HURST SPIT STABILISATION SCHEME
ENGINEERS REPORT
1
EXECUTIVE SUMMARY
Hurst Spit is a barrier beach which provides protection to an extensive area of low lying
land in the Western Solent. It is susceptible to breaching during severe storm events
and has declined in volume over a number of years. The decline in volume of the Spit is
due largely to the reduced shingle supply from the west resulting from coast protection
works, and to the increasing frequency of wave overtopping which results in the loss of
large volumes of material from the system. The declining volume of the Spit has
resulted in overtopping under more frequently occurring events than previously. This
trend will continue without appropriate remedial works and will result in extensive
damage to property and land in the lee of the Spit.
A long term stabilization strategy has been developed for Hurst Spit in parallel with
development of the Shoreline Management Plan for Poole and Christchurch Bays. The
proposals presented in this report will prevent the breaching of the spit under severe
storm action and will provide the standard of service necessary to protect the extensive
area of low lying land which lies in its lee (Figure 1) from damage during the 1:100 year
return period storm.
The assets to be protected include houses, Hurst Castle, an important recreational area
and sites of international conservation value.
A large scale coast protection scheme, comprising shingle renourishment, a rock
breakwater and rock revetments has been designed. A programme of maintenance,
which includes shingle recycling and maintenance of the rock structures and a
monitoring programme are planned to follow the main phase of works. The works will
extend from the Marine Café at Milford on Sea to Hurst Castle, a distance of
approximately 3km (see Figure 2).
The works will essentially be divided into two types of construction:
(a) Shingle renourishment
(b) Rock structures
The scheme costs may vary significantly according to the source of beach recharge
materials and rock.
It is anticipated that both the rock armour and the shingle will be brought to site by sea.
The shingle will be provided by an offshore dredging source and will be discharged by
pumping onto the beach. It will then be redistributed by mechanical plant on the land.
The estimated works cost of the preferred option is between £5.2 million and £7.6 million
if the aggregate is purchased from an existing licensed dredging area. However, there is
clear evidence that current licensed dredging areas are unable to provide shingle of the
appropriate grade and quality in the quantity required.
A marine aggregate production licence application has been made for the Shingles
Banks area, which can supply beach recharge material of the correct grade, quality and
quantity. If this application is successful then the estimated works cost of the preferred
option will be between £4.6 million and £6.4 million.
A third source of shingle, from inland gravel pits, has been considered but has the
disadvantages that a suitable size grading cannot be achieved, and that transport of the
material by road is environmentally unacceptable. This option is the most expensive
way of carrying out the scheme and has an estimated cost of between £7.8 million and
£8.4 million.
The average benefit cost ratio for the preferred option, using the Shingles Banks as a
source is 1.38 whilst the average benefit cost ratio for the same scheme but using
commercial licensed area is 1.25.
The average net present value for the Shingles Banks source is £2864414 and is
£2097582 for the commercial licensed area alternative.
The aggregate production licence application was made to the DOE in 1992, but
processing of the application through the government view procedure has been delayed,
this is due to the unusual step of the DOE requiring the NFDC to provide an economic
justification for the aggregate production license, by dual tendering for shingle from
various sources. This requirement has been made by the DOE in response to a request
from the MAFF. A government view on the application will not be made until the MAFF
have provided their formal comments on the application, which will then be reviewed by
the DOE.
The possibility of the award of the dredging licence, and hence the final choice of
scheme, will depend upon the coordinated response of the MAFF MEPD and flood and
coastal defence divisions to the DOE, following completion of the multiple option
tendering procedure. The tender results will be available in February 1996 and this
report should be reviewed in conjunction with the tender assessment. Benefit cost ratios
will need to be modified in parallel with the tender evaluation procedure and an
addendum to this report will reflect modifications in costs and the consequent
modifications of the benefit cost ratios.
The scheme had been designed to include a new rock revetment around the southern
face of Hurst Castle. However, at a late stage in the scheme planning English Heritage,
who manage the Castle decided to withdraw funding for this revetment, which is not now
included in the scheme proposals. Suitable modifications have been incorporated into
the design to allow the revetment to be built at a later date.
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. 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 due not only to their own erodibility, but also because they overlie
poorly cemented sands and sandy clays.
The predominance of winds from the south west results in a net longshore transport of
sediment from west to east. Hurst Spit which lies at the eastern end of Christchurch Bay
forms a major sink 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 mineral 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, which were acting like groynes,
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 on to Hurst Spit.
2.2 Hurst Spit
Hurst Spit forms the eastern boundary of Christchurch Bay and provides protection from
wave attack to an extensive area of low lying land in the Western Solent (Figure 1). The
Spit is composed largely of shingle and is approximately 3.5 km long (Figure 2). The
Spit is part of the Hurst Castle and Lymington River SSSI, is of considerable geological
interest in developing an understanding of coastal geomorphology and, despite the
extensive engineering works which have been undertaken, the beach still retains its
classic form.
The origins of the Spit data back to the time that the Western Solent and Christchurch
Bay were linked, when sea levels were approximately 20 metres lower than they are
now. The Spit could not therefore have started to form any earlier than about 7000-8000
years ago. It is the result of a series of marine transgressions which have eroded the
gravel sediments of Christchurch Bay.
The predominant easterly littoral drift in Christchurch Bay and the complex pattern of
offshore sand bank ridges has influenced the alignment and the form of Hurst Spit. The
development of Christchurch Bay has also played a significant role in the evolution of the
Spit. Construction of coastal defences at Highcliffe and Barton has resulted in a reduced
supply of sediment from the west, although the primary supply of shingle to Hurst Spit
was largely derived from the Pleistocene gravel terraces in the cliffs at Milford, which dip
below the surface at Hurst Spit. The system remained largely in balance until the
construction of the concrete sea defences and groyne systems at Milford in the 1960s,
after which erosion of the cliffs was halted and the supply of shingle was greatly
reduced. Consequently, the sediment transport system has been thrown out of balance
and the rate of erosion at Hurst Spit has increased.
It is estimated that the main body of Hurst Spit is declining in volume by approximately
7000-8000m3 per year. The littoral transport rate has been estimated at 1500030000m3 per year. Much of the material is lost from the system at Hurst Point, and is
taken offshore in the fast ebb currents and deposited on the Shingles Banks. The
material remaining in the shoreline system is transported around Hurst Castle and is
deposited on the active shingle recurve known as North Point (Figure 2).
2.3 Hurst Castle
Hurst Castle lies at the eastern end of the Spit and dates back to 1544 when the central
section of the castle was constructed. It has since been modified and extended on a
number of occasions, and now extends over a frontage of approximately 550m. The
Castle and its coast protection works are managed by English Heritage and considerable
expenditure has been made on these works since the 17th century. The Castle is a
national asset which is classified as an English Heritage scheduled monument. The
castle relies upon the presence of the spit to protect it from wave attack on its western
and northern flanks.
2.4 Keyhaven and the River
The Keyhaven River and Mount Lake lie in the sheltered lee of Hurst Spit and provide
moorings for about 600 small sailing boats, and for the small fishing fleet which operates
from the harbour at Keyhaven. The harbour and river, which also provide an important
recreational area for water sports, are leased and managed by New Forest District
Council.
The whole area is protected from south westerly gales and storm waves by Hurst Spit
and the saltmarshes, which provide an excellent natural defence. The Keyhaven River
system forms part of the Keyhaven – Lymington saltmarsh SSSI and is a designated
national nature reserve.
2.5 Keyhaven Marshes
Hurst Spit provides shelter to an extensive area of salt marsh and mudflats which lie to
the north of the spit. This area is of particular ornithological interest forming part of the
Hurst Castle to Lymington SSSI. The saltmarshes have been proposed as a Special
Protection Area under the EC Birds Directive, and as a candidate Wetland of
International Importance under the Ramsar Convention, which recognizes their particular
importance to overwintering wildfowl populations. The marshes also form part of the
South Hampshire Area of Outstanding Natural Beauty.
The marshes support Spartina Anglica which first colonized the area in 1895. By raising
the level of the saltmarsh, and thereby maintaining an artificially high crest level,
Spartina has masked the decline in volume of the shingle spit. The raised saltmarsh
trapped beneath the spit after roll back is subject to settlement and rapid erosion.
2.6 Lymington to Keyhaven Sea Defences
The land between Keyhaven and Lymington is generally low lying and is protected from
flooding by an earth embankment sea defence reveted on the seaward face, which was
raised and rebuilt following extensive flooding and damage in 1989. This sea defence,
which is protected from severe wave attack by Hurst Spit, is the responsibility of the NRA
who have redesigned it to resist severe storm surges but not wave action.
Without Hurst Spit acting as a natural breakwater across the eastern end of Christchurch
Bay the sea defence would have to be raised by a further three metres to maintain the
same standard and would require further armouring of its seaward and leeward faces.
2.7 The Western Solent and the Isle of Wight
The Western Solent is protected from severe wave action by Hurst Spit and provides an
extremely important area for recreational sailing. The spit is the closest point on the
mainland to the Isle of Wight and its presence controls the tightly confined tidal stream
which flows through Hurst Narrows. The currents are extremely fast and have resulted
in the formation of a deep scoured channel between the Isle of Wight and Hurst Spit.
The spit also provides considerable protection to the north western shores of the Isle of
Wight, from south westerly storms. The deep channel through Hurst Narrows is an
important navigational channel which is used by commercial shipping.
3
THE PROBLEM
It is estimated that the Spit is now at risk of destruction by 1:1 year return period storms,
and the loss of the protection provided by the Spit will expose and extensive length of
coastline to wave attack. This will put the existing sea defences of the NRA at
considerable risk of overtopping and breaching and will also destroy the saltmarsh in the
lee of the Spit, which is of international environmental importance. It will certainly
destabilize the Western Solent, but the effect of this is difficult to quantify. The amenity
value of the Spit will be destroyed and Hurst Castle will be subject to wave attack from
all sides.
3.1 Deterioration of the Spit
Since 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 onto the Spit in
the littoral drift has declined and, as a result, the spit has decreased in size; a process
which has accelerated markedly since the 1940s when large scale groyne construction
began at Bournemouth and Christchurch. In 1954, for the first time, the Spit suffered
breaching due to storm induced overwashing, although non damaging overtopping must
certainly have occurred before that time.
The Spit has been breached many times since 1954, notably in January 1962 when the
recently constructed timber groynes were outflanked as the spit rolled back during
storms which caused widespread flooding in Milford and Keyhaven. The increasing
frequency of storm damage and sharply rising maintenance costs since the 1970s
indicate that a threshold of stability has now been passed, and that the Spit is no longer
able to withstand even moderate storms and tidal surges without suffering severe
damage. Sediment transport calculations confirm this, and indicate that the potential
rate of loss from the Spit, between Cut Bridge and Hurst Point, is more than double the
rate of shingle movement onto the Spit.
During the winters of 1981/82, and again in 1989/90, storm induced overwashing
lowered extensive areas of the Spit. However, in the aftermath of the 1989/90 event,
tidal breaches formed for the first time through the remains of the Spit, allowing water to
flow through at all states of the tide and resulting in rapid erosion of the salt marsh in its
lee. In all cases restoration works, which were put in hand before the storms had
ceased, were successful in preventing further damage.
