Coasts (DOC) by liamei12345

VIEWS: 39 PAGES: 19

									Coasts




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Waves
How waves form?

Waves are usually formed by the wind blowing across the sea. This causes ripples on the top of the
surface and these develop into waves. The stretch of open water over which the wind blows is called
the fetch. The longer the fetch the more powerful the wave.

Video showing wave formation - http://www.bbc.co.uk/learningzone/clips/understanding-wave-
formation/4018.html




The above diagram shows you what happens when a wave moves closer towards the coast and
crashes onto a beach.

Crest - this is the top part of the wave and eventually topples onto the beach.
Swash - this is the water that rushes up the beach
Backwash - this is the water that flows back towards the sea

Types of waves

There are two types of waves; constructive and destructive waves.
Constructive waves:




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                                            Constructive waves are waves that surge up the beach
                                             and have a powerful swash
                                            They carry large amounts of sediment and 'construct'
                                             the beach making it more extensive


Main characteristics: They have a strong swash and a weak backwash and are smaller in height
compared to destructive waves. They are waves which contain a small amount of energy.

Destructive waves

                                            Destructive waves are named because they 'destroy'
                                             the beach.
                                            When the waves hit the beach they rear up and smash
                                             down onto the beach.
                                            There is very little swash when the wave breaks but
                                             has a powerful backwash.
                                            The backwash removes the sediment which leads to
                                             the 'destruction' of the beach


Main characteristics: They have a strong backwash, lots of energy and are waves which are high in
height




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Processes influencing our coastlines
Coastal processes can be split into two different categories:


                                               Land processes influencing our coastlines
                                               Marine coastline influencing our coastlines


Video showing the processes of erosion - http://www.bbc.co.uk/learningzone/clips/weathering-
erosion-and-coastal-features/4022.html

Excellent animation showing coastal processes and defences -
http://www.as.uky.edu/academics/departments_programs/EarthEnvironmentalSciences/EarthEnviron
mentalSciences/Educational%20Materials/Documents/elearning/module14swf.swf

Weathering

The processes of weathering affects rocks exposed at the coast. Freeze-thaw weathering in
particularly effective if the rock is porous (contains holes) and permeable (allows water to pass
through it). This can lead to dramatic rockfalls as shown below.


Full explanation as to how freeze-thaw weathering works can be found at the following link:

http://onlinegeography.wikispaces.com/Ice+as+a+shaping+agent

Mass movement
Mass movement is the downhill movement of material under the influence of gravity. In 1993 60m of
cliff slid onto the beach near Scarborough in North Yorkshire, taking with it part of Holbeck Hall Hotel.




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Marine Processes
Coastal erosion

Erosion – use the acronym CASH
Corrasion – (abrasion) is caused by large waves hurling beach material against the cliff
Attrition – is when waves cause rocks and boulders on the beach to bump into each other and
to break up into small particles
Solution – (corrosion) is when salts and other acids in seawater slowly dissolve a cliff
Hydraulic pressure (power/action) – is the force of waves compressing air in cracks in a cliff
One final process of erosion is:

Abrasion: this is the 'sandpapering' effect of pebbles grinding over a rocky platform, often causing it to
become smooth.

Coastal transportation

Processes of Transportation


                                               Traction – rolling stones along the sea floor (needs the
                                                most energy)
                                               Saltation – sand-sized particles bounce along the sea
                                                floor in a ‘leap frog’ movement
                                               Suspension – silt and clay-sized particles are carried
                                                within the water flow·
                                               Solution – some minerals dissolve in the water (this needs
                                                the least energy)


Longshore Drift
Video showing how Longshore Drift works - http://www.bbc.co.uk/learningzone/clips/landforms-
created-by-longshore-drift-and-coastal-deposition/9965.html

                                               Waves approach the beach in the same direction as
                                                the wind
                                               When the wave breaks, swash carries material up the
                                                beach at the same angle as the wind
                                               The backwash carries the material straight back down
                                                the beach under gravity·
                                               This process slowly moves material along the coastline




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Remember when drawing:


                                            To draw in a beach
                                            An arrow showing the prevailing wind
                                            An arrow showing the direction of longshore drift
                                            To draw in the movement of the pebbles
                                            To use the terms ‘swash’, ‘backwash’ and ‘gravity’ when
                                             describing the movement of the pebbles


Coastal deposition

Deposition occurs in areas where the flow of water slows down. The sediment can no longer be
carried or rolled along an has to be deposited. Coastal deposition most commonly occurs in bays,
where the energy if the waves is reduced on entering the bay. This explains the presence of beaches
in bays and accounts for the lack of beaches at headlands, where the energy of the waves is uch
greater.




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Landforms of erosion
Very useful BBC videos:
Formation of landforms - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-and-
landforms/9966.html
Wave-cut platforms and headlands/bays - http://www.bbc.co.uk/learningzone/clips/wave-cut-
platforms-and-headland-erosion/4021.html
Weathering and erosion - http://www.bbc.co.uk/learningzone/clips/weathering-erosion-and-coastal-
features/4022.html
An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-
dorset/3244.html
An example of stump formation - http://www.bbc.co.uk/learningzone/clips/old-harry-rocks-coastal-
processes-and-landforms/3245.html
More landforms - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-west-wales/3084.html
Cliff slumping - http://www.bbc.co.uk/learningzone/clips/coastal-landforms-blowholes-and-cliff-
slumping/3085.html


Headlands and bays

Cliffs rarely erode at an even pace. Sections of cliff that are particularly resistant to erosion stick out to
form headlands. Weaker sections of coastline that are more easily eroded form bays.




