- Created by: Russel Guittap
- Created on: 11-06-11 18:18
The coastal zone - The coastal zone is the transition zone between the land and the sea. Coasts are always changing. The way it looks and how it changes is a result of the land and sea working together. Coastlines are very places for people. Being on the coast has many advantages: access to the sea for fish and other resources, as well as access to farmland, good access for trade, and for connecting to other places, recreation and tourism oppurtunities.
Geology and rock type - The most important feature of a coast is often the type of rock in the area. Some rocks are resistant to erosion, whereas other rocks are more easily eroded: very resistant rocks are igneous rocks, such as granite and basalt,some sedimentary rocks like sandstone, limestone and chalk, are fairly resistant, weaker sedimentary rocks, such as clay and shale are the least resistant and will erode fastest.
Revise the diagram on page 72.
On coasts with hard, resistant rocks, erosion is slow. It may only be a few millimetres or centimetres a year. Most erosion happens during big storms, when waves are powerful. Gradually, erosion produces certain characteristic landforms that give the coast its shape: Wave power is concentrated at the base of the cliff, where the abrasion forms a wave cut notch, above this notch there is a cliff overhang, as the notch grows the overhanging cliff becomes unstable and eventually collapses, the resulting pile of rock debris at the base of the cliff protects the cliff from the further erosion, overtime the loose rock is eroded by attrition exposing to erosion again.
READ MORE ON PAGE 73 SPECIALLY THE DIAGRAM.
Geology at the coast
Rock structure - it simply means the way a different rock types are arranged. Rocks are generally found in layers, called strata. This means there may be several types of rock in one cliff. The cliff will only be as a resistant as its weakest layers. Rock starta can be arranged in two ways along coastines; If the layers are parallel to the coastline, the coast is concordant, if the layers are perpendicular (90°) to the coast, the coast is discordant.
Corcodant coasts have the same type of rock all the way along the coastline. Discordant coasts lots of diiferent rock types. When these two types of coast erode, different landforms are produced.
LOOK AT THE DIAGRAMS ON PAGE 74.
Concordant coasts, coves and cliffs: The most famous example of a concoradant coast is at Lulworth on the Jurassic coast in Dorset. This is a world heritage site because the geology here is so important. A resistant layer of hard portland limestone runs along the coast at Lulworth, hydraulic action and abrasion have eroded this and 'punched' through, exposing the less resistant rock behind, where the waves have been able to reach the softer rock, a cove has quickly widened, the erosion at Lulworth slows when the waves reach the more resistant chalk at the back of the cove, a steep chalk cliff has formed at Lulworth.
READ MORE AT TOP OF PAGE 75.
This cliff is faulty
Another factor which influences erosion is weakness in the rock forming the cliffs. There are two types of weakness; joints are small, natural cracks, found in many rocks and faults are larger cracks caused in the past by tectonic movements. The more joints and faults there are in a cliff, the weaker the cliff will be. Hydraulic action attacks the faults and joints causing erosion.
Discordant coasts, headlands and bays: In south west Ireland there is an unusual coastline, with very long headlands and bays. This is a discordant coast. Layers of resistant sandstones and less resistant limestones are found along the coast. Waves have eroded the limestones to form bays, leaving the harder sandstone as headlands.
READ MORE ON PAGE 75 SPECIALLY THE DIAGRAMS.
What causes waves? - When wind blows across the sea, friction between the wind and water surface causes waves. The size of waves depends on: the strength of the wind, how long the wind blows for, the length of water the wind blows over called the fetch. Some waves have fetch of thousands of miles. Waves can start near Florida and travel right across the Atlantic ocean before hitting cornwall, a fetch of about 6000 km.
LOOK AT DIAGRAMS ON PAGE 76.
Summer waves and winter waves - In the summer waves are small. They are called spilling waves. They have long wavelengths and low amplitudes. These are the type of waves everyone wuns away from on summer holiday, when they break they spill up the beach. They have a trong swash, this transports sand up the beach, the sand is deposited as a bank of beach berm.
In the winter, when storms and stong winds are more common, waves are different. They are taller and larger amplitude and shorter wavelength. Theseb are called plunging waves. They have strong backwash, this erodes sand from the beach, this creates a steep beach profile, the sand is carried offshore by an underwater rip current, the sand is deposited out at sea forming an offshore bar.
READ MORE ON PAGE 77 SPECIALLY THE DIAGRAMS.
Beach sediment - The material eroded from cliffs by hydraulic action and abrasion is called sediment. Sediment comes in many sizes, from tiny clay particles to larger sand and silt, right up to pebbles, cobbles and boulders. Sand is material that is 0.06 - 2 mm in size. Over time, attrition will make sediment smaller and rounder. Beach sediment is also transported from where it was eroded to new locations.
The drift - The main way sediment is transported is by longhore drift. This happens when waves break at an angle to the coast, rather that parallel to it. Because prevailing winds are are mostly from one direction, longshore drift is usually in one direction too. Longshore drifts transports sediment along coastlines - sometimes for hundreds of km's before it is eventually deposited.
Depositional landforms - When rocks are eroded, creating sediment, it will first be deposited very close to where it eroded. In a partly sheltered area such as a cove or bay, a beach will form because the sediment is trapped in the bay. Sediment transported by longshore drift will create new landforms where it is deposited.
