Along this length of coastline, the only
significant area of development, where coast defence works have
been carried out, is at Swanage. Here there is a length of
approximately 1.8km of coast defence works which principally
involve concrete or stone sea walls and timber groynes.
Around the southern part of Swanage bay the
coastline is protected by a stone wall and stone groynes to
stabilise the beach. These were in place prior to 1900.
The central section between the Mowlem and
Ocean Bay was protected by a sea wall but, in about 1925, Shore
Road was widened and a new sea wall constructed in front of the
existing one. Moving the sea wall seaward resulted in an
increase rate of erosion and a timber groyne system was carried out
as a sea defence scheme in about 1930. This scheme was effective in
retaining sand on the central section of the beach but had the
effect of starving the beach further to the north.
The section of the beach between Ocean Bay and
Sheps Hollow had development on the cliff to the rear and there was
a need for protection and a scheme involving extension to the sea
wall and timber groynes was carried in about 1962. The cliff
to the rear of the sea wall was not protected, otherwise than at
its toe, and is still eroded by the normal weathering process.
There are slips along the cliffs most years
during the winter period, when the cliffs become very wet.
The winter of 2000/2001 yielded the highest total rainfall for many
years and as a consequence there were more severe and frequent
slips on the cliffs.
A survey of the whole length of North Beach
was carried out to establish the feasibility of carrying out
protection works to prevent material falling on to the promenade,
which is used by the public. The estimated cost, in 1987, was
in the order of £350,000. As the primary protection by the
sea wall had already been carried out, the sum did not attract
grant aid and consequently the Council could not afford to
undertake the work
In 1989 the Council carried out a scheme, to
the south of Swanage, at Durlston Bay. The scheme provided
protection to an area of land on which the closest building to the
edge of the cliff was a block of flats, known as Durlston
Cliff. The problem was caused by a combination of ground
water and wave action. The cliffs below the flats are formed
mainly in layers of Purbeck Stone and marl. Slips
occurred in the material when it became excessively wet and the
marl lost its strength. This provided a lubricated surface on which
the blocks of Purbeck Stone could slip. The slipped material
then moved down to the shoreline where it was washed away, at the
foot of the cliff, by wave action. To protect against the
wave action of the sea, large armour stones, weighing between eight
to ten tonnes each, were placed at the bottom. The cliff was
then, in effect, rebuilt with rock fill using the armour as a
gravity retaining structure. In addition, ground water was
drained from the cliff and was discharged at the bottom to improve
stability.
In December 2000 a significant slip occurred
in an area just to the south of the area where the 1989 scheme was
carried out. The slip took away the coastal footpath and the
gardens of adjacent properties. Changes in policy and the
legislational framework mean it will be more difficult to undertake
a major scheme to protect the built assets now at a greater
risk. Unless some remedial measures, such as deep drainage or
shear piles, can be implemented to control and slow the rate of
erosion within a managed retreat policy, then an accelerated rate
of erosion may be expected to occur in response to predicted
changes in climate. Some emergency works have been carried
out which involved re-routing a stream which, following the slip,
was flowing into the slip area. This should reduce the rate
of erosion but is by no means a total solution.
Since the late 1990s there has been a
significant change in the way that issues concerning the coast have
been managed. This arose from the increased awareness of the
need for a closer working relationship with those dealing with the
coast and neighbouring Authorities. Shoreline
management plans were implemented which developed specific policies
for dealing with issues on the coast. This Council was
involved with the neighbouring authorities in undertaking these
plans. The district’s coastline falls within two separate
plans. The plan for the western length, defined as sediment
sub-cell 5g, is for the length of coast from Durlston Head to
Portland Bill. The eastern section, from Durlston Head to Hurst
Spit, is defined as sediment sub-cell 5f. Both plans are
complete and are available in this office for inspection.
The purpose of these plans is to provide a
strategic framework for the long-term management of the
coast. The plans have assessed the processes acting on the
coast, how these processes affect the sediment movement within the
cell and the influences on the natural and built environment. The
data collected has been used to establish the lengths of coastline
where coast defence work would be appropriate and, alternatively,
where works would be detrimental to the protection of the coast as
a whole.
These plans were undertaken in two
stages. The first stage involved gathering all existing data
on the coastline and establishing what further data was required,
as well as formulating a brief for the second stage.
The second stage developed a generic
management option for each specific length of coast. This
could be:
§ to do nothing and
allow erosion to continue;
§ to control the
amount of erosion by engineering techniques but retreat from the
existing line of the coast;
§ to prevent
further erosion by maintaining existing or providing new defences
on the existing line of the coast.
§ to prevent
further erosion by providing new defences in front of the existing
line of the coast.
The preferred options have been set out in the
plans which have now been adopted by the relevant authorities and
will provide the framework for the management of the coast in the
future.
