Sea Defense Background

People have tried to control erosion and flooding for many hundreds of years, with varied success. There are a number of alternatives when there is an erosion or flooding problem

1. Do nothing
2. build sea walls etc at the edge of the sea
3. build forward e.g. groynes, beach nourishment
4. build off shore defences e.g. offshore bars or breakwater

FACTORS TO CONSIDER

1. Cost of the scheme and up keep costs
2. Benefits of the scheme e.g. homes, farmland, roads and lives
3. How effective is the scheme
4. What effect will it have on other areas
5. What will it look like
6. Who pays. In low-lying areas the N.R.A. is responsible and in high areas it is the local council. The government will give a grant if it accepts that the scheme is needed.

DEFENCE	COST (2005)	ADVANTAGES and DISADVANTAGES
Sea wall	£6000 per m	Very strong,but may reflect waves,causing turbulence and undercutting . Very expensive
Rock revetment	£4500 per m	Absorbs the energy of the waves. Over time the rocks break up. Unsightly
Offshore bar	£5000 per m	Protects the base of the cliff. Must be large rocks or will be moved by storms
Groynes	£5000 each	Trap sand if there is any to trap. Steal sand 100m apart from beaches downdrift increasing erosion
Beach nourishment	£3500 per m	Absorbs wave action. May need replacing

 

SHORELINE MANAGEMENT PLANS

Until recently coastal management plans were piecemeal. Each area looked after its own part of the coast, which caused problems for adjacent areas. In 1995 MAFF encouraged the development of a more integrated approach in the form of SMP's/ They are made of sediment cells e.g. the coast from Hengistbury Had to Hurst Castle spit in Hampshire.

Stage 1 Collect data about processes and defenses, land use and ecosystems
Stage 2 Management objections set.
Stage 3 Plan preparation & Consultation.
Stage 4 Review.

MANAGED RETREAT OF THE COAST

In the past high flood tides and surges were absorbed by salt marshes along the coast. Over the years, these areas were reclaimed to create fertile farmland e.g. in the Fens, Romney Marsh and Pett Level, Thames Esturary, polders of the Netherlands etc. This was seeen as very important after the food shortages of the Second World War.

From time to time these areas are flooded by the sea, and have been protected by higher and higher sea walls and other defences . As sea level rises owing of the Greenhouse effect it will cost increasing amounts to protect this land. There has been a food surplus in the EU recently so the fertile land is less highly valued. In some places managed retreat is being used.

This allows part of the sea wall to be destroyed and the land behind is flooded during high spring tides until it reverts to salt marsh. This takes about 50 years. This land is seen as being expendable.

TYPES OF DEFENSE

Hard Defences

Static shoreline structures such as those constructed from timber, steel, concrete, asphalt and rubble.
These involve linear structures such as sea walls, revetments and breastwork and control structures of artificial headlands, offshore breastwork and groynes.

Soft Defences

Mobile/ responsive defence measures which seek to work with nature rather than control it. Such structures may consist of sand or shingle beaches and dunes or banks) which may be natural or constructed, and may include control structures. These can include soft solutions of beach nourishment, cliff/dune stabilisation, bypassing and managed retreat.

Tyre Structures

With 100,000 tonnes of scrap tyres available each year in Britain alone, of which some 8 million tyres went for landfill disposal, there is a lot of scope for people to try to find an alternative use for the 8 million tyres which cannot be retreaded. The tyre industry, which currently recovers some 75% of this 100,000 tonnes deserves a lot credit for its recycling and recovery programme. Which other industry achieves this level of recycling? I expect some one will tell me, but for the moment I am impressed. The use of scrap tyres, in the design of Norfolk Reef, came about quite naturally. The flexibility they offered seemed tailor made in the reef's construction and environment. The fact that they are in abundance and practically free, becomes a big plus for future sea and coastal defence work. Additionally, scrap tyre constructions, placed on the sea bed, become the hottest spot for miles around, offering a habitat to sea creatures from molluscs to fish. Perhaps, for once, we may be able to put something worthwhile back into the sea, which actually helps marine life to thrive and prosper. Scrap tyres might not have immediately sprung to mind, when seeking to invest in our ocean`s future, but I encourage you to think on and start designing structures