As a consequence of its declining volume, Hurst Spit has become subject to frequent
over washing under storm action. The Spit is a transgressive feature, moving landwards
due to the processes of overtopping and overwashing. The rate of transgression had
increased from approximately 1.5m per year (1867 – 1968) to 3.5 m per year (1968 –
1982). Since then, the spit has been subject to frequent overwashing and crest lowering
during storm action. Extensive throat and over wash fan systems have formed and the
volume of the bank above low water has declined further due to the displacement of the
shingle into the Mount Lake river channel, in the lee of the Spit.
Several recent severe storms have resulted in extensive damage to Hurst Spit. In
particular the storms of October and December 1989 caused dramatic crest lowering
and roll back across the salt marshes, and outflanking of the rock armouring. Crest
lowering in excess of 2.5 metres, roll back of the seaward toe by up to 60 metres and roll
back of the lee toe by up to 80 metres resulted in displacement of more than 100,000
tonnes of shingle overnight (Figure 3).
Maintenance work has been carried out on Hurst Spit by New Forest District Council
(NFDC) and its predecessor authorities since the early 1960s. Following events such as
those discussed above, sections of the Spit have been artificially reformed using
imported beach material. A number of attempts have been made in the past to stabilize
the western end of the Spit by armouring with rock, and by the construction of a groyne
at the junction of the rock armour and the shingle beach. These schemes have not been
entirely successful, serving only to transfer the problem further to the east. Costs of
maintenance have increased dramatically, particularly since 1980. The reinstatement of
the Spit following the 1989 storms cost approximately £450,000.
Physical model testing has shown that the Spit in its present form is very sensitive to
water level, and that a south westerly storm with a return period of once a year,
combined with a tidal surge of 0.5 metres above mean high water springs, would
generate the same degree of damage as the 1989/90 event. A sensitivity analysis
carried out by Hydraulics Research at the time of the scheme design (1990-91),
indicates that this combination of storm return period and tidal surge level is, on average,
likely to occur once every five years, which corresponds well with records of major
damage to the Spit. The declining volume of the spit since this assessment was made
has resulted in a reduction in the return period of the existing defences to 1:1 year .
3.2 Stability of the Spit
The main problems affecting the stability of the Spit are:
(i)
Sediment transport – more shingle is being lost off Hurst Point than is being
supplied from the Christchurch Bay beaches.
(ii)
Damage from overwashing – as the Spit shrinks in size, overwashing occurs
more frequently causing greater damage and accelerating the rate of roll
back or landward transgression
(iii)
Weak points – earlier attempts at stabilization caused wave energy to be
focused at the rock armour/shingle junction near Cut Bridge, leading to
frequent breaching.
(iv)
Rock armour stability – the existing rock armouring between Milford Beach
and Cut Bridge has a single layer rock armoured seaward face which is too
steep to provide good long term stability
(v)
The return period of a storm event for breaching of Hurst Spit is 1:1 year.
3.3 Effects of the without project option
If the stability problems of the Spit are ignored, and no further maintenance nor any form
of long term stabilization is undertaken, (the “do nothing” option) Hurst Spit will be lost.
This will have the following effects (see figure 4):
3.3.1
Recreational use of the Spit
Approximately 800 metres of the shingle spit between Cut Bridge and Hurst Castle, will
be permanently lost preventing recreational walking, angling, bathing and access to the
castle on foot.
3.3.2
Hurst Castle
Hurst Castle will, over a period of five years, become vulnerable to wave attack along its
north and north western flanks. Eventually it will become isolated as an island and will
require the construction of substantial protection works all the way round it in order to
guarantee its long term existence.
3.3.3
The saltmarsh and Keyhaven River
In the event of a breach of the Spit the saltmarsh will erode rapidly, due to direct
exposure to wave activity. Moorings in Mount Lake would be lost and Keyhaven
Harbour would eventually become vulnerable to wave attack. The rate of erosion
predicted in this area is shown in Figure 4.
3.3.4
NRA Sea defences
With the loss of the Spit and the saltmarshes the sea defences between Hurst Spit and
Lymington will suffer increased wave action, and overtopping of the wall will occur. The
wall will become subject to conditions which it has not been designed to withstand and it
will breach very soon after exposure. Flood levels for the sea defence design conditions
are shown in Figures 8-11.
3.3.5
The Western Solent and the Isle of Wight
In the absence of Hurst Spit, much of the extensive area of low lying land will become
subject to severe wave attack and hence rapid erosion. The north western shores of the
Isle of Wight will also become subject to severe wave action and accelerated erosion.
Whilst it is impossible to determine the effects on the tidal regime of the Western Solent,
there is no doubt that there will be some significant changes, due primarily to the
widening of the entrance channel. This may have serious implications with respect to
navigation of commercial shipping. Those areas that will eventually become vulnerable
are shown in Figure 7.
4
ALTERNATIVE SCHEMES
4.1 Design Criteria
Whereas most beaches form part of an integrated defence system, which include solid
features such as sea defences or cliffs, Hurst Spit is a barrier beach acting as the sole
line of defence to the land in its lee. Hurst Spit is a dynamic structure which can suffer
catastrophic failure, leading to extensive damage in a single storm event. Unlike many
restrained beaches, the changes effected by storm action are irreversible processes and
leave permanent changes and weaknesses within the defence. The beach performance
is extremely sensitive to small changes in geometry and wave and water level
conditions. Probabilistic risk analysis and management of such features is therefore
very complex. The fault tree shown in Figure 12 includes the various failure models
which are included in the risk assessment and indicates the complexity of the
performance of the beach.
Probabilistic risk assessment procedures have been applied to a range of management
scenarios for Hurst Spit. The fault tree model has been used as a basis for assessment
for potential beach 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 may provide
alternatives to beach management are far easier to assess in terms of stability, and
preliminary designs have been based upon the extreme conditions derived from the
wave climate and water level studies.
The alternative management strategies have been assessed with respect to the
following criteria:
a)
b)
c)
d)
ability to solve the potential problems identified in Section 3.3
acceptable environmental impact
technical soundness
reasonable cost
Alternative schemes which could not provide technically sound solutions have not been
fully costed and included with cost benefit analyses. Those which are environmentally
unacceptable but which might be fully or partially functional have been included in the
economic analysis.
4.2 Do nothing
Doing nothing will cause the potential problems identified in section 3.3, and accepts that
the Spit will be destroyed at some time. 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 important environmental site would result, as opposed to the
development of an environmentally beneficial habitat.
Managed setback would not increase the intertidal habitat and extensive areas of
important intertidal areas would be lost due to the increasing incidence of severe wave
activity. The area which the Spit currently protects and which would be lost almost
immediately if managed setback were to be introduced is an SSSI; a candidate
RAMSAR site, and is an internationally important nature reserve.
The introduction of managed setback would certainly not create a more stable and
sustainable coastal or flood defence, as recently constructed flood defences in the lee of
the Spit would soon become subject to increased wave activity in exceedence of the
design conditions.
Any attempt to artificially realign this section of coast would have far reaching effects
within the Western Solent and Hurst Castle would immediately become vulnerable to
wave attack around its whole perimeter.
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 Maintenance to a declining standard
The “do minimum” option is based on maintaining Hurst Spit with existing NFDC
resources to a declining standard and topping up from time to time.
Until recently the management policy at Hurst Spit has been to maintain the Spit to a
declining standard, using revenue finance from the coastal maintenance budget.
Approximately 4000m3 per year of land won material is placed to fill holes formed by
storm wave action. The net loss of volume from the Spit is about 8000m3 per year,
leaving a short fall of 4000m3 per year, hence the declining standard of protection
offered. These works are not able to restore the spit to the pre-storm standard which
has been declining gradually . The declining standard of protection is also due in part to
the high losses from the system due to rollback into the Mount Lake river channel, which
lies in the lee of the Spit, during severe storm events. Rollback of beach material into
this channel results in a reduction of beach material in the active storm protection zone
of the beach. There have been five series of emergency works since 1987 and the
incidence of severe storm damage is becoming increasingly frequent due to the
declining standard of the Spit. Recent severe storm damage has occurred under
conditions which have a higher frequency of return period than the 1:1 year return period
storm. Maintenance to a declining standard is therefore an inefficient method of
maintenance and will result in economic, recreational and conservation losses.
Continued maintenance to a declining standard will result in avertion of some of the
potential losses, but only on a short term basis. The recreational value of the Spit will be
retained for a period of no more than 4-5 years longer than the do nothing scenario.
Similarly, flood damage will be averted for a similar period, but will thereafter follow the
same pattern as the do nothing scenario. The value of Hurst Castle will also be lost.
This scheme does not meet the technical requirements required to protect against the
problems discussed in Section 3.3. The environmental impact of this option is not
satisfactory since large areas of salt marsh will continue to be lost however, this option
has been carried forward for financial assessment in the benefit cost analysis section.
4.5 Low level of investment – Upgrading rock structures and maintenance of
beach with emergency works
This option is based upon upgrading of the rock structures, maintaining the Spit to a
higher standard with MAFF grant aid and topping up with emergency works at
appropriate intervals.
Upgrading of the beach management system with appropriately designed rock structures
will protect the most vulnerable areas at the western end of the Spit. These will be to the
same standards as for the other options, which include rock structures capable of
withstanding a return period storm of 1:100 years.
The beach management element of the works requires 8000m3 shingle per year, to
maintain the annual shortfall in the sediment budget needed to sustain the same volume
of the Spit, and approximately 20,000m3 shingle for each of the emergency works
operations which would be needed every 2-3 years. This design is based upon results
of the physical model studies of the performance of Hurst Spit. A series of design cross
sections have been determined which will withstand the 1:1 year return period storm
along the full length of the Spit. The cross section geometry varies fro west to east due
to the varied wave climate along the length of the Spit.
This option does not allow a coherent management strategy to be developed for the
shingle spit. The Spit must be maintained to a high standard at the junction of the rock
revetment with the root of the Spit, in order to maintain the existing line of defence. The
introduction of a rock breakwater near to this junction will assist with this. Overtopping
must be limited below the overwashing threshold along the full length of the Spit. The
critical freeboard threshold parameter suggested in the parametric model for prediction
of the profile response of shingle barrier beaches (Ref 9) must not be exceeded, due to
the volatile natures of the response of barrier beaches to overtopping. This cannot be
achieved with certainty by this strategy.
The current short term interim beach management strategy is similar to this option;
emergency works which are capable of withstanding the 1:1 year return period storm are
carried out in combination with provision of appropriate reserves of shingle at the up drift
end of the Spit, to supply the equivalent of the annual net sediment transport budget
deficit. The emergency works typically require approximately 20,000m3 of material
during each operation. Given the severity of wave attack at this site, this needs to be
carried out as an emergency works operation every 2-3 years. During the interim
periods additional materials are currently placed from revenue maintenance budgets and
typically 3000-8000m3 of material are placed each year to provide the appropriate short
fall in the long shore feed element of the defence. Typical costs for this type of operation
are £440,000 for the emergency works, and £80,000 for the annual maintenance. The
current strategy also includes maintenance of the rock structures which are nearing the
end of their life.
