The Earth’s Crust is Changed in Structure by Coastal Processes – Waves Essay Sample
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Introduction of TOPIC
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.
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.
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 by land, winds from the north cause longshore drift movement from the north to south on the east coast.
Northerly winds have the greatest influence, they do not blow as frequently as westerly winds but they have crossed a long stretch of open sea. Longshore drift is very important in the formation of all landforms of coastal deposition.
Erosion means ‘the wearing away of rock, soil, etc by the action of water.’ There are four processes of coastal erosion:
Hydraulic power: This applies to the weight and impact of water against the coastline. Under storm conditions hundreds of tonnes of water may strike the rock face, the weight and voracity of which causes maximum damage. When the water strikes the rock face, air and water are trapped and compressed in the cracks and caves. This increases the pressure on the rock. When the sea moves away the trapped air expands causing weaknesses in the rock enlarging cracks. Pieces begin to break off. With time and pressure the cracks increase and the cliff face crumbles away.
Corrasion: This is also known as Abrasion. It is the process whereby the breaking waves throw sand and pebbles against the rock. Undercutting is caused by the broken off pieces of rock. During violent storms the breaking waves are strong enough to pick up large boulders and batter the rock and cliffs with them. Corrasion is an extremely effective process of erosion.
Attrition: This process occurs when particles cirried by the waves collide with each other and the rock face, they grind against each another and are reduced in size. Pebbles, and even larger boulders are worn down into smaller smoother pebble, shingle, and finally into sand- sized particles which are later deposited on beaches.
Corrosion: This process involves a chemical reaction on the rocks by the sea water. It is most effective on limestone rock, it dissolves in the water to form corrosive chemicals which erode the cliff face.
Of the four processes of erosion, hydraulic power and corrasion bear the most consequence. Under storm conditions both processes can harness sufficient power to cause maximum erosive damage. The highest rates of erosion are recorded during storm conditions. The speed of erosion on any coastline is dependent upon the type of rock it is built on. If a rock face is cracked, has faults and bedding planes, or is riddled with joints, it will erode far quicker than if it is a dense, massive block. The waves constantly attack any lines of weakness in the rock. Cliffs which are built of a hard rock, such as granite, resist the process of erosion much more than cliffs which consist of soft sediments, such as boulder clay.
Feature of Coastal Erosion
The most wide spread type of landform caused by coastal erosion is the sea cliff. The formation of cliffs begin when the destructive waves attack the bottom of the rock face between the high and low tide marks. Processes of erosion, such as hydraulic power and corrosion, cause waves to undercut the face forming a wave-cut notch. This forms a structure where the rock above hangs over the notch. As the wave continues to attack the notch increases in size and the weight of the overhanging rock increases constantly. Eventually it will make the structure collapse altogether. All of the remaining loose rocks and stones from the cliff are pulled and removed by the waves. Once the rocks a
nd stones have been cleared away the new rock face is exposed and the waves begin the process again.
The new landform created at the bottom of the cliff face when the cliff retreats is known as the wave-cut platform. This, between the high and low tides marks, is the gently sloping rocky area. It is covered by water at high tide but exposed when the tide recedes. The surface of the wave-cut platform is broken up by ridges and grooves. Many people will recognise this
area as the ‘flat rocky’ area that is used by tourists to walk out to sea or to search for crabs and other sea-life. The wave-cut platform is formed where the rock above has been cut away by the force of the waves to form the cliff. The wave-cut platform is greatly affected by wave erosion, because it is a low and flat area of rocks, where erosion is concentrated, and where the waves break between high and low tide marks.
These features are created by the processes of erosion and deposition. They form along the coastlines where there are alternative bands of resistant and less resistant rock. 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. The collapsed arch becomes a stack. The stack is a piece of rock which has become isolated from the main coastline. Once the stack has been isolated it too is attacked by the waves, but because of its position, it’s attacked from all sides. It is eroded and eventually collapses into the sea. All signs of where the coastline used to be disappear, no trace is left visible. Coastlines today are many kilometres further back from where they used to be, this is as a direct result from the unremitting energy of destructive waves, coastal erosion.
Features of Coastal Deposition
Sand, shingle and pebbles carried along by the waves is deposited by constructive waves. Due to their type of strength, they have a strong swash but weak backwash, they deposit more material on the coastline than they remove.
In geographical terms a beach is a gently sloping area of land between high and low water marks. Beaches are found along the coastlines where eroded materials have been deposited by the sea. The shapes and sizes of beaches differ, some are curved and located in sheltered bays, while others are straight and can stretch over many kilometres. Most common beaches are made up of sand, shingle or pebbles. The most popular tourist beaches are carried by longshore drift. Beach fragment size depends on the local rock type and the wave energy level. A coastline of weak rocks results in waves heavily laden with eroded material. A bend in the coastline usually results in deposition of materials by constructive waves. This is because a sheltered area has been created where material can build up on the beach.
