Coasts revision

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  • Created by: Laura P
  • Created on: 31-05-13 11:15

Key Words

Coast - where the land meets the sea.


Waves - the wind pushing the water.


Crest - top of a wave.


Trough - bottom of a wave.


Marine Erosion - wearing away of rocks by the action of the sea.


Weathering - breaking down of rocks by weather, plants or chemical action.


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Key Words (2)

Prevailing Wind - the dominant wind in an area.


Swash - movement of water travelling up the beach.


Backwash - movement of water travelling away from the beach.


Fetch - distance the waves have travelled .


Constructive wave - a wave with a stronger swash .


Destructive wave - a wave with a stronger backwash.

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Landforms (Hard Coastline)

- Headlands and Bays

- Caves

- Arches

- Stacks 

- Stumps

- Wave cut platfoms 

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Retreat of Soft Coastlines

Slides - material shifts in a straight line.

Slumps - material shifts with rotation.

Rockfalls - material shifts vertically.

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Waves

What causes waves?

  •  
    • When wind blows over the surface of the sea.
    • Friction with the surface of the water causes ripples to form.
    • With stronger winds the ripples will turn into waves.

The height & strength of a wave depends on:

  •  
    • The speed of the wind.
    • The time the wind blows for.
    • The length of water the wind blows over- called the fetch.

(http://rescuevoices.com/Stories%20By%20Author%20Name/Tim%20Wiley/wavediagram.jpg)

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How does a wave break?

  • The seabed interrupts the circular movement of the water (friction).
  • As the water becomes shallower, the circular movement becomes eliptical.
  • This makes the top of the wave rise up and topple over to 'break' on the beach.
  • Water rushing up the beach is called swash.
  • Water returning down the beach is called backwash.

(http://geographyas.info/Images/coasts/2waves/waveMotionDiagram.png)

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Constructive Waves

  • Constructive waves deposit
  • Strong Swash.
  • On beaches with gentler gradient.
  • More frequent in spring and summer.
  • Less steep with low energy.
  • Constructive waves are formed by distant storms.
  • Well spaced when they reach the coast.
  • Powerful enough to carry sediment in the swash, which is then deposited to construct the beach.
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Destructive Waves

  • Destructive waves erode.
  • Strong backwash.
  • Occur on beaches with steeper gradient.
  • Local storms.
  • More frequent in autumn & winter.
  • Steeper with high energy.
  • Closely spaced with a powerful backwash which removes sediment from the beach (erosion).
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Wave Refraction

  • Refract means to change direction or to bend.
  • Wave refraction refers to wave fronts changing direction as they approach the shore.
  • In deep water the wave fronts are parallel.
  • As waves approach the headland the speed reduces.
  • This causes them to bend towards the headland and attacks it from the side.
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Marine Erosion/Weathering

Corrosion - The weak carbonic acid in the sea water dissolves rock like chalk and limestone.

Hydraulic action - The force of the water against rocks makes cracks and eventually bits of rock break off.

Corrasion/abrasion - Where the water throws sand and pebbles against the rock which gradually wears it away.

Attrition - Rocks are smashed together and made into smaller pieces by waves and their edges are rounded off.

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Weathering

Weathering is the break down of rocks in their original place (in situ).

There are 3 types weathering:

Mechanical (physical) - No chemical change.

Chemical - Acid rain.

Biological - Caused living organisms.

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Mechanical Weathering

Freeze thaw 

Freeze thaw weathering involves the action of water as it freezes and thaws in a crack or hole in the rock. It happens when there is plenty of water and where temperatures fluctuate above and below freezing point.

1) Water collects in cracks in rocks.

2) Water freezes to form ice.

3) Expansion causes stresses and cracks are enlarged.

4) After repeated freezing and thawing, rock fragment breaks off and collects as scree at the foot      of the rock face.

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Chemical Weathering

Carbonation

1) Limestone is made of calcium carbonate.

2) When carbon dioxide is dissolved in water, it makes a weak acid called carbonic acid.

3) When carbonic acid comes into contact with limestone it reacts with the rock to form calcium       biocarbonate.

