Transcript File
What do all these pictures have in common? 3 answers required Coastal Defences? • Coastal Defences are ways of protecting the coastline from erosion. • They are normally man made, and generally very expensive. • But they have to be built in many places to stop the cliffs eroding. PREDICTION: Which do you think is the most and least effective defence, and why? How can the coastline be protected? Generally speaking, the most effective protection is a large beach. Why? Because the beach acts like a natural barrier between the waves and cliffs. The beach breaks the wave. This means that it absorbs the most of the wave’s energy before it reaches the cliff How can the coastline be protected? Sea Wall Rip Rap Gabions Groynes Beach Nourishment Offshore Reef Managed Retreat Large rocks placed in front of the cliff. Gets rid of energy, can be cheap depending on rock type, effective for many. Can make the beach difficult for tourists to get to, unattractive, doesn’t work in storm conditions. Cost £300 per metre RIP RAP Walls usually made up of concrete. The modern ones have a curved surface. Reflects & absorbs energy, very visiblemakes residents feel safe, effective for many years. Ugly-puts tourists off, expensive to build. Cost £3,000 per metre RECURVED SEA WALL This is placing more sand and pebbles on the beach. Looks natural, provides a beach for tourists and is a natural form of defence, cheap. It may affect the animal & plant life. The beach needs regular top ups as sand gets washed away. Disruption to homes whilst sand is being replaced Cost £5,000 per 100m BEACH REPLENISHMENT Usually made of wood that stretch out into the sea. Prevents longshore drift as sand builds up on one side of the groyne, keeps the beach in place and is effective for many years. Unattractive & makes it difficult to walk along the beach, they disrupt the natural processes of the beach Cost £5,000 each GROYNE These are wire cages filled with stone used to reduce erosion. Cheaper than other forms of protection, rock cages absorb energy. Wire cages can break & need to be securely tied down, not as efficient as other methods. Cost £11 per metre GABIONS This means allowing the sea to gradually flood the land or erode the cliffs. Creates new habitats for plants and birds. Cheap method. It is upsetting for land owners to lose land. Cost – depends on coastal area MANAGED RETREAT These are enormous blocks and natural boulders which are sunk offshore to change the wave direction and absorb energy from waves and the tides. Cost £1950 per m OFFSHORE REEF Coastal Defences can be put into two categories…Hard and Soft engineering Hard Engineering Soft Engineering Involves major construction work Works with natural processes - unobtrusive Large Scale, immediate impact, expensive Does not involve major construction Man Made Interference with natural processes Cheap, Small Scale, slow impact Rip Rap • Large rocks are place in front of a cliff £300 per metre • Normally hard rocks which are very resistant to erosion. This means they are long lasting. • This dissipates (breaks up) the wave energy so their erosive force is reduced when they hit cliffs. However, not during storms. • Relatively cheap • Can create unpleasant views and access problems for beach visitors Recurved Sea Wall • Usually made of concrete with a recurved face to reflect and absorb wave energy back out to sea. • This can sometime deflect waves onto one particular area – wave scoruing • £3,000 per metre • A very visible reassuring defence and long lasting. However this can spoil views • They can add to Longshore Drift Gabions • Wire cages filled with stone to reduce erosion. The cages absorb the wave energy and reduce erosive power. • £11 per metre (cheaper than others) • The cages will eventually break so they need tying down effectively. • Compared to others Gabions are not that effective, and certainly damage the view. • When the water drains through them cages, the material is deposited which can create beach Beach Replenishment/Nourishment • Placing of sand and pebbles on a beach artificially. Sand is a natural (and the best) defence as it absorbs energy. • Creates a cheap and natural appearance to beach for tourists • This is an unnatural addition and may damage wildlife. • The sand be easily be washed away which means it needs replacing again. • £5,000 per 100m Managed Retreat • Do nothing • Allow the sea to erode the cliffs naturally. This normally involves councils making a decision as to the value of land. • The only cost may be compensation for landowners • New habitats are created and it is cheap in the long run, but predicting rates of erosion can be hard. Offshore Reef • Enormous concrete blocks and boulders are sunk offshore to alter wave direction and dissipate energy. This means waves break further from sea and waves reaching the cliff are weak. • This will mean more constructive waves to create a beach • £1,950 per metre and difficult to install Groynes • Usually made of wood, although sometimes stone. Stretch out into sea to trap material caused by Longshore Drift. • This retains the beach (which is a natural absorber of waves energy) and prevent s erosion as well as for tourists. • £5,000 each • Effective for many years (20-30) but wood will eventually rot. • Effective, but can disrupt beach walks and interferes with natural processes which means beaches further downcoast will be starved of sand. • EG, Mappleton (first 10 mins)