Coasts
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Coasts Document Transcript

  • 1. The Specification The earth's crust is modified by coastal processes which result in distinctive landforms. Constructive and destructive waves. Processes of erosion (hydraulic power, corrosion, corrasion; attrition; transport (traction, saltation, suspension, solution, longshore drift) and deposition. Landforms and characteristic features and formation of wave cut platforms, cliffs, caves, arches, stacks, beaches, spits in the context of an example of an erosional coastline and a depositional coastline. Candidates should study two small stretches of coastline to illustrate erosional and depositional features and processes e.g. a headland as in the Lulworth Cove area, Marsden Rock, Flamborough Head, Hurst Castle, Spurn Point etc. The use of photographs and Ordnance Survey maps would prove helpful in this context. Candidates may be expected to name examples and describe the characteristics of the features and explain their formation with reference to the appropriate processes that have been at work. Recognise and describe coastal features on Ordnance Survey maps. The interaction between people and coastal environments. Coastal management issues, coastal erosion and tourism. The social, environmental and political problems caused by coastal erosion and tourism e.g. cliff collapse, coastal flooding, problems of resorts. Students should be aware of small scale exemplars of coastal areas where management issues occur. Such examples may include the Holderness coast (erosion), a Spanish resort and Bangladesh (coastal flooding). Students should be aware of the problems caused and their impact on different groups such as locals, governments and visitors. The strategies used to solve problems such as coastal defences e.g. groynes, sea walls, extending the tourist season and providing new facilities. The different values and attitudes of interested groups to coastal protection strategies and tourist developments and which strategies constitute sustainable development Students should be aware of the advantages and disadvantages of the strategies to different groups of people such as the impact further along a coast of building groynes and the possibility of overcrowding when new facilities are provided. Mechanical weathering This is the breakdown of rock without changing its chemical composition. The main type of mechanical weathering is freeze-thaw weathering:
  • 2. • The temperature alternates above and below 0ºc • The water flows into cracks in rocks • When the water freezes, it expands and puts pressure on the rock • When the water thaws, it contracts, which releases the pressure on the rock • Repeated freezing and thawing causes the cracks to widen and the rock to break up Chemical weathering Chemical weathering is the breakdown of rock by changing its chemical composition. Waves wear away the coast using four processes of erosion 1) Hydraulic power: Waves crash against rock and compress the air in the cracks. This puts pressure on the rock. Repeated compression widens the cracks and makes bits of rock break off. 2) Abrasion: Eroded particles in the water scrape and rub against rock, removing small pieces. 3) Attrition: Eroded particles in the water smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together. 4) Solution: Seawater dissolves rock like chalk and limestone because it’s a weak carbonic acid. Coastal Landforms caused by erosion Wave cut platforms: 1) Waves cause most erosion at the base of the cliff. 2) This forms a wave cut notch which is enlarged when more erosion takes place. 3) The rock above the notch breaks down and collapses. 4) This broken rock is washed away and another wave cut notch is formed. 5) The collapsing is repeated and this causes the cliff to retreat. 6) A wave cut platform is left behind. Headlands and Bays: 1) Some types if rock are more resistant to erosion than others. 2) Headlands and bays form where there are alternating bands of resistant and les resistant rock along the coast. 3) The less resistant rock is eroded quickly and this forms a bay – bays have a gentle slope. 4) The resistant rock is eroded more slowly and it’s left jutting out, forming a headland – headlands have steep sides. Coastal Management
  • 3. There are two methods to deal with coastal erosion: Hard engineering: man-made structures built to control the flow of the sea and reduce flowing and erosion. Soft engineering: schemes set up using knowledge of the sea and its processes to reduce the effects of flooding and erosion. Stategy What it is Advantages Disadvantages Prevents It creates a strong A wall made out erosion of the backwash which Hard of concrete that coast and acts erodes under the Sea Wall Engineering reflects waves as a barrier to wall. Very back to sea. prevent expensive to build flooding. and maintain. The boulders Can be moved by absorb wave strong waves, and Boulders that are energy, and so need to be Rock Armour piled up along the therefore reduce replaced. coast. erosion and flooding. Cheap. They deprive beach Groynes create Wooden/stone further down the wider beaches fences that are coast, making the which slow the built at right narrower. waves. This Groynes angles to the coast. Narrower beaches gives greater They trap material don't protect the protection from transported by coast as well, flooding and longshore drift. leading to grater erosion. Cheap. erosion and floods. Wider beaches Expensive. Has to created which be repeated. slow waves. Soft Beach Sand and shingle Greater Engineering nourishment added to beach. protection from erosion and flooding Dune Creating or Sand dunes Protection is regeneration restoring sand provide a limited to a small dunes by barrier between area. Nourishment nourishment or by land and the is expensive planting vegetation sea. Wave to stabilise the energy is sand. absorbed which prevents flooding and
  • 4. erosion. Stabilisation is cheap. The vegetation Marsh creation is stabilises the useful where mudflats and erosion rates are helps to reduce high because the Planting the speed of the marsh can't vegetation in Marsh creation waves. This establish itself. mudflats along the prevents expensive. coast. flooding and erosion. It also creates new habitats. Over time the People may land will disagree over what Removing an become land is allowed to existing defence marshland- flood. Managed retreat and allowing the creating new land behind it to habitats. flood. Flooding and erosion are reduced. Cheap. Waves erode weaknesses in the rock. The greatest line of weakness creates the largest cave when eroded. The size of the cave increases by further erosion by methods such as corrosion for example. This continues until the headland is opened on both sides. This creates an arch. Base erosion at the bottom of the arch causes the roof to collapse, leaving behind a stack, that has been separated from the mainland. Waves waves contribute to the majority of erosion along the coastline. WInd blown across the sea surface causes small ripples which eventually grow into waves. When a wave approaches the coast, it slows down due to friction with the sea bed. However, the upper part of the wave continues to move forward. As the upper part of the wave is unsupported, gravity acts upon it toppling it over. It then breaks forward against the cliff face or surges up the beach. Waves which erode the coastline are Destructive Waves.
