sea defences revision
TRANSCRIPT
The aim is to protect the coastline by preventing the waves eroding the coastline
Involves the use wood, concrete or stone
Sea walls Revetment Gabions Groynes Rip-rap Offshore breakwater http://www.think-energy.com/ThinkEnergy/1
6-18/case-studies/case-study-5.aspx
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
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
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
Advantages◦ Fairly cheap
Disadvantages◦ Are laid on a beach
which can disrupt access to the sea and tourist activity
◦ Do not last that long
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
Advantages – dissipate the energy of the waves before hitting the cliffs thus preventing erosion, cheap,
Disadvantages – ugly, may not last a long time
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
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
Material is trapped here, so the land is higher up
This is the last groyne, no more sand is trapped which leaves these cliff vulnerable to erosion
Large interlocking man-made boulders which protect the coast by breaking up the waves.
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
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
Advantages – protects the shore from all waves
Disadvantages -ugly, will ruin the surfing tourist industry
Cliff collapse, beach nourishment
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.
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
Replaces material lost by erosion each year.
•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
To protect or not to protect?
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
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