Continued adoption of this alternative is high risk and many combinations of storm
sequences may result in breaching of the Spit. The following example scenarios, which
have a high probability of occurrence, demonstrate the high risk of this strategy:
a) Sequences of 3-4 storm events of low intensity (with a return period of about
three months) may occur in rapid succession. These would result in a breach of
the Spit due to reduction of the beach width.
b) A storm surge of 0.8m on top of mean high water coupled with a storm of return
period of 6 months would result in overtopping and breaching.
c) The 1:5 year return period storm has a probability of exceedence of 0.18% during
a single year. If the 1:1 year standard alarm profile is maintained this will
certainly be breached under such conditions.
The current strategy of maintaining to a higher standard with MAFF grant aid and
topping up with emergency works at longer intervals is very costly in terms of providing
and placing materials. It is also difficult to plan for and poses a considerable drain on
staff resources during emergency work periods.
Materials for both revenue maintenance and emergency works are usually supplied from
inland gravel workings. The costs of materials for such operations are approximately
double those of dredged material. The volumes of material placed (typically 15,000 –
20,000 m3 of shingle at a time) do not however warrant the mobilization of aggregate
dredgers. Mobilisation of dredging plant and discharge pipelines would typically cost
something of the order of £250,000 prior to placement of any materials. Alternative
methods of discharge, such as bottom dumping over high water periods with later
reclaim using land based plant, are neither practicable nor desirable. The narrow tidal
range makes retrieval of dumped beach recharge material very difficult and the soft
substrate of the nearshore bed, which is relict mudflats, is easily damaged by such
operations. Whilst dredged material is cheaper, rapid response to an emergency
situation cannot be reliably achieved on projects with a short lead in time. A minimum
lead in time of 6-8 weeks is normally required for mobilization of piped discharge
dredging operations. There are of course further complications which increase the time
scale for works to commence, such as negotiations with local fisheries offices and the
MEPD on transshipment of materials, and also the availability of appropriate dredging
plant. On the basis of these arguments, a dredging based operation is clearly not viable
for emergency works situations and the alternative high cost land based solution is the
only practicable solution under the circumstances.
This type of crisis management is also fraught with other practical difficulties which
include availability of appropriate grades and quantities of land won materials.
Experience has demonstrated that only about 2000-3000m3 of material of good quality
shingle is available from local gravel workings on each occasion. Alternative supplies
have to be sought from wide spread sources, at inflated rates, to ensure delivery of
appropriately graded materials. Increased construction costs often have to be borne
during the emergency works due to downtime at the site during stormy periods. Suitable
plant is often not available at short notice and less efficient but more costly plant is often
used to carry out the emergency works.
The effectiveness of this “low level of investment” beach management option is very
difficult to assess in a high risk situation. It does not provide a suitable level of
protection. Analysis of the many possible scenarios demonstrated in Figure 12 indicates
the complexity of the responses, and shows how rapid development of failure situations
can occur as a result of various sequences of typical but relatively low energy storms.
Because of the high risk of failure this alternative is technically unacceptable and it does
not meet all the technical requirements to protect against the problems discussed in
Section 3.3. It does provide considerable additional protection however and will protect
against most of the economic losses. The main losses which would not be averted by
this scheme are environmental loss of saltmarsh during storms, recreational losses due
to frequent enforced closures of the Spit, and loss of some moorings due to the high risk
of damage. Despite these problems this option has been carried forward to the
economic assessment.
4.6 A Concrete Sea Defence from Milford to Hurst Castle
This would provide the protection required instead of the shingle spit. However, it would:
(i)
pose considerable technical problems with respect to founding the wall on the
saltmarsh
(ii)
cause scour and loss of beach material
(iii)
not be in keeping with the existing environment
This scheme is not acceptable on environmental grounds. All of the local and national
conservation agencies would strongly resist any proposal to construct a sea defence.
This alternative has been discarded on the basis of environmental grounds, technical
construction difficulties and it would also be very expensive to construct. This scheme
has not been considered further in the detailed economic assessment.
4.7 Extension of the Rock Revetment from Saltgrass Lane to Hurst Castle
The rock revetment from Milford to Cut Bridge has been fairly successful in resisting
wave attack and could be a practical and functional solution to the problem. However, it
would :
(i)
(ii)
(iii)
and
(iv)
not utilize the existing shingle beach to full effect
be subject to a high risk of foundation failure and large scale subsidence, and
therefore reduced armour interlock, due to the saltmarsh foundations;
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 the western end of the Spit, would strongly be
resisted by English Nature between the present root of Hurst Spit and Hurst Castle, due
to the drastic change from the barrier beach and saltmarsh environment, which provides
both the morphological and ecological importance of the nature reserve and SSSI.
Although this scheme could provide a functional solution to the problem at a high level of
investment, there are some technical doubts, 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 Hurst Spit, they have a
very high risk of becoming outflanked due to the roll back processes which result in crest
lowering and landward shifting of the crest. They would, therefore be inappropriate and
non functional at this site. Previous experiences of the failure of the groyne system at
both Milford on Sea in the 1960s and at Hurst Castle has demonstrated that these
structures are inadequate (Refs 3,5). In any case, the construction of groynes alone
would not prevent overwashing and accelerating roll back which are due to the
diminishing size of the Spit. This option has been discarded as it does not provide a
functional option and is technically unsound.
4.9 Offshore Breakwaters
Offshore breakwaters provide effective protection to beaches at many sites and could be
designed to be effective offshore from Hurst Spit. However, construction would not be
practicable at this site as the structures would have to be founded on the steeply
shelving foreshore which runs into the North Channel. If the breakwaters were to be
functional they would need to be located in approximately 8 metres of water near to the
centre of the North Channel. Since the costs of rock breakwaters are approximately
proportional to the square height of the structure, this option would cost far more than
any alternative option. As this channel is heavily used by small craft, the construction of
rock breakwaters in it would introduce a considerable hazard to safe navigation and is
therefore not environmentally acceptable. This option would cost several orders of
magnitute more than the alternative schemes, and has not therefore been considered in
the economic assessment.
4.10
Shingle Renourishment
Shingle renourishment has the advantage that the energy absorption characteristics of
the system will not change significantly. The onshore – offshore interchange of
sediments will not change significantly, provided that the near shore beach slope and
bathymetry is unchanged. The beach will therefore 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.
However, beach recharge has the following disadvantages:
(i)
(ii)
(iii)
limited life
high maintenance commitment
a complex design procedure is needed to ensure that wave overtopping and
crest roll back can be controlled
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 option has been carried forward to the economic assessment.
5
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 risk and indicates that damage will occur following a 1:1
year storm. Some of the economic losses are assumed to commence therefore, in year
1 of the economic analysis (see Appendix III).
5.1 Assumptions
The physical model testing carried out in 1990 indicated that the Spit, in its repaired and
rebuilt form following the storm damage of 1989/90, would be severely breached again
by a combination of wave height and tidal surge height with a statistical probability of
recurring once in five years. This proved to be accurate with breaching occurring in
winter of 1994. Following this storm season, emergency works were carried out to
ensure that the Spit could just withstand a 1:1 year return period storm, but would suffer
serious damage in the 1:2 year return period storm. For the purpose of economic
assessment it is assumed that serious breaching would next occur in 1996, or year one
of the assessment.
When breaching occurs shingle is lost offshore from the seaward face of the Spit, and
pushed back across the adjoining saltmarsh on the landward side. As the Spit is only
about 20 to 40 metres from Mount Lake another storm event like the one in 1989, when
the Spit rolled back by 40 to 80 metres, would result in it being pushed back into the river
channel. Without immediate restoration works the remains of the Spit and the adjoining
saltmarsh would be exposed to direct wave attack and rapid erosion. This situation was
predicted by the model tests, and confirmed by the post storm events in 1989, when
normal tidal movements were carving the remains of the Spit into a series of small
eroding shingle “humps” with strong tidal flows between and which were rapidly eroding
away the salt marsh edge, prior to emergency works.
The erosion scenario described in Figure 4 demonstrates the initial losses due to erosion
during the 20 year period following a breach. The total area of land protected by the Spit
is shown in Figure 7 and the distribution of land use within this area is shown in various
categories in Figure 8-11. The categories used are: agricultural, residential, commercial
and amenity.
The number of domestic properties, commercial properties and area of agricultural land
protected is discussed in section 6.7.
5.2 Assessment of Economic Benefits
Middlesex University were commissioned to carry out an assessment of the economic
benefits of stabilizing Hurst Spit in 1991. They identified five potential categories of
benefit, as follows:
(i)
(ii)
(iii)
(iv)
(v)
Recreation
Mooring of commercial and leisure craft
Sea defence (Lymington to Keyhaven sea defence)
Hurst Castle
Environmental
The financial benefits quoted reflect the Middlesex figures 1991 which have been
updated to 1995 price base using RPI. Figures in brackets indicate the 1991 values.
The greatest contribution to the total benefit is given by the recreational losses which
would result from the loss of the Spit. The average loss per visitor was determined
through a Contingent Valuation survey as being £4.15 (£3.72) per visit. The annual loss
then depends upon the number of visitors using the Spit.
What is particularly notable is that virtually all respondents reported that if the Spit were
to be lost, then they would stop visiting the site and would visit elsewhere instead.
Estimates of usage of the Spit varied between 60,000 – 100,000 visitors per year. From
daily counts undertaken at all access points to the Spit during the Contingent Valuation
survey, and by reference to the patterns of visiting over the year at other sites, the likely
number of visitors to the Spit was estimated to be in the range of 87,000 to 162,000.
To estimate the annual recreational benefits, the conservative figure of 87,000 visits was
used.
There was a potential risk of double counting between these recreational benefits and
those resulting from continuing to protect Hurst Castle. Consequently, the £4.45 million
(£4.0 million) value of protecting the Castle has been attributed to non use value as a
scheduled monument which is classed as a national asset.
There are some additional unquantified benefits. Of these, the main potential loss is to
recreational sailing as a result of worsening of the wave climate in the Western Solent.
There are no statistics available however, about the total number and frequency of use
by dinghies and yachts of this area. There would also be a relatively small loss through
the effects on the small fishing fleet at Keyhaven. A major and, in the current state of
economic science, unquantifiable loss would be the negative effect on the Sites of
Special Scientific Interest currently protected from erosion by the Spit.
A full explanation of Middlesex University’s work can be found in the accompanying
document entitled “Hurst Spit – An assessment of the Benefits of Coast Protection”. It
should, however, be noted that Middlesex University derived their net present value of
£11.36 million (£10.2 million) on the assumption that all losses would occur as soon as
the Spit was breached. This is clearly not going to be the case, and the benefit cost
analysis has been based on the more gradual timetable of losses detailed in below. The
benefits accruing from the breaching of the Lymington to Keyhaven sea defence due to
flooding have been reassessed to accord with accepted benefit criteria. These are
discussed in Section 6.7.
5.3 Recreation
Recreational use of the Spit is concentrated on the shingle section between Cut Bridge
and the Castle. The rock armoured section between the footbridge and Milford beach is
difficult to walk on and much less popular with visitors.
The immediate result of breaching of the shingle section would be the loss of use of
most of the Spit for recreational purposes. About 800 metres of the Spit between Cut
Bridge and Hurst Castle would be permanently lost. If restoration work was not
undertaken the remains of the Spit between the footbridge and Castle would be eroded
away and the recreational benefits completely lost.
