Swanage: Coastal Defence Case Study

Question 1

Coastal erosion

Answer 1

• High incidence of cliff collapse on the Wealden Clay cliff series that threaten key tourist attractions such as The Grand Hotel and Pine Hotel (lost a beer garden and 15m of land in Winter 2012).
• Halcrow Group, an engineering firm, estimate 500 cubic metres of material is lost from Swanage Bay each year.

-Wealden cliffs are vulnerable to gullying (flows of water eroding soil), cliff-foot erosion, and resulting translational slides (landslides).
The risk of these translational slides is intensified by groundwater seepage and mudflows.

• Since the 1920s, cliff-foot erosion has been slowed by the sea wall but sub-aerial erosion remain rapid.
• Around 50% of all eroded material is deposited onto the beach, but the remaining material is transported away.

Question 2

Defences

Answer 2

• A sea wall was built in Swanage in the 1920s.
• 18 groynes (originally built in the 1930s) were rebuilt with fresh timber in June 2006 at a total cost of £2.2m.
• 128,000 cubic metres of sand were imported from Poole Harbour for beach replenishment in Swanage.

-12 metre soil nails were placed in the Wealden Clay cliffs near the Pine Hotel to reduce risk of
translational slides.

• Cliff regrading has taken place to reduce the gradient of cliffs and therefore reduce risk of landslides.
• The total cost of the coastal defences amount to £5m.

Question 3

Evaluation

Successes

Answer 3

• High benefit to cost ratio (around 7 to 1)
• Coastal defences protect 84 properties and 4 hotels worth a combined £35m, which produces a high benefit to cost ratio considering the total cost of the defences is only £5m.
• Beach nourishment ensures the survival of tourism in Swanage. This is vital, as 39% of the population is employed in catering and services (according to Purbeck Labour Profile) – suggesting a high dependency on tourism for economic prosperity.
• Beach nourishment is also a good example of using ‘natural defences’ for coastal defence, which goes in line with the principles of integrated coastal zone management.
• Agenda 21 of the 1992 United Nations Development Report suggested that good ICZM means using natural defences (soft engineering) over hard defences where possible

Question 4

Evaluation

Failures

Answer 4

• Despite defences, landslides still occurred in Winter 2012/13 as a result of 160mm of rainfall.
• These landslides resulted in loss of 15m of land for the Pine Hotel which lost a key tourist amenity (its beer garden).
• This goes to show that soil pinning has failed.
• Beach nourishment will have to repeated every 20 years due to continual erosion.

Question 5

Background Information

Answer 5

• This might just be background information, but including some of this detail in your exam answers will be key to scoring good marks.
• Swanage is located in South Dorset and its coastal management is decided by the Poole and Christchurch Two Bays Shoreline Management Plan (SMP)
• Swanage is part of Sediment Cell 5 spanning Portland Bill to Selsey Bill.
• When making decisions about coastal management, the SMP must consider that the Department for Environmental, Food, and Rural Affairs (DEFRA) expects a benefit to cost ratio of 5 to 1. For example, this means £7000 spent on groynes must yield a benefit of £35000 to be justifiable to the taxpayer.