READ MORE ON PAGE 78 AND 79 SPECIALLY THE DIAGRAMS.
Coasts and changing climate
Rising sea levels - many scientists fear that gloabl warming will cause sea levels to rise, between 30cm and 1 metre by the year 2100. Sea level is rising today as the sea is warming up and expanding, melting ice sheets are likely to speed up the process.
For people who live on very low lying land next to the sea would be troublesome. There are many areas around the world at risk. In Bangladesh if sea rose by 1 metre up to 15% of the country will be flooded, in the UK, London and essex are at risk, because they are low lying. Many small coral islands in the pacific and indian oceans like the Maldives and the Tuvalu could disappear underwater.
Flood risk - Sea levels are constantly changing. Twice a day, due to the gravity of the moon, high tides cause raise sea levels, a few times every year there are exceptionally high tides called spring tides. If spring tides occur when there are large waves, the sea will be even higher. Worse if spring tides and waves combine with low air pressure then a storm surge can form.
READ MORE ON PAGE 80 AND TOP OF 81.
With higher sea levels in the future and possibly increased storms some coastlines will experience faster erosion rates. This is a real problem for the east and south coasts of the UK, where the rocks are less resistant. Clays, shales and mudstones are soft and easily eroded. Sea level rises of 50 cm would increase erosion rates and make existing sea defences useless. The only choice would be to try and build new, expensive defences, or abandon some areas to the sea.
Falling into the sea
A complex problem - Some coastlines are eroding quickly by over 2 metres a year. In the UK the holderness coast in Yorkshire, the north Norfolk coast and some coastal areas of Hampshire and Dorset have very high erosion rates. All of these locations have weak rocks that easily collapse. They all suffer from erosion, but weathering and mass movement are making the problem much worse. Weathering and mass movement are called sub aerial processes. Water movement is often important in causing weak cliffs to collapse.
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Impacts - Erosion in Christchurch bay affects many people: home owners could lose their homes to the sea, house values will fall and insurance might be impossible to get, Rapid cliff collapses are dangerous for people on the cliff top and on the beach, Sometimes roads and other infrastructure are destroyed, Many people would say the erosion makes the area unattractive.
People in the area have argued that they need sea defenses to protect their coast. These are expensive, and there is no agreement about which type of defense works the best.
Managing the coast
Coastal management - Hard engineering: using concrete and steel structures such as sea walls to stop waves in their tracks. Soft engineering: using smaller structures, sometimes built from natural materials to reduce the energy in waves.
The hard method is the traditional method of coastal management. However it has two major problems: it is very costly, it usually makes the coast look unnatural and often ugly. On the positive side, large, very strong sea defenses may be the best way to stop erosion. Engineers have many different types of sea defenses they can use. They often use several types of sea defenses together.
MORE ON PAGE 84.
When a wave breaks on a wide beach, the energy of the wave is dissipated by the beach before it hits the cliff. On coasts with rapid erosion, there are often very narrow beaches or no beach at all. The traditional way of solving this problem is to grow a beach. This is done by building wooden or stone groynes across the beach at right angles to the coast. Each groyne cost £200,000-250,000. Groynes stop longshore drift by trapping sediment, building up the beach. But building groynes in one place stops sediment reaching other places further down the downdrift, so the beaches elsewhere can disappear. Solving the beah problem in one place creates problems somewhere else. This can lead to conflict.
Sea walls - It is likely that sea walls built today will have to be made higher in future to cope with rising sea levels. On low lying coasts, settlements could end up in a race to build higher defenses in the future. This will be very expensive.
Managing the normal way
Managing the whole coast - The modern way of managing the coast is called holistic management. This means managing a whole stretch of coast, like all Christchurch bay, not just one place such as Barton on sea. Holistic management takes into account the: needs of different groups of people, economic costs and benefits of different strategies today, and in the future, environment, both on land and in sea.
Intergrated coastal zone management (ICZM) - is the name for this approach to managing coasts. For long stretches of coast is drawn up, called a shoreline management plan (SMP), which sets out how the coast will be managed. This should prevent one place building groynes, if this will then cause more erosion downdrift.
Soft engineering - On many coasts, soft engineering is replacing hard engineering. This works natural processes, and tries to stop erosion by stabalising beaches and cliffs and reducing wave energy. Soft engineering solutions can be cheaper than hard, and often less intrusive.
READ MORE ON PAGE 86 AND TOP OF PAGE 87.
Here comes the sea
Holistic coastal management means that some places are not protected from flooding or erosion. This is because: protection would be to expensivem, as the value of the land and buildings does not justify the cost, building defences might cause more erosion somewhere else, it might become impossible very soon because of global warming and rising sea levels, it might be better for the environment to create new areas of marsh, for instance.
This type of management is seen as sustainable. In some places in the UK, sea defences have been abondened and nature is taking its course. In the next few decades, the UK will face many difficult decisions about how best to protect the coast: at the moment the government thinks its too expensive to protect farmland and isolated houses, residents, councils and businesses often disagree, it is very hard to persuade people who have lived by the coast all their lives that protecting their property is not sustainable, planning new sea defenses will be difficult as we dont know the impact of rising sea levels will be.