67 terms

The Coastal Zone


Terms in this set (...)

Processes of Coastal Erosion
Hydraulic Power, Corrasion, Abrasion, Solution, and Attrition.
Hydraulic Power
This involves the sheer power of waves as they smash onto a cliff. Trapped air is blasted into holes and cracks in the rock, eventually causing the rock to break apart as the gaps are widened. This explosive force acting in a crack is called cavitation.
This involves fragments of rock being picked up and hurled by the sea at a cliff. The rocks act like erosive tools by scraping and gouging the rock.
This is the 'sandpapering' effect of pebbles grinding over a rocky platform, often causing it to become smooth.
Some rocks are vulnerable to being dissolved by seawater. This is particularly true of limestone and chalk. which form cliffs in many parts of the UK.
This is where rock fragments carried by the sea knock against each other, causing them to become smaller and more rounded.
Constructive Waves
These waves surge up the beach with a powerful swash. They carry large amounts of sediment and 'construct' the beach, making it more extensive. These waves are formed by distant storms, which can be hundreds of km away. They are well-spaced apart and are powerful when they reach the coast. They also have a weak backwash (backwash erodes the beach).
Destructive Waves
These waves are formed by local storms close to the coast. They 'destroy' the beach. They are closely spaced and often interfere with each other, producing a chaotic, swirling mass of water. They rear up to form towering waves before crashing down onto the beach. There is little swash when this wave breaks, but a powerful backwash. This explains the removal of sediment from the beach.
Mass Movement
Mass movement is the downhill movement of material under the influence of gravity. They occur in 4 ways: rockfall, landslide, mudflow, and rotational slip.
Fragments of rock break away from the cliff face, often due to freeze-thaw weathering, and fall to the bottom to form scree. Rockfall can also be the collapse of a cliff face.
Blocks of rock that slide downhill. These blocks occur on the bedding plane and fall down the hillside on the slide plane forming detached, piled rock at the base.
Saturated soil and weak rock flows down a slope, e.g. clay. A stream is often present which contributes to the saturation. Mudflow usually occurs on bedrock and forms a lobe when flowing downhill.
Rotational Slip
Slump of saturated soil and weak rock along a curved surface. For this to occur the slip plane must be curved. The top of the slump is called the head and the bottom, the foot and toe.
Coastal Zone: Erosional Landforms
Erosion at the coast produces distinct landforms, such as headlands, bays, cliffs, wave-cut platforms, caves, arches and stacks.
Headlands are formed because they are made of rock that is particularly resistant to erosion. This means that, because they are usually surrounded by less resistant rock such as clay, they jut out from the mainland into the water, while the clay is eroded. The headlands then protect the space in between because they are subjected to the force of the waves.
Bays are formed by the erosion of the band of clay rock. When surrounded by bands of more resistant rock, they become sheltered due to the breaking up of waves by the headlands. These waves can then construct beaches in the coastal inlet as they deposit sediment due to their weakened backwash.
Wave-cut Platforms
When waves break against a cliff, erosion close to the high-tide line makes a small indentation in the cliff to form a feature called a wave-cut notch. Over time, the notch gets deeper until the overlying cliff collapses under its own weight. This process continues to form a gently sloping rock platform called a wave-cut platform as the cliff line retreats.
Lines of weaknesses in a headland, such as joints or faults, are vulnerable to erosion. Waves gouge out the rock along a line of weakness to form a cave. Hydraulic action and corrasion is significantly effective here. Erosion widens the joints and cracks in the headland.
Formed when back-to-back caves breakthrough the headland creating a large space in the rock. Through widening of its base by erosion, the roof of the arch becomes to heavy to be supported and collapses into the sea. Weathering processes, such as fauna and flora, weaken the roof causing it to crumble and break up.
Formed when the roof of an arch collapses to leave an isolated portion of the cliff. This stack is then subjected to erosion and wave-cut notches are formed on around the base of the rock.
Formed when the wave-cut notches on the stack become deep enough so §that the mass of the stack can no longer be supported, causing the rock to tumble into the sea as the rock has been undercut. This leaves a small stump which is covered at high tide.
Old Harry's Rocks
Made of chalk. Refer to erosional landforms for formation.
Swanage Bay
Based on Wealden Clay band. Bay formed due to being surrounded by more resistant chalk and limestone. Waves broken up by headlands causing a beach to form. Groynes present to stop longshore drift. For formation refer erosional landforms.
Coastal Transportation:
Coastal transportation occurs in 4 main ways: Solution, suspension, traction and saltation. Longshore drift is also considered a means of transportation.
The transportation of dissolved chemicals, usually derived from limestone or chalk.
The transportation of lighter particles that are suspended in the water.
The transportation of heavier objects, such as pebbles, along the seabed.
The transportation of particles that cannot be suspended along the seabed through a hopping/bouncing motion.
