• Erosion - The wearing away and removal of material by a moving force, such as a breaking wave.
• Mass movement - The downhill movement of weathered material under the force of gravity. The speed can vary considerably.
• Sliding - Occurs after periods of heavy rain when loose surface material becomes saturated and the extra weight causes the material to become unstable and move rapidly downhill, sometimes in an almost fluid state.
• Slumping - Rapid mass movement which involves a whole segment of the cliff moving down-slope along a saturated shear-plane or line of weakness.
• Waves - Ripples in the sea caused by the transfer of energy from the wind blowing over the surface of the sea. The largest waves are formed when winds are very strong, blow for lengthy periods and cross large expanses of water.
Waves are essentially the movement of water molecules within the ocean, and are restricted to the surface layers of our oceans and seas.
They involve the circular orbit of water molecules and are the agents of coastal change. Waves vary enormously in size and character, from ocean to ocean.
What causes waves?
Waves are created by the action of the wind blowing over the sea or ocean. The friction from the wind causes the surface water to move in ripples which eventually form full waves. The stretch of ocean water over which the wind blows is called the FETCH.
Generally, the longer the fetch the larger the wave, and the faster the wind speed the larger the wave (this is why we generally get the largest waves during and just after storms). The South West of Britain is affected by waves that have an incredibly long fetch, as the South Westerly winds which blow the sea there travel uninterrupted for thousands of miles across the Atlantic Ocean. It is for this reason that the waves are large in Cornwall and generally great for surfing!
As the water approaches the coastline it encounters increasing contact with the shelving sea bed, which exerts a frictional force on the base of the wave. This changes the normal circular orbit of the wave into an elliptical orbit. As the waves get closer and closer to the coast the impact of friction grows, with the top of the wave moving faster than the base of the wave. Eventually a critical point is reached where the top of the wave (the CREST) curves over and creates a breaking wave. This breaking wave can be further disrupted by water returning down the coastline back out to sea.
Waves and interaction at the coastline
The movement of water and sediment up a beach is known as the swash, and is the direction of swash is largely determined by the prevailing wind. Whereas the movement of water back down the beach is known as backwash, and its direction is determined by the slope of the beach, and the water will move back at right angles to this slope. These 2 wave actions interact to give us LONGSHORE DRIFT, which moves sediment in one general direction along the coastline in a zig-zag fashion, governed by the prevailing wind. This action is important, as it moves or erodes sediment from an up current area and moves it down current, changing the shape and sizes of beaches.
Constructive and Destructive Waves
When the swash is bigger than backwash material gets pushed up to the back of beaches rather than removed. These waves are called CONSTRUCTIVE waves, and these waves tend to have low wave heights, lower wave frequencies (they break less often) and the waves are less steep. These waves are created by storms far out to sea which create a large swell which eventually reaches the coast.
Where backwash is larger than swash more material is being eroded from the beach profile than is being accumulated. This carries material out to sea and makes for a steeper beach profile. These waves are called DESTRUCTIVE WAVES which have steeper wave profiles, larger and higher wave crests and come more frequently. These waves are generated during large storms, such as the depressions which affect the British Isles.
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