Middlesex University’s survey indicates that about 87,000 people visit the Spit each year,
and that the recreational value of each visit is £4.15 (£3.72). The total loss of
recreational benefit in a full year if the Spit was permanently breached is therefore
£360,731 (£323,640), and this figure has been used in the benefit cost analysis. A
reduced figure of £180,365 ( £161,820) has been assumed for years one to three, as not
all recreational benefit would be lost immediately.
The large scale shingle recharge options would avoid this loss and the full recreational
value of the benefits would be maintained.
The rock revetment alternative would result in a significant reduction in the recreational
value of the area and an estimated 50% loss of recreational benefit has been assumed if
this option is adopted, due to the lower use by visitors. This reflects the significantly
lower use of the existing rock armoured section of the Spit. The current users, which
include anglers, bathers and walker, would all have much more limited use of the site.
The maintenance to a declining standard option would provide only limited recreational
benefits, by increasing the recreational life of the Spit by no more than five years. The
recreational benefits are curtailed after this period within the benefit cost analysis for this
option.
The low level of investment option would also result in a reduction in the recreational
benefits, due to the high frequency of storm damage and the frequent need for closure of
the Spit during emergency works programmes. An estimated reduction in the benefit
figures of 20% due to lower potential use has been made for this option.
5.4 Hurst Castle
Permanent breaching of the Spit would prevent access by land to the Castle although,
except for the winter months, visitors could reach the Castle by ferry from Keyhaven.
However, within five years of breaching, accelerating erosion is likely to open up a
channel between the Spit and the main Keyhaven River. This would limit the ferry
operations to calm weather periods in spring and autumn. Further erosion would expose
the ferry to increased wave attack and further limit its operating periods.
Loss of the Spit and erosion of the saltmarsh would effectively create an island around
Hurst Castle. Without a supply of shingle the Castle beaches would suffer accelerating
erosion bringing the south west edge of the “island” to the Castle’s timber defence works
by year seven. Further erosion would threaten the foundation of the Castle walls and
extensive protection works would be required around the west and south west side of
the “island” to preserve the Castle and to retain the relict recurve to the north west which
would provide shelter and protection from westerly gales. Further works would be
needed to extend the protection around the south wall and the north west shore of the
island by year nine. The cost of these additional works has been estimated at £4million.
It is the view of English Heritage that such expenditure would be justified to ensure its
continuing existence because of the status of the Castle as a scheduled ancient
monument.
Although English Heritage do not wish to proceed with the Castle revetment at the
moment, the existing defences only have a limited remaining life. However, as the cost
of the work will be borne by English Heritage and will not be eligible for grant aid under
the Coast Protection Act, it has not been included in the benefit cost assessment.
Either the rock revetment option, major beach recharge options or the low level of
investment option would ensure the preservation of the Castle in its current state. The
maintenance to a declining standard would however result in loss of the Castle on the
same timescale as the do nothing option.
5.5 Saltmarsh
The saltmarshes would erode rapidly if the Spit is breached, due to the exposure of the
soft sediments to severe wave attack. The projected erosion rates for the without project
option are shown in Figure 4.
Loss of the saltmarsh, which is a Site of Special Scientific Interest, a National Nature
Reserve, and of international importance for migrating and nesting birds, is
unquantifiable in economic terms, but would be disastrous in ecological terms. Although
no attempt has been made to value the salt marsh or include it in the benefit cost
assessment, protection of the marshes is one of the major reasons for seeking to
stabilize the Spit. A fuller description of the importance of the marshes and the impact
on them of the Spit breaching is given in Chapter 10 of the Environmental Impact
Assessment.
Either the rock revetment or any of the large scale beach recharge projects would
provide the necessary protection to the salt marsh areas. The maintenance to a
declining standard option would result in a delayed loss of marshes, but would still
eventually result in the same losses as the without project option. The low level of
protection option would provide protection to most of the salt marsh area, but some
areas would be lost following severe storm events (typically those with a longer return
period than the 1:5 year storm).
5.6 Keyhaven and the River
Loss of the saltmarsh will inevitably lead to loss of the Keyhaven River system, which
includes Keyhaven Harbour, Mount Lake and Hawkers Lake. 609 commercial and
leisure craft are currently moored in this river system. Middlesex University’s figure of
500 craft is therefore an underestimate.
Keyhaven River and Harbour provides one of the very few non marina mooring areas in
the Western Solent, and is therefore an important recreational and commercial resource.
Because of the fragile nature of the inter tidal areas in the Western Solent and
Southampton Water planning policies at both district and county level seek to prevent
further large scale provision of moorings and marina construction along the shoreline .
In any case continuing erosion by the sea is reducing the areas of sheltered water,
provided by inter tidal marshes and mudflats, which have traditionally been used for non
marina moorings. In these circumstances moorings lost from Keyhaven River and
Harbour could not be replaced elsewhere on the New Forest coast and are therefore an
economic loss to the area.
Following the 1989 storm, the Spit was restored to a line about 20 metres back from its
original position, over approximately 200 metres around the “elbow” leaving 10-15
metres of saltmarsh exposed on the seaward side. Regular monitoring of the profile of
the Spit following emergency repairs showed that this relict saltmarsh was eroded away
completely in 4 months, between March and June 1990. Flume tests of mud samples
carried out at Southampton University confirm this rate of erosion. From this it has been
estimated that, under direct wave attack, the saltmarsh would erode at about 30-45
metres per year.
In calculating the loss of moorings due to saltmarsh erosion the lower erosion rate of 30
metres per year has been used; indicating that the whole Keyhaven River system would
be lost in about 20 years. Predicted erosion lines are shown in Figure 4. Table 6.1
below summarises the number of moorings lost each year over this period. The year
numbers match those in the benefit cost analysis and the values are based on lost
revenue of an average £167.2 (£150) per mooring per year, derived from the Middlesex
document. Physical model testing showed that if no restoration work was carried out
after breaching between points B and D (Figure 4), the standing remains of the Spit
between points A and B would be washed away in 3 years. This result was repeated
over 20 times in wave/surge sensitivity testing, and the predicted erosion lines, therefore
reflect this timescale.
On the seaward face of the Spit the protection by the offshore banks varies, being more
exposed between points C and D than elsewhere. Wave heights are therefore greater in
this area resulting in higher rates of erosion. Loss of material over the winter period is
always greater along this length, and roll back more severe when breaching occurs.
The predicted erosion lines reflect the higher erosion rate in this area.
Either the rock revetment scheme or the large scale beach recharge options would
ensure the safety of the moorings and would avert all of the potential mooring losses.
Maintenance to a declining standard would provide a short increase in the life of the
moorings and thus some income losses would be averted for 10 years. Beyond this
point however, the moorings would be lost at the same rate as the without project option.
The low level of investment scheme would avert most of the losses in income from the
moorings but the Mount Lake channel which runs parallel with the Spit would still remain
vulnerable to severe storm events and a total of 40 moorings would be lost during the
scheme life.
The differences in the number of moorings lost for the various options and the
consequent benefits used for the benefit cost analysis are shown in Table 6.1.
Table 6.1 Loss of Moorings (1995 price base)
Without
Project
No of
moorings loss of
lost
revenue
(£)
2
20
3344
3
25
4180
4
40
6688
5
65
10868
6
70
11704
7
80
13376
8
90
15048
9
100
16720
10
105
17556
11
115
19228
12
125
20900
13
130
21736
14
135
22572
15
145
24244
16
165
27588
17
590
98648
18
600
100320
19
609
101825
20-50 609
101825
Year
No
Maintain
to
Declining
Standard
No of
moorings
lost
10
20
25
40
65
70
80
90
100
115
125
130
135
145
165
590
600
609
609
Low level
of investment
Revenue
benefit
(£)
1672
836
2508
4179
835
1671
1671
1671
835
0
No of
moorings
lost
0
5
15
25
30
40
40
40
40
40
40
40
40
40
40
40
40
40
40
revenue
benefit
(£)
3344
3344
4180
6687
6687
6687
8359
10031
10867
12539
14211
15047
15883
17555
20899
91955
93627
95131
95131
5.7 Sea Defences
The Spit protects the saltmarsh from direct wave attack, and both of them provide
protection for the Lymington to Keyhaven sea defences. These have recently been
rebuilt by the National Rivers Authority (NRA) , as a result of severe damage inflicted
by the 1989/90 storms. The NRA (Southern) has confirmed that the new sea
defence has not been designed to withstand the direct wave attack and increased
water depths which would result from loss of the Spit and saltmarsh in the event of
adoption of the without project option. The defence is an earth embankment
protected on the seaward slope by a concrete block revetment.
Without the protection provided by the Spit and saltmarsh (the no project option),
approximately 6km of sea defences would eventually become vulnerable to
increased wave attack, susceptible to breaching and would ultimately result in
serious flood and erosion damage to property currently protected by the sea
defence. Those areas which would ultimately become at risk are identified on Figure
7 and shown in detail in Figures 8-11.
The benefit cost analysis, design procedures and protection standards adopted by
the NRA (Reference 10) have formed the basis for the assessment of the damages
averted by the sea defences. The performance of the newly constructed sea
defences has been analysed in conjunction with breaching and erosion scenarios for
Hurst Spit. A complex procedure has been followed to determine the benefits which
can be achieved by carrying out any of the alternative options, (with the exception of
maintenance to a declining standard which would have only minimal benefit for a
limited period of no more than five years). This procedure is outlined below:
(a) Determine the annual incremental change to wave heights, periods and
water depths at a range of locations along the sea defence, which result
from breaching of Hurst Spit. The predicted marsh erosion scenario
(Figure 4) is used to determine changes in incremental exposure
(b) Determine the overtopping discharges that would result from the
incremental changes in exposure, derived in (a). These are calculated for
the full range of design conditions and protection standards provided by
the recently constructed sea defence (see Appendix IV) and tabled in the
NRA Engineers report
(c) Determine the frequency of occurrence of overtopping discharges that
would result in failure of the sea defence by breaching. Standard design
criteria for paved embankment walls are used to assess damage and to
predict the timing of sea defence failure (see Appendix IV)
(d) Determine the incremental increases in damage to land and property,
caused both by overtopping discharge and by flood inundation, due to
breaching of the sea defence
(e) Assess the values and frequency of damage to land and properties at risk
from flooding and or erosion (see Appendix IV). Baseline losses reflect
FLARE 1990 values where appropriate and 1988 Update Manual values
for other land use values not covered by FLARE. These values have
subsequently been updated to 1995 base prices using RPI.
Figure 4 demonstrates the likely erosion scenario in the event of a breach of Hurst Spit
and this has been used as the basis for recalculation of the incident wave conditions at
the sea defence. Allowances for the wave energy dissipation due to depth limitation and
bed friction have been made, although it is likely that the soft substrate will rapidly erode
to provide increased water depths and therefore potentially more severe depth limited
breaking wave conditions. This has been demonstrated previously by the rapid removal
of saltmarsh when exposed on the seaward side of Hurst Spit.