Longshore Drift
The transportation of sand and other light solids across the beach in a 'zig-zag' motion. This is most effective when the waves are 45º to the beach.
Depositional Landforms
There are 3 main landforms that occur because of deposition: beaches, spits and bars.
Beaches are the accumulation of sand and shingle. Sandy beaches are often found in sheltered bays where they are called BAY HEAD BEACHES. When waves enter these bays, they tend to bend to mirror the shape of the coast - REFRACTION. This is caused by the water becoming shallower. Wave refraction spreads out and reduces the wave energy in a bay, which is why deposition occurs. Berms are also common, particularly on shingle beaches, as they represent the different tides, such as storm and high tide.
Spit is a long, narrow finger of sand or shingle jutting out into the sea from land. Spits are common across world. As sediment is transported along coast by longshore drift, it becomes deposited at a point where the coastline changes direction or where a river mouth occurs. Gradually, as more and more sediment is deposited, the feature extends into the sea due to the presence of the prevailing winds. Away from the coast, the tip is affected by waves and wind approaching from different directions and the spit often becomes curved as a result.
Occasionally, longshore drift may cause a spit to grow right across a bay, trapping a freshwater lake or lagoon behind it. A barrier beach is sometimes formed when offshore bars have been driven onshore by rising sea levels.
Coastal Management
Coastal management comprises of soft and hard engineering techniques.
Coastal Management: Hard Engineering
This involves the construction of sea walls, groynes, and rock armour. Soft engineering consists of sustainable management of coast without using artificial structures.
Sea Walls: Description
Sea walls are concrete or rock barriers placed at the foot of cliffs or at the top of a beach. They have curved faces to reflect the waves back into the sea. Usually 3-5m high.
Seawalls: Advantages
1) Effective at stopping sea.
2) Often have walkways or promenades for people to walk on.
Seawalls: Disadvantages
1) Cost up to £6m per square km. C
an be obstructive and unnatural to look at.
2) Also very expensive and have high maintenance costs.
Groynes: Description
Timber or rock structures built on the beach and stretch into sea. They trap sediment being moved by longshore drift, thereby enlarging beach. The longer beach acts as a buffer to the incoming waves, reducing wave attack at coast. Cost £10,000 each per 200m interval.
Groynes: Advantages
1) Result in a bigger beach which can enhance the tourist potential of the coast.
2) Provide useful structures for people interested in fishing.
3) Relatively inexpensive.
Groynes: Disadvantages
1) In interrupting longshore drift, they starve other beaches downdraft, often leading to increased rates of erosion elsewhere.
2) Problem is not solved but rather shifted.
3) Groynes are unnatural and rock groynes are particularly unattractive.
Rock Armour: Description
Piles of large boulders dumped at the foot of a cliff. The rocks force waves to break, absorbing their energy and protecting the cliffs. Rocks usually brought in by barge. Cost £1,000 - £4,000/metre.
Rock Armour: Advantages
1) Relatively cheap and easy to maintain.
2) Can provide interest to the coast.
3) Often used for fishing.
Rock Armour: Disadvantages
1) Rocks are usually from other parts of coast or even abroad and so can be expensive to transport.
2) They do not blend with the local geology.
3) Can be very obtrusive.
Coastal Management: Soft Engineering
Soft engineering approaches try to blend with the natural coastline and processes. They do not involve large, artificial structures. They are more often 'low-key' with low maintenance costs, both economically and environmentally. Soft engineering such as beach nourishment are more sustainable. Includes beach nourishment, dune regeneration, and marsh creation (managed retreat).
Beach Nourishment: Description
Addition of san or shingle to an existing beach to make it higher/broader. The sediment is usually obtained locally so that it blends in with existing beach material. Usually brought onshore by barge. Also costs approx. £3000/metre.
Beach Nourishment: Advantages
1) Relatively cheap and easy to maintain.
2) Blends in with existing beach.
3) Increases tourist potential by creating a bigger beach.
Beach Nourishment: Disadvantages
1) Needs constant maintenance unless structures are built to retain beach, e.g. groynes.
Dune Regeneration: Description
Sand dunes are effective buffers to the sea yet they are easily damaged and destroyed, especially by human activity. Marram grass can be planted to stabilise the dunes and help them to develop. Areas can be fenced to keep people off newly planted dunes. Approx. £2000/100m.
Dune Regeneration: Advantages
1) Maintains a natural coastal environment that is popular with people and wildlife,
2) Relatively cheap.
Dune Regeneration: Disadvantages
1) Time-consuming to plant marram grass and fence off areas.
2) People sometimes dispute being prohibited from accessing certain areas.
3) Can be damaged by storms.
Marsh Creation: Description
Marsh creation involves allowing low-lying coastal areas to be flooded by the sea to become salt marshes. This is an example of managed retreat. Salt marshes are effective barriers to the sea. Cost depends on the value of the land as arable land cost approx. £5,000 - £10,000 per hectare.
