The Earth's crust is changed in structure by coastal processes - Waves.

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Geographical Theory

The Earth’s crust is changed in structure by coastal processes.

Waves

The coast is a narrow contact zone between the land and sea. It is constantly changing due to the effects of land, air and marine processes. These changes result in very clear landforms. Changes in the structure of the coastline have a wide and varied affect on the environment. Coastal management is often applied to ensure the changes can be both guided and controlled. On many coastlines the dominant process results from the action of waves. Waves are created by the wind blowing over the surface of the sea.

1. Water moves in a circular orbit.

2. Movement becomes elliptical due to friction.

3. The top moves faster than the bottom.

4. The wave breaks as the top of the wave is moving much faster, the weight and gravity pull it down.

5. The water returns to sea due to the pull of gravity.

Waves are responsible for most of the erosion along the coasts. The effect of the wind blowing over a calm, smooth sea surface causes ripples which grow into waves. The friction of the wave on the sea bed causes the lower part of the wave to slow down as it approaches the coast. This leaves the wave unsupported so it topples over and breaks forward against the beach. The waves causing the most erosion are called destructive waves. Destructive waves have three main features:

1. In proportion to their length they are very high.

2. The backwash caused is considerably stronger than the swash, this means the rocks, pebbles and sand are carried back out to sea.

3. They are frequent waves, breaking at an average rate of between eleven and fifteen per minute.

The height and destructiveness of these waves depend on the wind speed and how far they have travelled. Waves travelling a long distance have time to build up.

When they reach the coastline they release a tremendous amount of energy as they break, this can cause maximum damage. The length of water over which the wind has blown is called the fetch. Destructive waves have lots of energy, this is used by the backwash to transport material back down the beach. This action destroys the beach. Destructive waves pull more material away than they deposit, they erode the beaches with a weak swash and powerful backwash.

The other type of wave is called constructive waves. These have limited energy and operate in calm weather, they are usually only about a metre high. They have the opposite affect of destructive waves. They have a strong swash, which transports material up the beach and deposits material up the beach, but a weak backwash so the material remains where it was initially deposited.

1. In proportion to their height they are very long.

2. The swash caused is considerably stronger than the backwash, this means the rocks, pebbles and sand are carried up the beach.

3. They are sparse waves, and break at an average rate of between six and nine per minute. Constructive waves build up the beaches creating landforms.

Transportation

The loose, eroded materials are dragged along by waves and deposited in other area of the coast. Sediment carried into the sea by rivers is also picked up and carried away by the waves, and later deposited along the coastline. There are four processes of transportation:

Traction: large boulders, too heavy to be actually picked up by the waves, are rolled along the sea bed.

Saltation: smaller pebbles are bounced along the sea bed. As the flow of the sea changes they are picked up and dropped.

Suspension: sand and clay sized particles are carried along by the waves.

Solution: minerals are dissolved in the water and carried along in the flow. These include lime, chalk and limestone.

The process by which material, such as sand and pebbles, is transported along the coast by waves is called longshore drift. Longshore drift occurs when waves break at an oblique angle to the shore, not at right angles, due to prevailing winds. This means that waves push some material along the beach, and features are formed by this process such as spits.

Waves approach the coastline at an angle. Sand grain and pebbles always roll back down the slop at right-angles to the coastline because it is the steepest gradient. The swash is always at an oblique angle to the shore, while backwash is always at right-angles to the coastline.

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Materials are pushed up the beach at an angle and rolled down again at right-angles, this way they are transported along the coastline without settling. Most of Britain’s’ prevailing winds are from a south-westerly direction.

The general direction of longshore drift around the coasts of the British Isles is controlled by the dominant wind. In this way coasts are affected differently. Prevailing south-westerly winds cause the drift from west to east along the Channel coast, and from south to north along the west coast. Although the east coast is protected from the prevailing south-westerly winds ...

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