Although a breach of Hurst Spit would allow wave propagation across the saltmarsh
surfaces over high water periods, there is not likely to be sufficient energy in the waves
to cause overtopping of the sea defence until the marshes have eroded to within 10-20m
of the sea defence. Even under these circumstances green water overtopping is likely to
occur only under the most extreme water levels, due to the limiting water depths at the
toe of the wall.
The sea defences between Saltgrass Lane footbridge and Keyhaven are constructed to
a mean level of 2.4m ODN and are designed to withstand a maximum overtopping
discharge of 0.05m3/m/s. Design standards suggested by Owen (Ref 11) Goda (Ref 12)
and Fukuda et al (Ref 13) suggest that a sea defence with a paved crest and rear slope
is capable of withstanding overtopping discharge of 0.05m3/m/s without damage. A
structure with a paved crest and unprotected rear slope can withstand discharges of
0.02m3/m/s whilst a sea defence with a grassed crest and rear slope can withstand only
0.005m3/m/s without damage.
Calculations of changes in local wave heights, resulting from increased exposure and
increased water depths indicate that conditions which are likely to cause a breach in the
embankment type sea defence will occur very soon after the marsh area is lost from the
sea defence (due to failure of the lee face and subsequent collapse of the sea defence).
Overtopping calculations indicate that failure of the sea defence is likely, even under
very frequently occurring conditions (see Appendix IV). A range of possible scenarios
for wave overtopping are shown in Appendix IV, for the conditions at the sea defence
following breaching. These demonstrate the severe conditions that will occur on a
regular basis. The NRA sea defence design allows for a freeboard of only 0.3m at the
design water level between Saltgrass Lane and Keyhaven, but severe overtopping is
likely to occur at much lower water levels in the event of a breach of Hurst Spit. Whilst
inshore breaking wave heights of only 1 metre may occur when the wall is first exposed,
due to depth limitation the wave periods will be largely unaffected by depth. The
conditions generated by long period (7-10 second) storm waves from the south and
south west are likely to cause severe damage, due to the high volumes of water within
the waves; such waves have much longer periods than those currently generated within
the Western Solent, which are limited to about 4 seconds. As water depths increase due
to erosion of the soft substrate adjacent to the sea defence, wave heights may increase
further, although the limiting breaking wave height at the wall is unlikely to exceed 1.6m.
Sensitivity testing of conditions which would give rise to a breach of the sea defence
indicate that, once the sea defence if exposed to direct wave attack, breaching could
potentially occur several times within a year. It has been assumed therefore that the sea
defence is breached during the first year of exposure following the removal of the
saltmarshes. This is likely to be in year 7. The spreadsheet matrix shown in Appendix IV
examines a wide range of wave and water level combinations and indicates that the
flooding scenarios presented by the NRA are likely to come in to effect within 8 years of
an initial breach of Hurst Spit, and that all properties and land protected by the sea
defence would be vulnerable to flooding to the same degree as would have occurred
prior to construction of the sea defences.
The stability of the seaward face of the sea defence is more difficult to assess, given the
relative uncertainty of incident wave conditions at the sea defence. Much larger forces
are required to cause structural damage to the seaward face of a block revetment than
for failure due to overtopping on an unpaved lee face. As damage to the lee face by
overtopping discharge will result in breaching of the sea defence, it has not been
necessary to demonstrate the failure of the more resistant seaward slope. Despite the
uncertainty of the incident wave conditions at the sea defence however, it is likely that a
sea defence failure could also be induced by destruction of the block revetment
armouring of the seaward face under the more severe storm events.
The NRA makes the assumptions that flooding will occur within the flood zone inland to
the flood level contour, following a breach of the sea defence. This assumption has also
been adopted for this case to maintain consistency of approach (Figures 8-11). It is
likely that flood levels will be increased by the extremely high overtopping discharge
levels that result from the more severe wave attack, but no allowances have been made
for this within the benefit cost analysis. The standard flood damage assessment criteria
have been used to calculate benefits due to flooding. Flooding frequency figures and
flooding depths have been derived from the NRA benefit cost analysis. The revised
FLARE 90 analysis formulae have been used to recalculate the benefits which would
occur from flood damage (for those asset types included in the revised manual) and the
benefits have been revised into the appropriate years for this benefit cost analysis.
Properties which were not originally at risk due to flooding by landward ingress to the
flood level will become vulnerable to damage during certain of the more severe events.
A limiting water level of 2.1m ODN has been adopted by the NRA which excludes all
those properties between Saltgrass Lane footbridge and Hurst Castle sailing club from
the benefits. The floor level of these properties is consistently between 2m and 2.5m
ODN and they would certainly also be at risk from wave induced flooding in the event of
a breach of the sea defence. Aerial photographic evidence indicates that waves
propagated landwards across the saltmarshes well in excess of 200m following the
breach of Hurst Spit in 1989. This occurred until well after the peak of the storm and at
relatively low tidal levels. This indicates that similar wave conditions (those which occur
at least every 3 months) might be capable of reaching the strip of properties which lie
parallel with the most vulnerable section of reconstructed NRA sea defence, if the sea
defence becomes breached (Figure 8). As there is some degree of uncertainty about
the vulnerability of these properties however, they have been excluded from the
economic analysis.
The flood levels that will be reached as a result of breaching and wave run up are
strongly dependent upon the combination of waves and water levels. The simplistic
approach to the joint probability adopted by the NRA is not appropriate to assess this
effect, as the importance of wave activity is much increased and the total independence
of waves and tides assumed by the NRA is not the case. Allowing for partial
independence of waves and water levels, a range of conditions may occur which would
result in flooding.
The basis of the joint probability analysis for this assessment has been derived from the
MAFF funded study of waves and tides by HR Wallingford (Ref 8). Empirical calibration
of this study by field measurements over a ten year period suggests that the
mathematical derivation of regional joint probabilities underestimates the local frequency
and level of extreme tides, and indicates that more severe conditions are likely to occur
more frequently than has been suggested. The flood damage frequency analysis may
therefore underestimate the benefits resulting from flood damage. The HR analysis has
been modified to allow for the nearshore set up which is reflected in the NRA return
periods for water levels at the sea defence. The empirically derived water levels indicate
a local set up of approximately 0.6m above the offshore conditions. Table 6.2 below
indicates equivalent offshore conditions to those used to derive the NRA flood frequency
damage levels, based on a partially correlated joint probability of waves and tides. The
water level predominates over wave conditions as the most important process from the
joint probability analysis, due to the depth limiting effects of shallow water at the site.
Wave conditions have been modified to allow for wave breaking.
Table 6.2
Return
Period
5
10
20
30
40
50
Offshore
Hs
SWL
(m)
(mODN)
3.3
1.20
2.8
1.29
2.5
1.36
2.0
1.39
1.8
1.42
1.2
1.50
Nearshore
Hsb SWL
(m)
(mODN)
1.9
1.8
1.9
1.89
1.9
1.96
2.0
1.99
1.8
2.02
1.2
2.10
These conditions are relevant only to those areas within the active run up zone between
Saltgrass Lane and Keyhaven. Flooding is only likely to occur as a direct result of
overtopping at a few locations in the event that the sea defence has not breached. This
is unlikely to occur for at least 10 years after the initial breach of the Spit, by which time
wave attack will occur at the sea defence on a regular basis in front of several residential
properties at Keyhaven. Whilst those properties would be at risk, damage cannot be
predicted with certainty. They have therefore been excluded from the benefits. An
estimate of damage to properties that would be affected by flooding and ultimately
erosion damage in this scenario is shown in Table 6.3
Table 6.3
Unnamed at
Saltgrass
Old Saltgrass
East Saltgrass
Saltgrass Cottage
Keyhaven House
Sedge End
1&2 Keyhaven Cott
Tomorrow
Creek end
Long Forge
Coastguard Cott
Level
distance
To sea defence
first year
of damage
1.6
2.1
2.1
2.1
2.1
2.3
2.3
2.3
2.2
2.2
2.2
1.9
70
125
125
125
150
150
150
150
150
150
150
200
10
11
11
11
11
12
12
12
12
12
12
14
Further damage will occur during the next 35 years as salt marsh is eroded from the
area to the north east of Keyhaven. Figure 4 projects total loss of the Keyhaven River
saltmarsh system within 20 years of the formation of an initial breach of Hurst Spit.
Properties within the low lying land immediately to the North of the Keyhaven River
would then become vulnerable to both erosion and flooding (Figures 8-11). Further
benefits will occur due to this erosion but it is not realistically possible to quantify the
rates of erosion beyond 20 years. These benefits have not been quantified within the
benefit cost analysis.
Summaries of the damages and losses are shown below in Table 6.4. The damages in
this table have been derived from the NRA summary of damages and losses and the
revised property damages shown in Appendix IV of this report. All values shown are
indexed to 1995. The average annual damages averted are shown in Table 6.5 and
these have been carried forward to the benefit cost analysis.
6
PROPOSED SCHEME
6.1 Scheme outline
The main elements of the proposed scheme are as follows.
(i)
(ii)
(iii)
(iv)
Recharge of the Cut Bridge to Hurst Castle section with suitably graded
shingle
Construction of a rock revetment around the south western flank of the Castle
Construction of a single nearshore rock breakwater at the rock
armour/shingle junction
Reconstruction of the existing rock armouring between Milford beach and Cut
Bridge
The proposed stabilization scheme includes the following main elements
6.1.1
Shingle Renourishment
The beach will be renourished with 253,000m3 shingle material between Cut Bridge and
Hurst Castle. The beach recharge material will lie within the grading envelope given in
Figure 5. It will be constructed to a level of 7m ODN and with a crest width of 12m
between profiles HU6-HU16, and tapering to a level of 6m ODN at Hurst Castle.
Obtaining shingle of the correct size and grading is of paramount importance to maintain
the natural permeability, slopes and thus the long term stability of the Spit. The best
source of shingle would therefore be the Shingles Bank in Christchurch Bay, which is fed
by material lost off the Spit.
6.1.2
Detached Breakwater
Although large offshore breakwaters would be a hazard to navigation a single small rock
breakwater built close to the shore would effectively dissipate wave energy at the
shingle/rock junction without obstructing the navigable channel. A rock armoured
breakwater will therefore be constructed at the junction of the shingle Spit and the rock
revetment. The breakwater will be armoured with 6-10tonne rock. The crest of the
breakwater will be at 2.5m ODN.
6.1.3
Existing Rock Revetment
The existing revetment at the seaward end of the Spit has performed fairly well over the
past 30 years but needs to be modified to improve its long term stability.
The 550m long revetment, from the Marine Café to Cut Bridge, will be reconstructed and
armoured with 3-6 tonne rock and given an improved crest and toe detail.
6.1.4
Hurst Castle Revetment and Perched Beach
A rock revetment will be constructed at the western end of Hurst Castle. The end detail
of the revetment will be returned into the beach and it will be buried at either end. The
revetment will be constructed with 3-6 tonne rock armour. Shingle renourishment will
take place on the lee side of the revetment, between the Castle walls and the revetment.
The shingle for the recharge of this section will be recycled from the North Point on the
active recurve of Hurst Spit.
6.2 Design
6.2.1
Research and Monitoring
Discussions with several research organizations, MAFF, the recommendations of a
consultants desk study (Ref1), and the dramatic increases in maintenance costs on the
New Forest coastline prompted the NFDC to set up a research programme, to
investigate the causes and extent of damage, and to identify possible long term solutions
for stabilization of Hurst Spit.
