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Sea defences

Sea defences. The aim of sea defences. The aim is to protect the coastline by preventing the waves eroding the coastline. Hard engineering. Involves the use wood, concrete or stone. Hard engineering. Sea walls Revetment Gabions Groynes Rip-rap Offshore breakwater

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Sea defences

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  1. Sea defences

  2. The aim of sea defences • The aim is to protect the coastline by preventing the waves eroding the coastline

  3. Hard engineering Involves the use wood, concrete or stone

  4. Hard engineering • Sea walls • Revetment • Gabions • Groynes • Rip-rap • Offshore breakwater • http://www.think-energy.com/ThinkEnergy/16-18/case-studies/case-study-5.aspx

  5. Sea wall • Made of stone or concrete. • Reflects waves and withstands waves breaking on it • The wall prevents the base of the cliff being eroded away • May have a curved top to reflect more wave energy

  6. Sea wall • Advantages • Strong and sturdy will last a long time • Disadvantages • Ugly • Expensive • The base of the wall can be undercut by the sea resulting in it collapsing into the sea. • Often rip-rap is placed in front of the wall to prevent this

  7. Revetments • A sloping concrete or wooden wall with gaps between each piece • Can have stones piled up behind them • They dissipate (weaken) the strength of the waves beforeing hitting the cliffs

  8. Revetments • Advantages • Fairly cheap • Disadvantages • Are laid on a beach which can disrupt access to the sea and tourist activity • Do not last that long

  9. Gabions • Strong wire baskets filled with stones that gradually get covered with grass and sand • The stones dissipate the strength of the waves before they hit the cliffs • Can be placed on a beach or in front of a cliff

  10. Gabions • Advantages – dissipate the energy of the waves before hitting the cliffs thus preventing erosion, cheap, • Disadvantages – ugly, may not last a long time

  11. Groynes • A low wall built out into the sea • This traps sand that is moved by longshore drift • This reduces erosion as there is a larger beach, which absorbs more wave energy • This means if any waves do hit the cliff they have a much reduced energy, which results in less erosion

  12. Groynes • Advantages • Help to create a larger beach which is good for tourists • Traps sand and pebbles, making a bigger beach preventing less cliff face erosion • Disadvantages • Prevents the movement of sand down the coast leaving other areas unprotected and move vulnerable to erosion (see Holderness case study

  13. Material is trapped here, so the land is higher up Groynes This is the last groyne, no more sand is trapped which leaves these cliff vulnerable to erosion

  14. Rip-rap • Large interlocking man-made boulders which protect the coast by breaking up the waves.

  15. Rip-rap • Advantages • A cheap(ish) way of protecting the coastline when used with other sea defences. For example a sea wall. Rip-rap placed at the base to prevent undercutting of the sea wall • Disadvantages • Ugly

  16. Offshore breakwater • Located in the sea below the low water mark • It may be dumped interlocking material or concrete wall • Protects the shore from all waves Offshore breakwaters

  17. Offshore breakwater • Advantages – protects the shore from all waves • Disadvantages -ugly, will ruin the surfing tourist industry

  18. Soft engineering Cliff collapse, beach nourishment

  19. Cliff collapse – soft rock • Cliffs made of sand or clay are said to be made of soft rock • This makes them very susceptible to erosion • The soft rock absorbs rainfall and becomes waterlogged. This makes it heavy. When the base of the cliff is eroded by waves a portion of the cliff will slump.

  20. Slumping

  21. Preventing cliff collapse • Drainage pipes to remove excess water • A sea wall may be placed at the base of the cliff to prevent erosion • Geotextiles - this is a webbing that is nailed to the cliff and helps to hold the cliff together

  22. Beach nourishment • Advantages • Cheap • Uses natural materials • Disadvantages • Uses dredged material from out to sea. This means there is less material protecting the coastline and reduce the energy of incoming waves. This results in waves with more energy hitting the coastline. This could increase the rate of erosion • Replaces material lost by erosion each year.

  23. Coastal protection To protect or not to protect?

  24. This is a difficult question • Protecting the coastline costs a lot of money • The Government has to decide whether to protect the coastline • To do this it puts an economic (money) value on the land to be protected. • If this ‘value’ is more than the cost of the sea defences then the coast will be protected • If the value of the land is less than the cost of the sea defences then the coast will not be protected

  25. Generally • Farmland has low economic value so often is left to the sea in what is termed ‘managed retreat’ • Managed retreat is the letting of the land to erode into the sea • Houses and businesses are generally protected but it is not always the case • See Happisburgh case study

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