4) The calcium biocarbonate is soluble and is carried away in solution, gradually weathering the         limestone or chalk rock.

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Biological Weathering

  • Animals and plants can wear away rocks - For example, burrowing animals such as rabbits can burrow into a crack in a rock, making it bigger and splitting the rock.
  • Plant roots are effective at growing and expanding in cracks in the rocks.
  • People can also cause biological weathering by walking.
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Land Forms - Cliffs and Wave-cut notches

Cliffs

  • Cliffs are shaped through a combination of erosion and weathering.
  • Soft rock e.g. sand and clays, erode easily to create a gently sloping cliff.
  • Hard rock e.g. chalk, is more resistant and erodes slowly to create steep cliffs.

Wave- cut notch

1) Weather weakens the top of the cliff.

2) The sea attacks the base of the cliff, forming a wave cut notch.

3) The notch increases in size, causing the cliff to collapse.

4) The backwash carries the rubble towards the sea, forming a wave-cut platform.

5) The process repeat and the cliff continue to retreat.

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Land Forms - Cliffs and Wave-cut notches

Cliffs

  • Cliffs are shaped through a combination of erosion and weathering.
  • Soft rock e.g. sand and clays, erode easily to create a gently sloping cliff.
  • Hard rock e.g. chalk, is more resistant and erodes slowly to create steep cliffs.

Wave- cut notch

1) Weather weakens the top of the cliff.

2) The sea attacks the base of the cliff, forming a wave cut notch.

3) The notch increases in size, causing the cliff to collapse.

4) The backwash carries the rubble towards the sea, forming a wave-cut platform.

5) The process repeat and the cliff continue to retreat.

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Land Forms - Caves, arches, stacks & stumps

Headlands can be vulnerable to erosion because they stand out from the rest of the coast, so overtime other features may develop on a headland.


Caves- Occur when waves force their way into cracks in the cliff face. Hydraulic action.

Arches- If the cave has been formed in a narrow headland, it may be enlarged and break through                the headland forming an arch.

Stacks- The arch will gradually become bigger until the roof collapses and is seperate from the                 headland.

Stump- The stack will then be worn away until it collapses to form a stump.

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Bays and Headlands

  • Headlands are formed when the sea attacks a section of coast with alternating bands of hard and soft rock.
  • The bands of soft rock, such as sand and clay, erode more quickly than those of more resistant rock such as chalk.
  • This leaves a section of land jutting out into the sea called a headland.
  • The areas where the soft rock has eroded away, next to the headland, are called bays.
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Transportation

Solution- Minerals are dissolved in sea water and carried in solution - solute load.


Suspension- Small particles carried in the water.


Saltation- Particles too big to be suspended are bounced along the seabed.


Traction- Large pebbles rolled along the seabed

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Longshore drift

Waves carry eroded material away. Some is carried out to sea. But a lot is carried along the coastline. This process is called longshore drift.

1) Waves approach the beach at an angle.

2) As the waves break the swash carries material up the beach at the same angle as the wave        approached.

3) As the swash gets weaker, the backwash and any material carried by it falls straight back            down the beach at a 90 degree angle under the influence of gravity.

4) This means material is moved along the beach in a zig-zag route.

Longshore drift is the link between erosion and deposition along the coast- eroded material is transported and then deposited furthur along the coast.

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Stopping longshore drift

A common method of protecting beaches is the use of groynes.

These are like fences, running down a beach into the sea. 

They interupt the process of longeshore drift and so help prevent the beach material from being transported away.

Longshore drfit can cause parts of the beach to be heavily eroded and parts to be built up because longshore drift takes sediment away from parts of the beach which means the base of the cliff is no longer protected.

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Spits

Spits are long, narrow ridges of sand or shingle running out from the coast with one end attached to the land. They form when the direction of the coast changes. 

  • The sea must be relatively shallow.
  • There must be a change in the direction of the coastline.
  • There must be a good supply of sand and other material.
  • Waves must approach the coast at an angle, so that longshore drift can work.
  • The sea must be fairly calm with constructive waves.