A pilot study was set up to record wave conditions and beach cross sections in 1987
(Ref 2,3). This study has since been expanded to include measurements of tides,
currents, off shore surveys, and special surveys following storm events. Following the
severe storms of the winter of 1989 it was decided to accelerate the research
programme, to identify a long term management strategy for Hurst Spit.
It was apparent that the development of an effective stabilization strategy would require
a sophisticated programme of studies; to evaluate the unusual beach processes; to
quanitify 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 and 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 1.
6.2.2
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. Each must be
considered as separate design conditions due to the complexity and variation of failure
mechanisms that can result in breaching of Hurst Spit. Combinations which lie between
these two extreme combinations must also be considered. The conditions given below
have a probability of exceedence of 39% during the 50 year design life of the scheme
and represent the 1:100 year joint probability return period events.
Table 6.1 Design Conditions
Tide level
(mODN)
2.27
0.87
Offshore wave
Conditions
Hs
Tm
(m)
(s)
5.84
8.5
7.9
9.6
Inshore wave
Conditions
Hs
Tm
(m)
(s)
3.76 10.0
4.14 11.2
Probability of exceedence
during scheme life
0.39
0.39
The wave conditions shown are for the most exposed western end of the Spit. Less
severe wave conditions corresponding to the same return period have been used at the
less exposed locations.
6.2.3
Hydraulic Model Studies
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 overwashing of Hurst Spit.
Determine the rate of loss of shingle from Hurst Spit, under storm conditions
and under “average” conditions.
Compare the performance of proposed stabilization measures with the
existing Spit
Examine the effects of existing structures, such as groynes at Hurst Castle
and Milford on Sea, on shingle transport
Identify threshold crest levels and widths to provide alarm levels prior to
failure of the shingle bank
Evaluate the stability and hydraulic performance of existing and proposed
rock armoured structures
Identify a planned maintenance programme following beach renourishment (if
appropriate)
Identify the most cost effective and environmentally acceptable strategy for
the maintenance of Hurst Spit
Analysis of the beach profile field data indicated that damage to the Spit 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 Spit was modeled in four sedments at a scale of 1:40 to accommodate the problems
of scaling. These were linked together by mathematical modeling to produce an overall
picture of the performance of Hurst Spit. 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 Spit. These included the changes in alignment of the Spit and the effects of
sediment control structures, such as the rock groynes at Milford on Sea and the terminal
bastion at the eastern end of the rock revetment.
The test programme was broken down into the following elements:
(i)
(ii)
(iii)
(iv)
Mathematical modeling of the nearshore wave climate
Physical modeling of four overlapping sedments of Hurst Spit at a scale of
1:40
Numerical modeling of sediment transport
Interactive modeling of the results from the physical and mathematical
models
A detailed account of the hydraulic model studies can be found in Appendix 1
6.2.4
Detailed Design
At the time of the model design it was clear that the model would have some limitations
and that certain characteristics could not be reproduced. These included the response
of the saltmarshes to wave attack following breaching of the Spit, erosion of the bed and
fine sediments seawards of the Spit, the effects of sedimentation on the flow in Mount
Lake and the geotechnical response of the saltmarshes upon which Hurst Spit is now
founded. Whilst tests were carried out over a range of fixed water levels, the model did
not have full tidal control. These factors have been analysed separately using field
measurements and empirical methods.
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 and the maintenance
programme. This forms the basis of the long term stabilization strategy for Hurst Spit.
With the exception of the Castle revetment, it is essential that the proposed works are
constructed as an integrated scheme. Ad hoc construction of individual elements of
structures and replenishment will not provide an appropriate solution to the problems.
The scheme also requires that a planned maintenance programme is kept up throughout
its life t ensure that the alarm cross section levels identified at various locations along the
Spit are not reached. If the scheme is not maintained at or above these alarm
conditons, then failure of the beach may result under the design conditions. It is
essential to note that the design conditions vary along the Spit and that a different cross
section profile is required at each of the beach cross sections to accommodate this.
The natural shingle grading on Hurst Spit is very coarse, D50=15mm. The design cross
sections and sediment 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. Recent restoration work has utilized beach material
of a somewhat coarser grade than the natural material. Whilst this does not present a
problem, it does exhibit different hydraulic characteristics. Alternative gradings could be
used in the replenishment, but these will have significant effects on the hydraulic
performance of the replenishment.
6.2.5
Alternative design for finer wide graded materials
The application for a production licence to recycle material from the Shingles Banks area
to Hurst Spit has met with some resistance from the MEPD through the government
procedure. NFDC have been advised by the DOE that a dual tendering exercise should
be carried out to determine whether the Shingles Banks area presents a more economic
source of material for the beach recharge than commercially licensed areas. As the
commercial licensed areas are unable to supply material of the grade required, the
design process has been reviewed and an alternative design has been developed.
The basis for alternative recharge designs using materials with finer and wider gradings
made the assumptions that these materials would have the following effects on the
hydraulic performance of the beach.
1) The beach will form a dynamic equilibrium slope at a shallower angle for either
finer or more widely graded materials than for the indigenous beach grading.
This will require a larger quantity of material to form the capital recharge. As the
slope offshore of the existing beach toe is very steep ( approximately 1:25 at the
beach toe) this will inevitably result in the toe of the recharged beach forming
further offshore.
2) The longshore sediment transport rate will be faster for finer material than for
coarse material. Losses from the system will be greater therefore. This will
result in a requirement for more frequent and higher volumes of maintenance to
be included in the beach management plan.
3) The use of a finer grading or a more widely graded material will reduce the
permeability of the beach and will consequently result in increased wave run up
at the crest under certain circumstances
4) The environmental effects of using a finer graded material may be harmful, if
large quantities of fines are transported in suspension or along the shoreline into
the Western Solent either during the discharge operation or under the influence
of wave attack post placement
5) More widely graded materials will contain a higher proportion of fines which are
likely to be lost from the system at an early stage
The effects of sediment size on hydraulic performance of a beach have been
investigated by a number of researchers and their results have been applied to a range
of scenarios which might provide alternative solutions for recharge of Hurst Spit. These
are described in detail below.
Whilst the grading required has an average D50 of 15.5mm and a D84/D16 ration of
about 10, most of the dredged sources can provide materials with a D50 of about 910mm and a D84/D16 ration of about 50-60. The sand content which is typically 25-40%
of the dredged materials bulk volume leaves the remaining shingle with an average of
15mm and a D84/D16 ratio of about 6. The sand content within the dredged material is
therefore substantially larger than that found within a naturally graded Hurst Spit.
A number of alternatives may be taken in redesigning the beach to accommodate the
problems given by the use of alternative sources.
1) Use as dredged materials from existing licensed areas without screening and
place additional volumes of beach fill. Redesign the beach management
programme to take into account additional losses from the system.
2) Use dredged materials and screen either onboard or at a screening plant to
achieve the required design grading. This may not be practicably achievable.
3) Use dredged materials and screen either onboard or at a screening plant, to
achieve a compromise to the design grading, allowing for additional materials to
be placed and for additional materials in beach management.
On board screening at the dredging area with a screen of typically 10mm will reduce the
sand percentage, but the use of screening is of limited value.
Effects of grading on slope
Recent studies by HR Wallingford (Reference 6) have identified the effects of sediment
grading on the beach slope. These studies confim that by virtue of their lower
permeability, finer or more widely graded sediments produce flatter slopes whilst coarser
or more narrowly graded sediments produce steeper slopes. The NFDC coastal
monitoring programme has confirmed the validity of these studies by beach profiling of a
range of beaches within Christchurch Bay.
The formulae developed in the above studies have been used to assess the relative
cross shore equilibrium slope performance for a range of sediment gradings using
alternative material sources as well as those produced from the grading of the
indigenous material. The problem appears to be that the currently licensed areas
produce material with a lower D50 size than required and also more importantly with a
wider grading ratio D84/D14. A sensitivity analysis has been carried out using a wide
range of alternative design conditions and using the information derived from dredging
companies for existing licensed areas. The best area analysed was area 127, south
west of the Needles. This provides material with a grading ratio D84/D14 of 50 and D50
of 12mm at the mid point line of the grading envelope derived from sampling. This
compares with a design D50 of 15mm and a D84/D14 ratio of 9.4. The figures derived
from the dredging companies are for 80% screened material and therefore represent a
best case scenario for grading. The main problem with the grading is that the D84 value
achieved is 0.6mm, which has a significant effect on the grading ratio. The following
equation demonstrates the variation between the equilibrium slope performance of
materials of different gradings.
A shift in the grading from a D50 of 15mm to 12mm and with a D84/D14 ratio from 9.4 to
50 would result in a reduction of the mean equilibrium slope from about 18 degrees, for
the range of design wave conditions and would effectively result in an increased from the
crest to the new toe position. The steep seaward slope at the toe of the beach at an
angle of about 1:7 indicates that the new equilibrium slope toe would form a futher 2530m offshore than the present design section. The combined effects of the steep
gradient and the wider and finer grading result in a requirement for an additional
32,000m3 for the length of the recharge.
This in turn would result in substantially more fine workable material in the active beach
zone, thus increasing the potential for longshore transport of the material. The rate of
loss of beach material from the intertidal zone will increase until the beach recharge has
reached a new equilibrium grading and this will inevitably result in a significantly higher
proportion and volume of fine material in the active beach zone on a permanent basis.
Assuming that most of the recharge is placed initially on the leeside of the Spit, the new
equilibrium slope may not be achieved until 1-2 storm seasons following the beach
recharge, during which the beach is able to cut back from the current equilibrium slope
into the more widely graded and finer graded recharge material. This process is likely to
be accelerated at the western end of the Spit where beach material will be required
within the active zone immediately.
The alternative to providing no compensation for the smaller size of material is to allow
the beach to reach its own equilibrium slope angle by cutting back of the beach. This is
unacceptable as the natural equilibrium slope of the alternative beach material would
result in erosion behind the alarm threshold for breach formation. This may be
demonstrated graphically by projecting the equilibrium slope angle landwards from the
toe of the proposed recharge. The intercept of the equilibrium slope with the proposed
crest of the recharge is virtually at the junction of the lee slope and crest junction thereby
providing no crest berm at all.
Run up
Since Hurst Spit is a barrier beach wave run up is a critical parameter. The effect of an
increased grading width or reduction in median size is likely to reduce the permeability of
the beach. This in turn is likely to increase the wave run up. The effect of reduced
permeability is to some extent overcome by the reduction in equilibrium slope and the
two factors tend to balance themselves out under most conditions. No further additional
allowance has therefore been made for increased run up.
Longshore transport
The effects of longshore transport were derived from a combination of results from field
data, energy based longshore transport based formulae and physical model testing of
the original design. The design which utilized the Shingles Banks as a source assumed
typical gradings based around the indigenous beach material. Alternative sources of
material have been assessed by substitution of alternative values for less coarse
materials within the sediment transport formulae which reflect the change in size of the
size and distribution of the beach materials.