How a particular management option for a
length of coast is achieved may require a more detailed study and
these have been termed “strategy studies”. These would
normally follow from the recommendations of the shoreline
management plan. However, because there was a predetermined
need to study the erosion problems within Swanage Bay a strategy
study was carried out concurrently.
The purpose of the study at Swanage Bay was to
appraise the effectiveness of the existing groynes, sea walls, and
other coastal defences, as well as to look at where coastal
defences were required in other locations around the bay. The
report recommended a scheme to increase the width of the beach by
recharging with sand to make it more effective in dissipating wave
energy. The main concern about the scheme was whether sand
could be retained on the beach without control structures, the
possibility of sand blocking the outfalls which discharge into the
bay and the effect the movement of sand may have on other
facilities and the natural environment of the bay. For this
reason a further detailed study was carried out which involved the
collection of more extensive data to develop and increase the
reliability of the mathematical model of the coastal processes in
the bay. When this work was completed, the recommendation was for
replacement of the existing timber groyne field with rock groynes
followed by beach recharge. This was assessed by the
consultants as being the most economic scheme over a 50-year design
life. It was not, however, the cheapest scheme in terms of
initial capital cost.
Consultations were carried out on this scheme
and issues raised had cost implications for the proposals. A
reassessment of the scheme economics was therefore carried out and
this has resulted in a very small margin between the scheme costs
between rock and timber groynes. As the local community had a
preference for timber groynes, the consultant developed a timber
groyne and sand recharge scheme. The Council gave its
approval to proceed with the design stage of this scheme in October
2004. It was a requirement to carry out the works in the
winter to avoid disruption to the tourist and amenity use of the
beach during the summer.
Work on site started on installing the timber
groynes at the end of October 2005. 18 timber groynes were built
using green heart, a tropical hardwood. This material was
from a certified source and some material was salvaged from the old
groynes which were removed. The sand for the beach recharge was
originally programmed to take place in January 2006 but this was
brought forward to the end of November 2005. This allowed the
contractor to use the material to create bunds to protect the works
from inundation by the sea which allowed longer working periods
between tides. Approximately 90,000 cubic metres of sand were
deposited on the beach. The works were completed at the
beginning of June 2006. The final scheme cost has not yet been
established but it will be about £2.2 million.
The scheme was designed to cater for a one in
three hundred year storm and the performance of the beach and the
near shore sea bed is monitored as part of the “Strategic Regional
Coastal Monitoring Programme”. This is a programme of monitoring
along the whole of the south coast. This provides measurement to
assess the amount of movement of the sand and will provide the data
to establish when the beach will need to be recharged. It is
expected that there will be a loss of sand from the beach and that
it will periodically need recharging. At the time of the
design, it was estimated that there would be a need to recharge the
beach with about 40,000 cubic metres of sand every 20 years.
Sand being deposited on the beach
At the same time as considering a scheme for
Swanage a study was being carried out to look at the possibility
and impact of deepening the entrance to Poole Harbour and whether
the dredged material could be used for re-charging the beaches at
Bournemouth, Poole and Swanage. The study showed that there
would be little impact to the harbour and therefore this material
was used for the recharge of Swanage beach with the benefit of
considerable cost savings. The recharge contract was undertaken as
a separate contract to the groynes contract as it involved partner
authorities at Poole and Bournemouth as well as Poole Harbour
Commissioners. The quantity of sand that was taken to Swanage
was relatively small compared with the 400,000 cubic metres which
was pumped onto the Poole beaches and the 600,000 cubic metres
which went to Bournemouth.
Sampling of the sand was carried out as part
of the initial study to establish the grading of the sand and its
suitability for use on the beaches and the location of different
grades established. This was to try and match the grading of the
indigenous material. The finer grade material went to
Swanage and Poole with courser material going to Bournemouth and
the coarsest towards Hengistbury Head.
A strategy study in Poole Bay and Harbour has
also been completed and the final report was dated February
2004. This includes the coastline from Hengistbury Head to
Durlston Head as well as Poole Harbour and the islands. The
report can be found on Bournemouth Borough Council’s
web site. It is under residents/ environment/ assessment of
flood and coast defence options.
Consideration is now being given to
undertaking a second round of shoreline management plans and the
boundaries for these plans have been reviewed. This Council
will still be involved in two shoreline management plans. One
plan will run from Hurst Spit to Durlston Head and the other from
Durlston Head to Rame Head.
You are welcome to view any of this
information at these offices but it would be advisable to make an
appointment so that the documents can be made available to you. You
may have copies of this information at the Council’s standard rate
for photocopying.
Purbeck District Council is a member of the
Standing Conference on Problems Associated with the Coastline
(SCOPAC) and you may be able to
obtain more information from the web site.
Part of the district’s coastal boundary,
running from the chalk stacks at Old Harry to the district’s
western boundary, forms part of the world heritage site. More
information about the geology and geomorphology of this coast can
be found on the World heritage Site.