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

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Introduction

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. ...read more.

Middle

The more resistant, hard rocks, take longer to erode by destructive waves. Those areas of hard rock will be left protruding out into sea forming one or more headlands, the headland will usually take the form of cliffs. These headlands are then exposed to the full force of the waves. The less resistant, softer, rock will be eroded more rapidly by destructive waves. This erosion will lead to the formation of bays, which usually slope inland and create the right conditions for a beach to form. The shape and size of headlands and bays will be dictated by the local geology of the area. As time progresses the resistant rock headland will protrude and more, and eventually break away. The bays will be eroded back and beaches will be built up. Waves attack any weakness in rock, joints and grooves are constantly eroded and to great effect. Vertical lines of weakness may be gradually increased is size and eventually form a cave. Hydraulic power and corrosion in particular are the processes of erosion causing this formation. The rocks forming the cave have to be quite hard and resistant to withstand collapse. When waves break at the face of the cave the face is blocked off momentarily, trapping the air within it. This motion compresses the trapped air inside the cave, which then has the effect of putting extra pressure on the roof, back and sides of the cave. Caves forming part of a narrow headland can be pushed through by the sheer weight and power of this pressure. This creates an opening at both sides of the cave, it then becomes a natural arch. Waves attack the base of the arch, putting increased pressure on the top of the arch. The process of erosion continues and increases until the arch collapse. This occurs especially if there is a weak point at the top of the arch. ...read more.

Conclusion

Gabions: are steel mesh cages containing boulders, they are built onto the cliff face above a sea wall. They give stability to cliff structure. The rocks absorb some of the wave energy and cut down erosion. They cost about �350 per metre, but are ugly and need replacing on a regular basis. Armour Blocks: are large boulders piled on beaches where erosion is likely to occur. They are a cheap form of defence but not too effective as they can be undermined or moved by the force of the waves. Rip-rap is artificial blocks used in the same way as armoured blocks. There are five types of soft engineering: Beach Nourishment: is achieved by transporting material, such as mud or sand, to the coast to create an artificial beach. Because the beach is a natural flood defence the replacement of eroded sediment prevents flooding. This action costs approximately �800 per metre, which may sound relatively inexpensive for a sea defence, but as it requires to be replaced over and over again it becomes an extremely expensive option. Shoreline Vegetation: is the binding of beach sediment by planting things like marshbeds on the shoreline. This slows down erosion, and creates an area which encourages shoreline habitats to develop. This is a fairly inexpensive project and one that suits people concerned with the environment, like active conservationists. Dune Stabilisation: is the use of sand dunes as a defence against storm floods. Sediment is added and erosion is reduced by controlling footpaths and planting marram grass, which then supports the dune ecosystem. This is another very inexpensive form of sea and flood defence, and again one that would be popular with the local conservationists. Managed retreat: is about slowing coastal erosion but not trying to stop it. Buildings are either moved or lost to the sea. Compensating for this can be far cheaper than using other forms of coastal control. Set backs: simply means building houses that are set back from the coasts edge. Local authorities in areas known for extensive erosion do not give planning permission for houses to be built near these coastal areas. ...read more.

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