Kamphuis (1991) suggests a dependence of the longshore drift Q on median grain size
D50. This dependence is supposedly valid for both sand and shingle beaches and
indicates that:
The effects of sediment size on longshore transport have been examined by substitution
of various values of D50 into the CERC formula. The effects of sediment size are given
by the following equation
This indicates an annual increase in sediment transport rates of 6% for material of 12mm
D50 compared with material of D50 15mm, which will require either that a higher initial
recharge quantity is placed or that the rate and volume of managed beach recharge is
increased to offset this difference and maintain performance.
Given that the net annual drift rate is approximately 15,000m3 per year an additional
recharge of 900m3 is needed for each year. Put in context, over the 50 year design life
of the managed recharge, this means that approximately 45,000m3 additional material is
required to ensure the performance of the scheme is maintained. Similar factors may be
applied to alternative sources of material.
Additional materials
The analysis of the relative performance of alternative beach materials indicates the
following requirements.
Increased volume due to longshore transport 45,000m3
Increased volume due to equilibrium slope 32,000m3
Total additional material required 77,000m3
6.2.6
Materials for Renourishment
The alternative beach renourishment designs have examined a range of gradings of
beach recharge material, although there is a clear preference to use material with a
similar grading to the indigenous beach material. Previous maintenance of the Spit has
utilized both land won and dredged materials.
6.2.6.1 Inland Sources
Maintenance and restoration following storm damage has traditionally been carried out
using gravel rejects form local inland pits. Washed rejects have been used each year to
strengthen eroded areas and repair storm damage. The material is purchased through
the Council’s annual term tender procedure from a single local supplier. Quality control
can, therefore, be exercised at the source of supply in advance of the material being
required, minimizing the need to reject substandard material from site. The natural
grading of the Spit has a mean size of about 15mm. The nominal grading of the gravel
rejects is significantly greater, being in the range 40 to 75mm. In practice the grading of
the rejects is at the upper end of this envelope with little material below 40mm and a
significant amount in excess of 75mm. Despite the difference in grading between the
natural and land won materials the gravel rejects have generally performed well because
they maintain, or enhance, the high natural permeability of the Spit. The main
disadvantage is that when the coarser material is lost form the beach surface by storm
action it moves out of the system altogether, being too heavy to be re-deposited in
calmer conditions. It also has to be delivered by road through small coastal villages,
which can have serious environmental consequences if large quantities are required.
The current cost is about £13.5 per cubic metre delivered to site plus the costs of
placement. Following the 1989/90 storms about 12,500 cubic metres of gravel rejects
were used for emergency restoration works. The nominated local supplier could provide
only 40% of this total, the remainder coming from a variety of other inland sources.
Supply of the 253,000m3 of shingle from inland sources for this scheme would also
present a considerable problem within the short timetable of the contract period.
6.2.6.2 Commercial Dredging Areas
In 1985, following a series of storms which seriously damaged the Spit, restoration was
carried out using about 30,000 cubic metres of commercially dredged marine aggregate.
It has been suggested that shingle replenishment with a narrow grading, such as this,
may result in a significant “fining” of the beach due to migration of fine material from the
surface of the beach into the core; and that loss of the coarser surface fraction could be
linked to this “fining” of the core. Monitoring of the Spit following replenishment with the
marine dredged aggregate certainly supports this view, for the following observations
were made over a period of four years in the replenished area:
(i)
(ii)
(iii)
(iv)
Little fine material was lost from the Spit; instead it moved down through the
coarser shingle into the centre of the bank where it formed an impermeable
core;
Most of the coarsest fraction was lost to storm action, although a small
amount of this material did reappear further along the Spit during calmer
conditions
Accelerated scouring of the beach occurred along the seaward face of the
Spit; and
Destructive overwashing occurred with greater frequency
6.2.1.1 Alternative Source of Material – The Shingles Bank
Because of the problems experienced with the use of land won gravel and commercially
dredged marine aggregate for beach replenishment, and the high costs involved, an
alternative source of shingle was sought for the proposed Hurst Spit stabilization
scheme. A potential local resource was the Shingles Bank in Christchurch Bay which
was thought to be the destination of material moving off the end of the Spit. Due to the
shallow water over the bank, and its proximity to the Hampshire and Isle of Wight
coastlines, it has never been considered a suitable deposit for commercial exploitation
and little was known about its composition. In 1990 New Forest District Council began
discussions with the Marine Estate section of the Crown Estate, and with the Flood
Defence Divison of the Ministry of Agriculture Fisheries and Food (MAFF) about the
possibility of removing material from the Shingles Bank for replenishing Hurst Spit.
At that time the Crown Estate and MAFF were themselves looking into alternative
sources of material for beach recharge schemes which would take maritime authorities
out of competition with the aggregate industry for scarce supplies of marine aggregates.
They were seeking non commercially exploitable (non aggregate) marine deposits, and
considered that New Forest’s proposal could form a pilot scheme for further progress in
this area.
6.2.1.2 Licensing Requirements
Before a marine aggregate production licence is issued by the Crown Estate the
applicant’s proposals are subject to the Government View Procedure, which was revised
in 1989 and now incorporates the requirements of the EC Directive on Environmental
Assessment. Without a positive Government View a licence will not be granted. One of
the main criteria is the potential impact on adjacent coastlines of the proposed dredging
operations. Currently the procedure requires an applicant to demonstrate that his
proposals will have “nil effect” on the adjacent coast, based on the assumption that the
dredged material will go for concrete production, or similar, and will be lost to the marine
environment.
Clearly this presents a serious obstacle to the licensing of near shore sand and shingle
deposits, unless all the bodies consulted under the Government View Procedure can be
persuaded to use “net benefit in favour of the coastline”, rather than “nil effect on the
coastline”, as the criteria for coast protection related applications. With this in mind, this
Council, with advice and guidance from the Crown Estate and MAFF, developed a
strategy which, it was hoped, would prove successful in securing a production licence for
the Shingles Bank. The strategy consisted of four basic elements:
(i)
(ii)
(iii)
(iv)
Research to prove the resources of the Bank
Looking at methods of extracting shingle from the Bank and placing it on the
Spit
Evaluating the effects of this on the wave climate of Christchurch Bay and
Assessing the general environmental impact of the scheme
6.2.1.3 Proving the Resource
With advice from Hydraulics Research Limited a research programme was set up in
collaboration with Southampton University’s Department of Oceanography (SUDO) to
provide the following information:
(i)
(ii)
(iii)
The geological setting of the area, to define underlying geology
The bathymetry and the distribution of surficial sediments over the Bank,
together with an assessment of the mobility of this material, based on the
analysis of side scan sonar imagery; and
The thickness of the superficial sediment cover, based on the analysis of high
resolution sub bottom profiling.
The results of this study were very encouraging, confirming that the Shingles Bank is a
major sink for coarse grained sediments. The thickness of the deposit was shown to be
in excess of 12 metres on the eastern side, and the total volume of the bank in excess of
42 million cubic metres. The quantity required for replenishing the Spit, 253,000 cubic
metres, is less than one percent of this total reserve. Ground truth data, in the form of
vibracoring, supported these results and indicated four potential extraction areas where
the grading of the deposits closely matches the natural grading of the Spit. Following
consultation with MAFF a research licence was issued by the Crown Estate for this work
between December 1991 and December 1992.
6.2.1.4 Engineering Options for Extraction and Placing
In March 1992, Posford Duvivier were commissioned to undertake a desk study of the
engineering options relating to extraction of shingle from the four areas identified in the
SUDO report, and the subsequent placement of the material in the required locations
along Hurst Spit. This study considered the following dredging options:
(i)
(ii)
(iii)
Self propelled hopper dredger, pumping ashore
Static dredger, discharging into barges for pumping ashore or bottom
dumping; and
Static dredger, pumping ashore through a floating pipeline
Posfords demonstrated that it was feasible to remove material by dredger from only two
of the four areas studied, largely due to the very limited water depths over the Bank.
The low tidal range of 2.2 metres and rapid tidal currents also provide severe constraints
on discharge location. However, they concluded that a medium sized self propelled
hopper dredger could extract the required volume of material from either of the preferred
areas along the south east face of the Bank, and pump it ashore in a two to three month
continuous operation at an estimated cost of £12 per cubic metre, excluding royalties to
the Crown Estate. This is obviously a very economical means of obtaining beach
recharge material of a satisfactory grading.
6.2.1.5 Aggregate Production Licence Application
Following the offshore exploration programme, a formal application for a marine
aggregate production licence was made for 300,000m3 material from the Shingles
Banks. The application has been supported by an Environmental Impact Assessment
carried out by Wimpey Environmental. The licence application was submitted to the
Crown Estate Commissioners in accordance with the DOE guidelines and is presently
undergoing the formal consultation process.
The DOE have advised the NFDC that the MEPD have requested that the proposal to
use the Shingles Banks as a source is confirmed as an economically justifiable argument
before it can proceed further with the government view procedure. A dual tendering
exercise will therefore be carried out prior to DOE providing a government view on the
application.
6.3 Maintenance
A planned maintenance programme has been developed, in order that maintenance of
the renourished Spit 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 field surveys carried out since 1987.
6.3.1
Threshold Levels
The damage threshold condition, (defined by conditions giving rise to overtopping and
resulting in roll back of the crest), vary along the length of the Spit. At the western end
the wave climate is more severe and consequently the alarm or threshold damage level
of the crest is higher than at the eastern end where the Spit is more sheltered. 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 bank by crest lowering. The maximum run up levels and hence alarm
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, vary
considerably along the length. Values of +6.3m ODN between profile lines HU6-HU18
with a crest width of 8m, and +4.8m ODN with the same crest width between HU18HU20 provide the effective upper limits of wave run up for the conditions tested.
Sections constructed with crest levels in excess of 6.1m ODN were not overtopped at all
during testing and this should therefore provide a safe crest level. The vulnerability of
the beach to narrow crest widths was demonstrated in the model. Whilst a crest width of
8m at a level of 6.1m ODN provides a very safe situation against breaching in the design
storm, the crest width should be maintained at this level, to allow for a sequence of
storms occurring over a short period of time. The performance of Hurst Spit in 1989,
when subject to two severe events in a short space of time, demonstrates the
requirement for some considerable reserve in the design of the renourishment.
6.3.2
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 subsidence of the shingle into the very soft substrate. It is not possible to
quantify the volume of shingle lost by subsidence during the life of the scheme, since
ground conditions vary considerably along the length of the Spit. Recent experiences
with emergency renourishment following breaching have shown that initially considerable
loss of material may occur following resinstatement, due to subsidence into the
underlying saltmarsh. This is less likely to pose a problem on the section of the Spit
which lies above partially compacted saltmarsh.
The proposed renourishment
landwards of the beach may pose more of a problem in this sense. Evidence of rates of
settlement by leveling of datum poles, suggests that in exces of 0.5m settlement may be
expected during the first year following construction, in addition to the initial settlement
caused by the initial loading of the saltmarsh. Careful monitoring of the crest levels
following construction should identify those areas that will require maintenance due to
settlement. Allowance for topping up the crest to the design level has been made during
the first two years following construction. Additional beach material will be placed in
stock piles on the leeside of the rock revetment following construction, in readiness for
future maintenance.