A spit is a long and narrow ridge of sand or shingle, a beach, that sticks out into the sea. One end of the spit is attached to the land while the other end lies out in the sea. If the spit is of sand formation, sand dunes are normally found at the back of it. There is an area of standing water behind the spit, which becomes a haven for different marsh plants.
Spits take on different shapes and sizes. Spits along the coast of the English Channel have hooked ends, while others run parallel with the coast. Often spits are formed across river mouths, where tides meet calmer waters of an inlet or bay, and where the coast changes direction suddenly. Spits are formed the same way as beaches are, by eroded materials carried along the coast by longshore drift. The deposition of materials begins at the bend of the coastline. But unlike the build up of beaches, the deposition does not just build up on the beaches. In the case of spits, the materials accumulate out into the open sea away from the coast. This forms long ridge of sand or shingle. Both sea and fresh water get trapped behind the spit as it forms. Waves cannot reach the sea areas behind the spit, so there are often salt marches and mud flats covering that area. Once the
spit extends out into deeper water the end of the spit can be affected by strong winds, winds and sea currents curve the end of the spits.
The decision for coastal management is a strongly debated issue. There are views both for and against it. Although, in some countries like Holland whose main cities, industries and farmland lie up to 8 metres below sea level, coastal management is essential. In the British Isles the question has been whether to let nature take its cause, and allow the sea to erode parts of the coastline, which makes other parts of the coastline comparatively safe. Or to intervene by implementing coastal management strategies, which could have a ‘knock-on’ affect along other parts of the coastline. There is always the economical side to consider too, sea defences are not cheap to either construct or maintain. Coastal erosion has to be closely monitored so that local authorities are aware of dangers posed to local inhabitants and the environment.
There are numerous things to consider before a decision is made to use coastal management:
The type of problem a particular part of the coastline faces.
Are homes, businesses or farmland under threat?
Is there any threat to human life?
Will the environment be badly affected by the coastal problem?
Is it a conservation area where local wildlife etc, needs protecting?
Will the tourist trade be affected?
What pressure is coming from local residents?
Will coastal management be economically viable?
What types of strategy should be implemented?
Will the coastal management strategy used be cost effective?
What effect will it have on other parts of the coastline?
Coastal management is usually deemed necessary at popular coastal resorts, these are areas where a vast number of people choose to live or visit. Coastal management in this case is twofold, to keep the sea out and to keep the beach intact. Two different approaches are used to defend against coastal erosion and flooding. There are hard engineering and soft engineering. The term ‘hard engineering’ refers to a structure that is firm and solid, which is unyielding to pressure. The application of science has been used in the design of the structure. The term ‘soft engineering’ refers to a structure that lacks hardness or firmness and will yield to pressure.
There are five main hard engineering defences:
Sea Wall: Building a sea wall is the usual method used to keep the sea out. The sea wall is a thick, high, concrete construction built at the back of the beach on the seaward side of the promenade. The top lip of the sea wall is curved, this deflects the force of the wave and pushes any sediment carried by the waves away from the sea front. Sea walls protect against flooding in low land coasts. Building a sea wall is extremely costly, approximately 7000 per metre, so great consideration has to be given before embarking on the project. Apart from the enormous costs, there are other set-backs concerning a sea wall. They require extensive maintenance as they are constantly battered by tonnes of water. Also, the energy of waves that are deflected is released further along the coastline. The sea wall may be protecting one part of the coastline but it is helping to erode another part of the coastline.
Groynes: are wooden structures placed at right-angles to the coast where longshore drift occurs. These structures, build out at sea, slow down the effects
of longshore drift, and act as barriers with deposition occurring behind them. They reduce movement of material along the coast and hold the beach in place. In some parts the cliff will be protected from further erosion, and the beach will protect low areas from flooding. Groynes are placed 10 and 400 metres apart, and cost between 10,000 and 30,000 each.
Like the sea wall, they are an extremely costly defence. The problem that arises from using this defence is that more coastal erosion will occur further along the coastline, because the groynes prevent the beaches deposition. Groynes are effective in keeping the beaches intact, but they have a disastrous effect on the beaches on the downdrift side. The waves have been deprived of their load of sediment, so they remove material from the beaches and cliffs further along the coast.
Revetments: are slatted barriers build where a sea wall would prove too expensive. There are two types, timber and block filled. Timber revetments are a sloping fence built parallel to the coast. Block filled revetments are two parallel fences filled with concrete. The revetments break the force of the wave trapping beach material behind them, and protecting the cliff base. They coast around 2,000 per metre.
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.
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