How successful are the coastal defences in Christchurch Bay?

Key Questions: 1- Has long groyne successfully widened Solent beach? 2- Has the gabion revetment scheme prevented Hengistbury Head from becoming an island? 3- How sustainable are the coastal defences? Aiming for A/A* - then pick ONE of these as well: - How economically sustainable is the beach replenishment programme? - Has the coastal management had an impact further along the coast?

1- Has long groyne successfully widened Solent beach? Data collection: Beach profiles east and west of long groyne Groyne drops Aerial photographs

2- Has the gabion revetment scheme prevented Hengistbury Head from becoming an island? Photographs Secondary research

3- How sustainable are the coastal defences? Bi-polar analysis: BI-POLAR EVALUATION OF SEA DEFENCES NEGATIVE EVALUATION FACTOR -3 -2 -1 1 2 3 POSITIVE EVALUATION FACTOR Vulnerable to erosion (unable to 'hold the line')   Effective protection against erosion (able to 'hold the line') Vulnerable to overtopping (unable to control flooding) Effective against overtopping (good flood defence) Ugly (poor aesthetic value) Enhances natural environment (high aesthetic value) Poor access to beach Good provision made for access to beach High risk safety hazard to general public No obvious safety risk to general public Short lifespan &/or high maintenance costs Good life expectancy &/or low maintenance costs High levels of disturbance caused to local people during construction Low levels of disturbance caused to local people during construction Disturbs natural coastal processes & habitats Maintains natural coastal processes & habitats

Christchurch Bay Christchurch Bay comprises a 16km section of open coastline exposed to dominant waves from the south-west.  The coast has been formed and is characterised by cliff geology and continual erosion (at a historic rate of approximately 1m per year). On the coast outside the harbour, there are 50 groynes, 4km of sea wall, 2km of vulnerable cliffs and 3ha of special salt-tolerant grass sward.  The 1990 value of these defences was estimated at £8m and they protect, conservatively, about £100 million of real estate in the front line.

Natural Defence - Ironstone doggers As the cliff has receded, the Ironstone Doggers have been slowly slipping down onto the beach and rebuilding the defences stolen by the Hengistbury Mining Company and others. It can be seen, on the beach at Hengistbury Head, that where Ironstone Doggers are most prevalent, the cliff has regressed less.

Gabion Revetment Along with a set of new groynes either side of the original 1938 long groyne a Gabion Revetment has been built to protect the weakest point in the eastern end of Hengistbury Head, on the South beach just before Hengistbury Head rises up to Warren Hill. While this thing is hardly a picture of Beauty it does provide an effective defence and will hopefully prevent Hengistbury Head becoming Hengistbury Island. One major problem with the gabion revetments is their life span. Inevitably, as they exist in a very harsh environment, their useful life is limited

Beach Replenishment Shingle is less prone to long shore drift and reduces the overall loss of sand, however it does not completely solve the problem and has to be repeated approximately every ten years. It is also wildly unpopular with those who enjoy the sandy nature of the Hengistbury beaches. Major beach replenishment last took place at Hengistbury Head in 2005/6, focussing mainly on the vulnerable area to the south of Barn field In 2010 beach replenishment was conducted along Fishermans Walk and Southbourne Beaches. Here though the majority of the replenishment was sand.

Beach Replenishment

Long Groyne The first attempt to counteract erosion came in the 1930s when Bournemouth Council constructed a breakwater now known as "the Long Groyne". It’s aim was to widen Solent Beach to protect HH. Mudeford Sandbank which lies in the lee of Hengistbury Head suffered considerable loss of beach material, particularly since construction of the Long Groyne in 1938, leading to the threat of a breach to Christchurch Harbour.  Such a breach would lead to increased flooding of land and property around the harbour and the Council developed a scheme to improve the standard of defences along the seaward edge the sandbank 

A success? While the sea defences erected at Bournemouth and at Hengistbury Head have significantly slowed the erosion, regrettably it is difficult to see how full equilibrium can now be restored within this area, without the utilisation of new techniques. There has been concern of a breach across barn Field which would turn Hengistbury Head into an Island. There are mixed opinions regarding this possibility with some considering such a disaster almost inevitable within the next 50 years while others put the risk at 3% or less.

Barton on Sea