Saturday, January 21, 2012

COAST: Cliff Collapse

Cliff Recession 
Rates of coastal erosion are determined by a range of factors. These include the fetch of a wave, type of beach, the supply of beach material by longshore drift, slope of the cliff, vegetation cover, local hydrology, the rate at which cliff debris are removed from the foot of the cliffs and the material that cliffs are made of. These are each examined below: .
  • fetch of the wave - the longer the fetch of the wave the greater the erosive energy of the wave
  • type of beach - beaches dissipate wave energy. The higher the beach the lower the energy in the wave as it meets the foot of the cliff (if it does at all!)
  • the supply of beach material by longshore drift - if there is a consistent supply of new beach material by longshore drift this will help preserve the beach. If this has been stopped, by building groynes for example, this can increase the rate of cliff recession as there is no beach material to absorb the energy of the waves.
  • vegetation cover - cliffs with vegetation cover tend to be less resistant to recession as roots help bind and reinforce the cliff material.
  • local hydrology - if there is a large amount of surface run off and infiltration this can increase the rate of cliff recession.
  • the rate at which debris are removed from the foot of cliffs - if material that has formed at the foot of cliffs is rapidly transported away then the cliffs will be quickly exposed to erosion.
  • cliff material - the material that cliffs are made of has a significant impact on cliff recession. Soft boulder clay cliffs recede much quicker than cliffs formed from sedimentary rock such as chalk. These types of cliff recede in quite different ways.
Soft Cliff Material
Cliffs formed from boulder clay, material deposited by glacial periods, are susceptible to high rates of coastal erosion. The Holderness Coast is an example of a coastline formed from boulder clay and is the fastest eroding coastline in Europe. The soft boulder clay is quickly eroded through hydraulic action and abrasion. However this is not the only way it is being eroded. Sub-aerial processes, such as rainfall, also cause erosion. This often happens where layers of boulder clay, left behind by melting glaciers, become saturated and cause the cliff to slump. The debris on the beach is then eroded by the sea leaving the cliff exposed once more.

Slumping explained:
Stage1 - the soft boulder clay holds rainwater and run-off.

Stage2 - Waves erode the base of the cliff creating a wave cut notch. The clay becomes saturated and forms a slip plane.

Stage 3 - The weight of the saturated cliff causes it to slump.

The video below shows evidence of slumping at Aldbrough. The video shows the early stages of the process. Water will percolate down the large crack to lubricate the slip plane. This will cause the land to slump further down. 

Hard Cliff Material 
In areas of more resistant cliff material erosion is greatest when waves break at the foot of a cliff. This causes erosion at the base of the cliff. This creates a wave-cut notch in the base of the cliff. As the notch increases in size the weight of the cliffs above become too much and the cliff collapses. This material will provide temporary protection for the cliff behind. However, once it has been removed by the sea this process will occur again. Where cliffs are made of more resistant material, wave cut platforms will be created.

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