IntroductionSandbanks is in East Dorset. The beach is widely acclaimed as one of the best beaches in Britain. Historically fragile, Sandbanks is the responsibility of the Borough of Poole who aim to provide long term protection to the infrastructure and properties on the Sandbanks peninsula.To prevent the sea breaking through the Sandbanks peninsula and forming a second Harbour entrance, a series of 13 groynes was constructed in the 1890’s. From then until the 1950’s, Sandbanks beach steadily became wider, allowing much development.
In the 1950’s, the old groynes decayed, and were seen as a danger to public safety, so they were removed. Since then the beach has steadily shrunk again.From then on the construction of the along the Sandbanks stretch, together with the various cliff stabilisation schemes, have afforded protection and provided an essential and much valued amenity for both residents and visitors. However, as with most coastal engineering, problems have arisen. The sandy beaches are one of the most prized assets of both Bournemouth and Poole, and they were derived in the first instance from the erosion of the sandy material in the cliffs.Once the cliffs had been stabilised and seawalls and promenades built, an essential link in the coastal process system was broken, since the principal source of the sand was no longer available. Thus in recent years, beach replenishment has become an essential element in the coastal strategy programme of Bournemouth Borough Council.
Sand has been dredged from a number of locations offshore and pumped ashore along different stretches of the coast. The programme was first introduced in 1970 and is an ongoing operation, with the next replenishment due in 2003-2004.ManagementThere are two different types of coast defence works.
There are works which protect against flooding of the land. These are called sea defence works and are carried out in accordance with the Land Drainage Act 1991. There are also works which protect against erosion. This is where the land behind the works is higher than any tidal flooding level. These works are termed coast protection works and are carried out under the Coast Protection Act 1949. The Government, by way of the Ministry of Agriculture, Fisheries and Food, give maritime District Councils grant aid for carrying out works on the coastline. Surprisingly, the levels of grant aid differ; those for sea defence works being set at 45% and those for coastal protection work varying up to a level of 70% of the cost of the work.
Along this length of coastline area of development, where coast defence works have been carried out, Sandbanks defence works where introduced.Around the southern part of Sandbanks the coastline was protected by a stone wall and stone groynes to stabilise the beach. These were in place prior to 1900.
Further to the North the peninsula area was protected with timber groynes to stabilise the beach. This work was carried out in about 1962.In more recent times the sand dunes have been managed by the planting of marram grass and tree planting for shelter and to limit dune mobility. Artificial sea defences have been built along former dune coasts.Management implementation were introduced to Landowners to take advantage of the regression of the dune front by means of sand trapping fences and dune management, mainly the planting of marram grass.
Plans/ Evaluation ManagementThe plans introduced was to provide a strategic framework for the long-term management of the coast. The plans were assessed to 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 were 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 stageThe second stage was developing 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 orThe preferred options were set out in the plans which where adopted by the relevant authorities and provided framework for the management of the coast in the future.
To achieve the management option for sandbanks, a required detail study of that area was implemented. Shoreline management plan was recommended to be followed.The study enabled engineers and other staff to evaluate the effectiveness of groynes and other coastal defences , as well to look at where coastal defenses may have been required in other particular location. The report scheme recommended an increase in width of the beach by recharging with sand to make it more effective to dissipate wave energy.Cost are presented as net present value, discounted at 6% over 50 yearsThe cost of doing nothing have been calculated as the costs of doubl;ing or triplinf erosion or flood damage caused by climate change over the next 50 years.;