Many spits develop a hooked or recurved end. As the spit grows out into deeper water, the sand or shingle is more easily forced towards the land by the stronger waves.

Because the sea is so powerful, spits can change shape quite quickly.They can be breached and even destroyed by storms - but longshore drift will gradually rebuild them.

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Coastal Managment

Hard Engineering

  •  
    • Uses man-made structures to reduce erosion and flooding.
    • They are often expensive, ugly and not sustainable.

Soft Engineering

  •  
    • Works with natural processes, causing less damage than hard techniques which battle against nature, often relocating the damage.
    • They are often less expensive, attractive and more sustainable.
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Hard Engineering

Sea walls: Concrete or rock barrier, placed at the foot of cliffs or at the top of beaches.

Cost = £6 million per km.


+ Effective at stopping the sea.

+ Stops the sea from reaching valuable land.

- Can ruin scenery.

- Very expensive and high maintenance costs.


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Hard Engineering

 Groynes: Timber/rock structures built out to sea which stops sand being                                           carried away by longshore drift making a larger beach which acts as a                             natural sea defence - slowing down waves.

Cost = £10,000 each.

+ Results in a bigger beach which can enhance the tourist potential.

+ Prodive useful structures for people interested in fishing.

+ Not too expensive.

- Interupts longshore shift so beaches further are starved.

- Groynes are unnatural.

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Hard Engineering

Rock amour: A large barrier of rocks at the foot of the cliff - can reduce the impact of waves.

Cost= £1,000- £4,00 per metre.

                            + Cheap and easy to maintain.

                            + Can provide interest to the coast- used for fishing.

     - Rocks can be expensive to transport.

     - They do not fit with the local geology.

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Soft Engineering

Beach nourishment - The addition of sand or shingle to an existing beach to make it                                           higher and broader. 

Cost = £3,000 per metre.

+ Relatively cheap and easy to maintain.

+ Blends with existing beach.

+ Increases tourist potential.

- Needs constant maintenance.

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Soft Engineering

Dune regeneration - Marram grass can be planted to stabalise sand dunes.

Cost= £2,000.

+ Maintains a natural coastal environment that is popular with people & wildlife.

+ Relatively cheap.

- Time consuming to plant.

- Can be damaged by storms.

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Soft Engineering

Managed Retreat - Involves allowing low-lying coastal areas to be flooded by the sea to                                 become salt marshes which are effective barriers to the sea.

+ A cheap option compared with maintaining expensive sea defences.

+ Creates a habitat for wildlife.

- Land will be lost as it is flooded by sea water.

- Farmers or landowners will need to be compensated.

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Case Study- Happisburgh- Cliff collapse & erosion

Where? Village on the Norfolk coast line.

What? Stretch of coastline is being eroded away very quickly.

Why? Rocks were laid down 12,000 years ago therefore the rock is soft and easily eroded.

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Case Study- Happisburgh- Cliff collapse & erosion

1) The main problem is that cliffs are soft- sand on top and clay below. (geology)

2) Rain soaks into the cliffs and helps to weaken them (weathering).

3) Meanwhile, the waves erode the cliffs from below.

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Case Study- Happisburgh- Cliff collapse & erosion

What has been done to protect the coastline?

The coastline has been eroding for 12,000 years but no one was effected so no one cared. It's only recently that settlements have been threatened.

Happisburghs sea walls date from the 1950's and have been damaged by storms etc, so therefore have very little effect now. They are at risk of not being renewed because DEFRA (department of environment...) have analysed that it will not be cost effective.

  • DEFRA have decided that the coastline isn't worth protecting. This is mainly because the village is far enough away from the sea not to be in imediate danger. But also because new defences will cost more than the land and houses are worth.
  • No coastal defence system get the go-ahead if people object and there have been some objectors.
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Case Study- Happisburgh- Cliff collapse & erosion

What's the future for Happisburgh?

Without support from the government, the local council has spent £160,000 installing boulders in attempt to stop the waves. It is only a temporary solution.

