by Nichols
(1985) and NFDC who have recorded static water levels as high as 2.3m ODN, and
water levels above 1.5m ODN on at least three occasions during that same period.
Joint probabilities of waves and tides in Christchurch Bay have been calculated by
Hague (1992). The deep water wave conditions used by Hague are based upon
hindcast data derived from an earlier study (HR 1989a) in conjunction with the Hurst Spit
project. Bearing in mind the lack of long term site specific data and the relative location
of Naish Beach to the tidal data gathering site, the predictions are clearly open to
question. The water level conditions used for the design of the beach replenishment, on
the basis of local measurements would therefore appear to represent an over design if
Hague’s data is valid. The wave data used for the design of the stabilization scheme
corresponds with Hague’s data, but the extreme water levels tested are somewhat more
severe. The measured extremes however, perhaps represent more appropriate and
more reliable design conditions. It would therefore seem prudent to adopt a design
water level of 1.87m ODN together with the 1:1 year storm, or the 1:100 year storm at
MHWS, or a combination of waves and water levels lying between these two limits. This
range of conditions should satisfy a conservation design, allowing for an appropriate
factor of safety within the design. A design scheme life of 20 years, in conjunction with a
nominal 5mm/year increase in sea level rise will increase the design water levels by
approximately 100mm during the life of the scheme. The conditions tested in the model
are therefore close to the design condition plus a factor of safety, by the end of the
scheme life.
A radio tide gauge and weather station was installed in the lee of Hurst Spit during 1990.
This instrumentation transmits data to the Council Offices at Lymington, where on line
computer analysis of the data is carried out and data is stored. As the data set
increases in size, it will provide much better statistically valid local predictions of extreme
water levels. Analysis of this information will be incorporated into the post construction
monitoring programme.
APPENDIX II: HYDRAULIC MODEL STUDIES
1
Assessment of Existing Conditons
A series of preliminary model tests were carried out to provide an assessment of the
threshold of damage for the existing beach and structures. The results of these tests are
summarized below.
1.1 Shingle/sand beach
The existing beach will not provide protection to the cliffs from a storm with a probability
of a 1:1 year return period along the whole of its length without extensive erosion.
At present the beaches at Highcliffe to provide some sediment feed to Naish and Barton
beach, although the rate of transport along the toe of the rock armour is relatively slow.
It seems likely that the beach will continue to become more and more depleted, due to
the accelerated rate of scour in front of the cliffs if the existing beach and groyne plan
shape layout at Highcliffe is maintained. In this instance the supply of beach material to
Naish beach will be reduced and the proposed replenishment will not perform optimally.
The sea defence at Barton will also become vulnerable to undermining and will become
unstable as a result.
1.2 Rock Revetment
The existing 1550m length of rock revetment at Barton on Sea is of an appropriate
geometry to protect the land on its leeward side from wave attack. The stability
performance of the rock armour is adequate under the design conditions. The structure
is however at risk from deep seated slip planes within the cliffs and would benefit from
the dynamic toe weighting that would be introduced by a shingle beach.
1.3 Junction of Revetment and Shingle Beach
The problematic interface between the rock armour and the shingle beach results in a
weak junction. The current rates of erosion at this junction are unacceptable. The rate
of transport of shingle along the toe of the revetment to the west of the junction at
Highcliffe is slower than along the open beach. There will always therefore be a natural
shortfall of material at this junction.
2
Design of the Beach Recharge and New Structures
A range of possible renourishment options were examined in the model, together with
several alternative types of rock structure to link the replenishment with the existing
defences (HR, 1995).
2.1Proposed Beach Profiles
The wave climate along the length of the beach recharge frontage is very similar. If the
renourishment is to stabilize the cliff toe it must prevent green water overtopping of the
beach crest, which results in toe erosion. The crest level of the renourished beach must
therefore be above the level of maximum run up during the design storm. Alternatively,
the crest width must be both sufficiently high and wide to allow wave events which
exceed the crest to restrict erosion.
The test programme has identified an appropriate crest level along the length of the
beach. The design renourishment profiles are shown in the drawings. The hydraulic
model tests have identified that a crest level of 3m ODN will be exceeded infrequently by
green water under any of the combinations of waves and water levels tested, but will
provide sufficient energy to occasionally wash the toe of the cliffs and so maintain clean
exposure of the important stratigraphic sequence. The model tests have identified
threshold geometry conditions beyond which the shingle beach and cliff toe will be
vulnerable under the design storm. The design profiles are based upon beach
renourishment with sediment of similar grading to that which can be produced from the
Southampton Water capital dredging site.
The proposed beach renourishment cross section has a crest level of +3m ODN, with a
crest width of 10m. The proposed crest levels for the renourishment are lower than the
maximum predicted run up levels for any of the conditions tested and occasional
washing of the cliff toe will therefore continue.
The longshore transport tests and beach mathematical models have suggested that the
longshore transport rates will be essentially the same as those occurring at present.
Appendix III: Benefit Cost Tables
Appendix IV: Capital Assets At Risk