  • 5. Three features which give destructive waves the qualities enabling it to erode the coastline are: - They are high in proportion to thier length. - The backwash is much larger than the swash. This in turn, causes rock. pebbles and sand to be carried back out to sea. - They are frequent waves, breaking at an average rate of between eleven and fifteen per minute. The height and destructiveness is determined by the distance the the wave has travelled and the wind's velocity. If the waves have been driven by the wind for a prolonged period of time or have travelled across a large area of ocean, they have had time to grow to thier full height, this means that alot of energy can be released when the wave breaks against the coastline. The length of water over which the wind has blown is called the fetch. In order to get a wave with greater eroding power, a greater fetch and stronger wind is required. Ideal conditions for the formation of large destructive waves occur occasionally and can be seen in the Case Study: Cornwall 4th January, 1998. The onshore south-westerly winds were strong with the gusts over 160 km/h and had a long journey over the Atlantic Ocean. This very large fetch and strong wind, causes a destructive wave, with great potential for coastal erosion. Transport Any material of any size, that is eroded along the coastline, is transported by the waves, and is then deposited further along the coast. More eroded materials are carried by the rivers that feed into the sea. The sediment that is picked up along the river is then gathered by the waves, and carried along with the eroded material from the coastline. Larger boulders roll along the sea bed, and then the smaller boulders bounce along, which is called saltation. Sand particles are carried in suspension, whilst the limestone and chalk is dissolved in the water, and is also carried off, but this time in solution. This transportation of materials such as sand, pebbles and boulders along the coast by the waves is called longshore drift. This occurs as waves have a tendency to approach waves at an angle, but the sand particles tend to leave the beach at a right angle, due to the fact that it is where the slope has the steepest gradient. This means that as pebbles are pushed up the beach at an angle, it will always come back down at a right angle. This means that the material is transported along the coastline. Around the British Isles, longshore drift is controlled by the direction of whichever wind is prevailing. If the south-westerly winds are dominant, it causes the drift to move from west to east along the channel coast, and from south to north on the west coast. The land covers the east coast from the
  • 6. south-westerly winds. On the other hand, northerly winds cross a lot of sea before the reach the coast, and therefore, despite the fact that winds are less prevalent than the south-westerly winds, they have the greastest influence on the longshore drift. Longshore drift has great importance over the formation of all the landforms of coastal deposition. Deposition The load of the waves, which are sand, shingle and pebbles, are deposited by constructive waves. These waves tend to deposit more than they remove in terms of material on the coastline. The main features that these waves exhibit, and are distinguished by are: - They are long in proportion to their height. - They break gently on the beach, which means that the swash is stong in comparison to the backwash. - They tend to be infrequent waves, with only 6-9 waves per minute. These waves are tradtionally associated with calm sea conditions, where the winds are light, and are not blowing directly onto the shore. This means that they occur more frequently in the summer, than in the winter. Constructive waves occur most effectievly when in a sheltered coastal location, such as a bay, which is sheltered by rocky headlands on both sides. Beaches A beach is the area of gently-sloping land between the high water mark and low water mark. This means that at high tide, the majority of this land is submerged with water. Physically, beaches can be straight and stretch along the coast for several miles. Other beaches, such as those found in bays, are curved. Beaches are most commonly made up of sand, shingle and pebbles. Many beaches are composed of whatever is deposited on the shore by the swash of the waves. More deposition can be seen in an area where the waves have a stronger swash than backwash. However, this deposited material does not stay for long and is moved along the beach due to longshore drift or carried back out to see by a waves' backwash. The materials from which a beach is formed are carried by longshore drift. If a coastline has weak rock, susceptible to erosion, the waves will be heavily laden with material. Whenever there is a bend in the coastline, it is more likely that deposition, via constructive waves, will take place as a more sheltered area has been created. More material builds up at this point and this causes a beach to build up. Spits
  • 7. A spit is a narrow ridge of sand or shingle (see picture). One end of the spit is attached to the land and the other end extends out to sea. In theory all a spit is, is a beach that extends out to sea rater than being completely attached to the coastline. If a spit is made from sand, sand dunes are usually produced at the back of it. The area of water behind this sand is usually muddy and marshland. Case Studies Scarborough
  • 8. cliffs, , transport + deposition , formation of (nikesh) beaches + spits , case study left to do (me)