Monday, August 20, 2007

Coastal Defence

When managing the coastline there are two main options:
- this is where man made coastal defence structures are used to reflect large amounts of wave energy and hence protect the coastline.
- this is where beaches or naturally formed materials are used to control / re-direct erosion processes.

You need to know examples of coastal management techniques and their advantages and disadvantages:

Hard Engineering Techniques:

1. Re-Curved Sea Wall
- concrete wall which is curved on the underside to deflect the power of the waves
- these can be very expensive (up to £1-2 million per km) and the deflected waves can scour material at the base of the wall causing them to become undermined
- these are however a very effective means of preventing erosion and they reflect rather than absorb wave energy.

2. Rip Rap
- large boulders on the beach absorb wave energy and break the power of the waves
- although movement of the boulders is expensive this can be a much cheaper method than some other solutions
- the boulders can however be undermined easily by waves washing away sand and shingle beneath them. They also can be quite ugly, changing the appearance of a coastline.

3. Groynes
- these structures (usually either wooden or steel) are designed to top longshore drift and therefore act to build up and anchor beach material, protecting the base of cliffs.
- they are effective at reducing erosion in the area they are constructed in by causing significant build up of beach material
- groynes may however starve areas further down the coast of material by stopping longshore drift, resulting in an increase in erosion in these areas

4. Gabions
- these cages of boulders are built into cliff faces to protect the cliff from the force of the waves;
- they are cheaper than sea walls and can be very effective where severe erosion is a problem
- they are however visually intrusive

5. Revetments - these wooden structures break the force of waves and beach material builds up behind them
- they are cheap and effective at breaking waves
- as well as being visually intrusive however they do need replacing more frequently than most other defence methods.

Soft Engineering Techniques

Soft engineering includes beach replenishment in which beach material is added to provide a "natural solution". Environmentally this is a preferred option as it maintains the beauty of the landscape and avoids visual intrusion, however it can be expensive to maintain as longshore drift continues to move beach material down the coast and therefore regular replenishment is required.

Sand Dunes and salt marshes can also be encouraged to act as natural barriers to the waves.

Case Study of Coastal Defence: Aldeburgh (Suffolk)

Aldeburgh is just south of Dunwich, here a large scale coastal defence scheme is in place to control erosion. Aldeburgh is a busy town and tourism is very important to the local economy in this area. It is therefore seen as cost effective to have a coastal defence scheme in place to protect the economically valuable land.

Hard engineering is in place with a combination of sea wall, rip rap and groynes. There is also an area of salt marsh to absorb some of the waves energy should it breach the sea wall.

Another area severely affected by coastal erosion and where significant coastal defences are in place is Overstrand on the North Norfolk Coast, pictures of the coastal defences in this location can be seen here.

Further north, the small village of Dunwich is also severely affected by coastal erosion, yet very little, apart from some small soft defences and a recent limited and unsuccessful attempt at hard engineering is in place - here is a very useful account of the reasons for the differences in the extent of coastal defence between Dunwich and Aldeburgh.

Wooden Revetments and Gabions:

Follow up links:
Download a Coastal Defences summary sheet (includes advantages and disadvantages)
Coastal Management (BBC Bitesize)
Coastal Management Strategies (S-Cool) - very good!

Podcast: Coastal Defences
You can listen to a podcast of this post below - to download a copy to listen to on your .mp3 player click here.

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