6.3.3
Routine Maintenance Requirements
Routine maintenance requirements include the following elements:
(i)
(ii)
(iii)
Bulldozing of material from the plugged breakwater gap will be required
annually. The gap between the breakwater and the shingle bank/revetment
is only ten metres at the toe. In view of the longshore transport from the west
it is likely that the breakwater gap will plug with sand or shingle after a fairly
short period of time. It will, on occasion, be cleared naturally by wave action,
either by overtopping or by waves driving through the gap at high water
levels. However, it is expected that the breakwater will slow the transport
rate considerably, consequently requiring some artificial force to drive
material through the gap and onto the main body of the Spit. This area may
take 2-3 years before an equilibrium rate of transport is reached. Better
estimates of long term maintenance commitments will be made with the aid of
the monitoring programme.
An accumulation of the coarser fraction of material is likely to occur at the
rock revetment west of Hurst Castle. This will provide a supply of shingle for
recycling to the areas between profiles HU6-HU8, which is likely to be the
most vulnerable to erosion. It is likely that annual maintenance of this area
by transport of material back to the west will be needed. Allowance has been
made to recycle approximately 5000m3 per year.
Following construction of the rock revetment at Hurst Castle, the rate of
shingle transport around the Castle is likely to reduce slowly.
The
accumulation of shingle at the North Point is also likely to be reduced as the
supply of material diminishes. This area will effectively be isolated from the
main system ( and lies within the adjacent coastal cell defined for shoreline
management). Wave action from the north east will however continue to
drive material along the shingle recurve. As less material will be entering the
system from Christchurch Bay there is a risk that beaches on the northern
side of Hurst Castle will be outflanked over a period of years. This problem
can be overcome by recycling material between the accumulating North Point
and the area immediately to the north of Hurst Castle. Any surplus material
can be transported further around onto the main body of the Spit. It is
expected that there will be an annual commitment to maintain the river
entrance channel, and to protect the northern flank of the Castle defences.
Recycling from the North Point would still continue when the new rock
revetment around the Castle was finally constructed.
The scheme has a design life of 50 years, during which there will be a requirement to
recycle or top up the renourishment and maintain the rock structures. Whilst it is difficult
to assess the quantities of material and losses from the system, estimates have been
made which suggest a preliminary programme of recharge maintenance.
This
programme will be revised in conjunction with the results of the planned monitoring
programme. The first planned recharge will take place in year 10 when an estimated
100,000m3 of shingle will be required. This will be followed by recharges of 100,000m3
at 15 year intervals until year 40. Appropriately increased volumes of material will be
placed if finer more widely graded material has to be used for the recharge.
Intermediate recycling of material and clearing of the breakwater gap will be carried out
in line with the details given above. An allowance for major maintenance of the rock
structures has been included in year 6, following the initial settlements and movements
which might be expected during the first few storm seasons. Further maintenance of the
rock structures is also planned at strategic intervals, during the life of the scheme.
6.4 Monitoring Programme
The design criteria for this scheme are somewhat unusual in that no green water
overtopping is permissible if the beach is not to be breached, 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, or from the
proposed dredging from the Shingles Banks to be identified.
6.4.1
Onshore surveys
Land surveys of the area 200 metres to the east of the proposed breakwater, and 100
metres to the west have been established to evaluate the performance of the
breakwater. 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 will be stored on a PC based database
and analysed using DGM3 software.
The active shingle recurve north of Hurst Castle is at present monitored on a quarterly
basis. An area immediately to the south of this, linking the North Point to the Castle, has
not been monitored as regularly. Quarterly surveys are proposed to fill in the gaps in
existing information. Knowledge of the performance of this area will provide valuable
information to evaluate the performance of the proposed rock revetment around Hurst
Castle. It is proposed that the whole of the beach north of the Castle from reference line
HU20 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.
Data will be analysed using PC based DGM3 software.
The existing monitoring programme provides adequate information along most of the
length of the Spit. Additional survey lines should be established between survey lines
HU16 to HU20. This will provide the same coverage along the full length of the Spit.
Additional post storm surveys will be required to allow validation of the numerical model
which is being developed from the physical model. These surveys should be carried out
in parallel with wave measurements and tidal measurements, and should be carried out
to the required format for comparison with the numeric model of beach profile response.
This data will provide confirmation of the validity of alarm threshold values provided by
the physical model studies and the numeric model.
In the unlikely event that the renourishment has an identical grading to the existing Spit,
the sediment transport rates and the profile response of the Spit to storms will not be
significantly different to the present. In the more likely event that the renourished Spit
will comprise materials of a slightly different grade (probably with a smaller mean size)
the sediment transport rates may well be increased and the loss of shingle under storm
attack will be greater. In this event it will be necessary to carry out regular sediment
sampling and size grading analysis 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. The results
of a recent study by HR on the profile performance of beach material of different grades
(Ref 6) may also provide valuable calibration information.
6.4.2
Offshore surveys
Offshore surveys extending 500 metres offshore will be established in conjunction with
the land surveys. Current metering surveys will be carried out at several stations over
full tidal cycles prior to construction of the breakwater, and will be repeated post
construction to identify any changes in current paths and magnitudes caused by the
breakwater. The frequency of land surveys will be increased during the first two years
after construction to monthly surveys of selected areas, and thereafter will be reduced in
frequency to the standard quarterly surveys, subject to satisfactory performance of the
structures.
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. Occasional offshore
hydrographic surveys of the Shingles Banks area will also provide better information
about changes in the offshore bathymetry.
A Datawell omni directional waverider buoy has been installed in the North Channel, on
the leeward side of the Shingles Bank, 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 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.
Clearly this is a very important factor in determining the design conditions at the site, as
the wave conditions reaching the Spit are largely determined by the depth of water
across the shallow offshore banks.
6.4.3
The Isle of Wight
Concerns have been expressed by South Wight Borough Council (now Isle of Wight
Council) with respect to the perceived possibility of adverse effects of dredging from the
Shingles Banks on the Isle of Wight beaches. Following discussions with officers and
councilors from South Wight and the MAFF regional engineer, a series of actions have
been proposed by NFDC to overcome these fears.
A monitoring programme has been designed by New Forest District Council on behalf of
SWBC, to monitor the performance of beaches on the west coast of the Isle of Wight and
to monitor changes to the bathymetry of the offshore Shingles Banks dredging area.
The surveys will include quarterly topographic line and level surveys of beaches, spot
height surveys and hydrographic surveys. Additional surveys will also be carried out
after severe storms. Data will be recorded by NFDC in a database and analysed.
Baseline surveys will be carried out prior to the dredging operations to establish the
existing condition of the Isle of Wight beaches. The coastal survey area will extend
along the foreshore of the Isle of Wight between the Needles and Fort Albert as agreed
locations and the offshore surveys will be carried out across the Shingles Banks.
Results of the surveys will be compared and will be analysed by reference to measured
and predicted wave conditions. If and when appropriate, a revised mathematical model
refraction grid of the Shingles Banks area will be produced for use with the refraction
models OUTRAY and INRAY (previously used in determination of the nearshore wave
climate) and these models re-run to identify changes to the wave climate resulting from
modified offshore bathymetry.
If beach changes occur which may possibly be attributed to dredging, and no informal
agreement is possible between SWBC and NFDC, then MAFF will be called upon to act
as arbitrator, using an agreed expert consultant to act as a technical advisor. In the
unlikely event that changes to beach levels can be attributed to dredging, NFDC will
promote beach recharge works on the appropriate beaches with grant aid from MAFF.
6.4.4
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 nourishment to be
assessed throughout its life, by comparison with the design conditions. The complex
wave climate, resulting in varied wave conditions along the length of the Spit, can be
assessed by substituting different wave conditions into the database for comparison with
given profiles. As the geometry of the beach changes with time, either as a result of
subsidence or to wave action, the possibility of wave overtopping can be assessed. 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.
6.5 Impacts of the proposals
6.5.1
Influence of the Proposals on Coastal Processes
The construction of a large scale stabilization scheme including beach renourishment,
revetments and a breakwater 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.
The proposed groynes and beach nourishment at Milford on Sea will not have a
significant effect on longshore transport to the west, provided that the groyne bays are
initially nourished with an adequate supply of shingle to maintain the existing transport
rate.
The proposed modifications to the rock revetment will not have any significant effect on
sediment transport rates or on toe scour, as the proposed revetment is hydraulically
virtually the same as the existing revetment. The existing structure has already reduced
the rate of transport onto Hurst Spit to a lower than desirable level.
The proposed breakwater at Cut Bridge will slow the rate of sediment transport onto the
shingle section of Hurst Spit. However, if the renourished beach on its down drift side is
maintained in accordance with the planned maintenance guidelines, it should perform
significantly better than at present. The breakwater itself will reduce the incident wave
energy at the junction of the rock revetment and shingle Spit, thereby slowing the rate of
transport away from the junction.
The main body of the proposed renourishment will have no significant effect on the rate
of sediment transport, as the foreshore equilibrium will not be altered significantly. This
is because the beach renourishment will take place on the lee face of the existing Spit.
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, due to the
erosion of the seaward crest of the Spit.
The beach processes on the eastern side of Hurst Castle are driven primarily by waves
generated in the Western Solent, resulting from easterly and north easterly winds.
Sediment is therefore driven from Hurst Point in a north westerly direction towards the
North Point. The processes driving shingle to the North Point will be unaffected by the
works. Sediment transport rates should therefore remain constant between Hurst Point
and the North Point, until Hurst Point becomes cut back due to shingle starvation. The
rate of transport to the North Point will then reduce. This is likely to be beneficial, as the
accumulation of shingle on the North Point is a problem at present, blocking the
navigation channel at the mouth of the Keyhaven River.
In the long term it is inevitable that Hurst Point will reduce in volume. Shingle which is at
present recycled from the North Point to Hurst Spit could then be efficiently recycled to
Hurst Point, to alleviate starvation in this area. In the mean time the excess material
from the North Point will be recycled onto the Castle beaches to mitigate the effects of
the proposed rock groyne. The proposed plan shape of the revetment around the castle
takes into consideration the possibility of shingle starvation at Hurst Point. It is
suggested that the eastern end of the revetment is returned parallel with the Castle and
is buried in the beach, when it is eventually constructed by English Heritage.
6.5.2
Environmental Impact Assessment
Although and EIA is not a specific requirement for a production licence application it was
clear that, because of the nature of this scheme, the Government View Procedure would
demand one. An EIA was also considered necessary because of the environmental
sensitivity of the Spit itself and of its surrounding area. Accordingly, in February 1992,
Wimpey Environmental Ltd were commissioned to undertake an Impact Assessment
with the following terms of reference to:
(i)
Review the existing environmental information available
(ii)
Examine the potential environmental consequences of extracting shingle form
the Shingles Bank
(iii)
Examine the environmental consequences of the Spit and its surrounding area
of carrying out beach recharge and construction of the various rock structures
(iv)
Identify areas of poor or non existent knowledge, and make recommendations
for further investigation
A copy of the EIA, bound separately, forms part of this application.
6.5.3
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 clear sediment transport drift divide, at the cell
boundary, the effects of the scheme have already been relatively easy to assess. 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 & 5c (the Western Solent and Southampton Water).
The comprehensive environmental assessment includes consultation with all
organizations which might normally be consulted in preparation of an SMP and there is
unaminous agreement that the preferred beach recharge option is the most appropriate
course of action at this site.