Marsh Creation: Advantages
1) Cheap compared with maintaining expensive sea defences that might be protecting relatively low-value land.
2) Creates a much-needed habitat for wildlife.
Marsh Creation: Disadvantages
1) Land will be lost as it is flooded by sea water.
2) Farmers or landowners will need to be compensated.
Managed Retreat
One other option for coastal management is to allow some retreat of the coastline. Sometimes called coastal realignment. Good option if there is considerable risk of flooding, cliff collapse, and where the land is relatively low in value. Poor quality grazing land, for example, is not worth protecting if the costs of the defences outweigh the benefits. This appraisal is known as coast-benefit analysis. This approach is likely to become more likely as sea levels continue to rise.
Case Study: Holderness Coast
Processes acting on the Holderness Coast: longshore drift, mass movement. This occurs because of destructive waves and boulder clay, respectively.
Protective strategies such as rock armour, groynes and managed retreat.
Holderness Coast: Coastal Erosion
This occurs on the beach because of longshore drift and destructive waves. In response to this, groynes are used to stop this process. Mass movement also occurs due to the easily saturated boulder clay land. Nothing can be done to stop this. Rock armour is also used in different places to reduce the energy of the waves. However, these are vulnerable to erosion because they contain air spaces that allow hydraulic action.
Rising Sea Levels
Rising sea levels are mainly caused by the thermal expansion of the seawater as it absorbs more heat from the atmosphere. The melting of ice on the land will increase the amount of water in the sea but will not significantly affect sea levels. Similarly, melting sea ice will have no direct affect on sea levels. However, the amount of sea level rise depends on the rate of global warming.
Case Study: East Anglia and the Thames
This area is most at risk in the UK of rising sea levels. Areas such as the Fens, Norfolk Broads, Essex and the Thames are at risk.
Case Study: East Anglia and the Thames
1) Settlements such as King's Lynn may be under threat as sea levels rise. Valuable agricultural land (the Fens) will be at a greater risk from flooding.
2) The Norfolk Broads are a popular tourist destination that produces £5m+ for the local area. Sea level rise would flood the broads.
3) As sea levels rise, coastal erosion is likely to increase, thus threatening coastal settlements such as Overstrand and Happisburgh. It will also be expensive to protect these as sea defences will require strengthening.
Case Study: East Anglia and the Thames
4) 1953, East Anglia suffered from a storm surge, which killed 300 people. People are therefore worried that this will happen again.
5) Low-lying mudflats and marshes in Essex are particularly vulnerable to sea-level rise. Areas of salt marsh are being squeezed between sea walls and rising sea. 22% of E. Anglia's salt marsh could be lost by 2050. In some places, managed retreat will breach the sea walls to allow deliberate flooding of salt marshes.
Case Study: East Anglia and the Thames
6) The Thames Barrier currently protects buildings worth £80bn. It will probably need to be replaced in the next 30-50yrs. As sea levels rise, large areas of the lower Thames estuary will be at risk from flooding, affecting housing, industry and farmland.
Salt Marshes and Coastal Habitats
1) Salt marsh begins as an accumulation of mud and silt in a sheltered part of the coastline, e.g. behind a spit or bar (the lee).
2) As more deposition takes place, the mud begins to break the surface to form mudflats.
3) Salt-tolerant plants such as cordgrass soon start to colonise the flats. These early colonises are called pioneer plants. Cordgrass also has long roots which prevent it from being washed away. These roots also stabilise and trap the sediment.
Salt Marshes and Coastal Habitats
4) As the level of the mud rises, it is less frequently covered by water. The conditions become less harsh as rainwater begins to wash out some of the salt and decomposing plant matter improves fertility of the newly forming soil.
5) New plant species such as sea asters start to colonise the area and gradually, over hundreds of years, a succession of plants develops. This is known as a plant succession as a new, more capable plant pushes less capable plants out.
Case Study: Keyhaven Marshes, Hampshire
Formed in lee of Hurst Castle spit. Supports a range of habitats including grassland, scrub, salt marsh, and reed beds. This variety accounts for a rich diversity of wildlife, including: plants, birds and insects.
Keyhaven Marshes: Wildlife
1) Plants - Cordgrass, Sea lavender (attracts wildlife).
2) Birds - Oystercatcher (feeds and nests in salt marshes), Ringed Plover (feed intertidally and nests on salt marshes).
3) Butterflys - Common Blue (resident butterfly commonly found on higher marshes).
4) Spiders - Wold Spider (clings for hours to submerged stems of cordgrass waiting for low tide and food).
Keyhaven Marshes: Risks
People are particularly concerned that the marsh will be come increasingly 'squeezed' between the sea and and sea walls. Furthermore, breaching of the spit in storms, for example, poses a significant threat to the marsh as a section 50-80m was exposed to the full force of the sea and consequently eroded in just 3 months.