The future remains uncertain. The government doesn't think it's right to spend tax-payers money on hard defences for such a sparsly populated part of the coastline. Meanwhile, a 'do nothing' approach is taking place.

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Case Study- London - Impact of coastal flooding

Where? River thames estuary, London.

When? In January 1953 there was a huge flood which let to the death of 300 people which is why there has now been the construction of the Thames barrier.

What? Thames barrier built in 1974 and took 8 years to build. Cost £1.6 billion.

Materials of the gates:

  • Concrete peirs that hold operating machines.
  • Gates are made of steel and are hollow.
  • Is there is a flood, they close and form a steel wall. 

With recent gloabl warming, there has been water levels rising and this meant that the barrier needs to be updated faster then expected. This will cost an extreme amount the goverment doesn't have, but will decrease unemployment.

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Salt marshes

Salt marshes are ares of coastal grassland that is regularly flooded by seawater.

Salt marshes support a range of habitats including grassland, scrub, salt marsh and reed beds, accounting for a wide diversity of wildlife.

(http://waylandscape.co.uk/assets/images/Salt-Marsh.jpg)

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Formation of salt marshes

Stage 1: Mud and silt accumulates in a sheltered coastal area, for example in the lee (behind) of a spit. Over time the mud builds up and breaks the surface to form mudflats.

Stage 2: Salt-tolerant plants (cordgrass) colonise the mudflats. These early colinisers are called pioneer plants and have long roots to stop them being swept away and stabilizes the mud.

Stage 3: The more the mud rises, it's less covered by water. The conditions are less harsh as rainwater washes out some of the salt and decomposing plant matter creating new soil. Sea lavenders and sea asters dominate.

Stage 4: The marsh uplands (oaks and shrubs) dominate so the smaller plants and species die out. This is known as vegetation succession. This area is rarely covered in water.

Key words

Pioneer plants: the first plant species to colonise an area.

Vegetated succesion: a sequence of vegetation species colonising an environment.

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Why are salt marshes important?

  • Important habitats for a variety of plant and animal species allowing a diverse ecosystem to develop.
  • They attract people to visit them, therefore providing an economic benefit to the local area.
  • In some locations, the salt is extracted for commercial usage.
  • Salt marshes can provide an important soft management tool when used as part of managed retreat. They're effective barriers against the sea and a cheap option compared to hard strategies.
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Case Study- Keyhaven Salt Marshes, Hampshire

Background:

  • Located behind Hurst Castle spit
  • Supports a range of habitats accounts for a rich diversity of wildlife in the area- including cordgrass, sea lavender, spiders, butterflies.

But they are at risk:

  • Retreating up to 6 metres a year, which threatens a squeeze of the salt marsh.
  • Human activities such as trampling, parking and pollution wear away the salt marshes.
  • With sea levels expected to rise by 6 mm a year, the big issue is the "squeeze" between the low sea wall and the rising sea.
  • Under threat from the severe storm breaching Hurst Castle Spit. In 1989, it was eroded in less than 3 months. 
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Case Study- Keyhaven Salt Marshes, Hampshire

What has been done to manage/protect the salt marshes?

  • In 1996, rock armour and beach nourishment helped increase the height and width of the spit, in attempt to help the creation of the salt marshes. Since the £5 million defences, the spit has not been breached and the Keyhavnen Marshes seems safe.
  • The area is officially a site of special scientific interest (SSSI) and partly a National Nature Reserve. This means the area is carefully monitered and managed. Access and development have been limited.
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How will rising sea levels affect the coastal zone

Global warming is causing sea levels to rise:

- Massive amounts of water are being added to the oceans because the worlds ice sheets and  glaciers are melting.

Global warming and rising sea levels mean:

  • There will be more storms at sea.
  • Erosion around the coast will be speeded up.
  • Homes and land in areas without strong sea defences will be lost.
  • Our existing sea defences won't be strong or high enough so there will be more sea flooding.

(In the UK, East Anglia will be hardest hit by rising sea levels which will threaten coastal defences, low lying areas and natural ecosystems.)

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