                                                 Form where there are alternating outcrops of resistant
                                                  outcrops of resistant (harder) and less resistant
                                                  (softer). ·
                                                 Destructive waves erode the soft rock move quickly to
                                                  form bays·



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   The harder rock is more resistant and are left
    protruding into the sea·
   The headlands protect the adjacent (next by) bays
    from destructive waves
    As the headlands protect the bays sand is deposited to
    form a beach. At the headlands there often wave-cut
    platforms and notches.

    Cliff and wave-cut platforms




   Wave erosion is greatest when large waves break against
    the foot of the cliff ·
   The waves undercut the foot of the cliff to form a wave-cut
    notch ·
   Over time the notch enlarges the until the cliff above it is
    unsupported and collapses ·
   The gentle sloping expanse of rock marking the foot of the
    retreating cliff is called a wave-cut platform ·
   Wave-cut platforms are exposed at low tide but hidden at
    high tide




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Caves, arches and stacks




When drawing a diagram label:


                                   What and where the different processes of erosion are
                                    occurring ·
                                   Where wave-cut notches may be formed ·
                                   Check you have the correct sequence·
                                   Explain what hydraulic power, corrosion and abrasion




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Landforms of deposition
Video showing all of the coastal landforms of deposition -
http://www.bbc.co.uk/learningzone/clips/depositional-coastlines/4023.html
Beaches



                                                They are usually found in sheltered bays between two
                                                 headlands
                                                The headlands protect the area from erosion
                                                Low constructive waves deposit material on the shore
                                                Gradually a beach is built up
                                                Material on a beach is well sorted – the biggest pebbles are
                                                 nearest the land with the smallest nearest the sea
                                                The larger the material the steeper the beach – pebble is
                                                 steeper than sand



Spits
http://www.geographyalltheway.com/ks3_geography/coasts/imagesetc/deposition.swf - an amazing
animation which explains the formation of a spit!!
Video showing the formation of a spit - http://www.bbc.co.uk/learningzone/clips/the-formation-of-
spits/3246.html




A spit is an area of sand or shingle that has been transported by longshore drift and then deposited
as the coastline has changed direction. It is attached to the land at one end. It is a depositional
landform. Hurst Castle Spit in Hampshire is a very famous example.

Where the coastline changes direction, sediment is deposited in the same direction as the original
coastline (i.e. in line with the prevailing wind direction). Where there is a break in the coastline and a
slight drop in energy, longshore drift will deposit material at a faster rate than it can be removed and
gradually a ridge is built up. The material is deposited in the deeper water offshore until the ridge is
built above the level of the sea. Drift continues along the seaward side of the spit extending it further

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down the coast while salt marsh develops in the slow-moving water on the landward side.

Spits can become a permanent feature. This happens when the prevailing wind picks up sand from
the beach and blows it inland across the spit to form sand dunes. These dunes will then be colonised
by vegetation, which stabilises them. It is common for a salt marsh to develop in the sheltered area
of water behind the spit. Water is trapped behind the spit, creating a low energy zone. As the water
begins to stagnate, mud and marsh begin to develop behind the spit.

A spit may grow out across a river estuary. Where the spit is crossing a river mouth, the river will be
diverted so that it follows the coastline for some miles before reaching the sea.

Bars




Bars can form in several ways:
(a) a spit grows the whole way across a bay
(b) a sandbank develops offshore, parallel to the shore, and is moved towards the coastline by the
waves and wind until it joins the mainland


Slapton Sands is an example of a bar. The lagoon of water than has formed on the landward side of
the bar is called Slapton Ley.


A tombolo is formed where a spit joins an island to the mainland. An example is the Isle of Portland
which is joined to the mainland by a shingle ridge known as Chesil Beach.




Useful weblinks:

BBC Class Clips video - Blakeney Point
BBC Class Clips video - Kaitorete Spit, New Zealand
BBC Scotland video about spits, bars and tombolos
BBC Bitesize - Spits
BBC Bitesize - Tombolos




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Sea-level rise
Videos on Youtube channels
The causes of sea-level rise:

One of the effects of global warming is sea-level rise. Over the last 15 years, global average sea
levels have risen by 3mm a year. The latest estimate from the Intergovernmental Panel on Climate
Change (IPCC) suggests a rise in global sea levels of between 28 and 43 cm by the end of the
century.

The main cause of sea-level rise is thermal expansion of seawater as it absorbs more heat from the
atmosphere. Also the melting of ice on land e.g. from Greenland and will increase the level of the
oceans.

The actual amount of sea-level rise will vary from place to place due to variations in the level of the
land and the amount of deposition of sediment occurring at the coast.

The effects of sea-level rises:

In the UK, East Anglia is likely to be hardest hit and this threatens coastal defences and natural
ecosystems. Elsewhere in the world, vast areas of low-lying coastal plains such as Bangladesh and
whole chains of islands such as the Maldives and Tuvalu could disappear. More than 70% of the
world's population live on coastal plains so the effects of sea-level rises are going to be devastating.




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Cliff Collapse
BBC videos on cliff collapse:
Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-dairy-farm-is-threatened-
by-coastal-erosion-at-holderness-east-yorkshire/3190.html
Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-farm-is-threatened-by-
coastal-erosion-at-holderness-east-yorkshire/3189.html
Buildings threatened by erosion - http://www.bbc.co.uk/learningzone/clips/a-house-threatened-by-
coastal-erosion/3188.html
Erosion at Hallsands - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-at-hallsands-causes-
and-effects/9967.html
Erosion at Holderness - http://www.bbc.co.uk/learningzone/clips/coastal-erosion-at-holderness-east-
yorkshire/3187.html
Erosion at Hallsands - http://www.bbc.co.uk/learningzone/clips/hallsands-destruction-of-
coastland/3242.html
Case study on Happisburgh - http://www.bbc.co.uk/learningzone/clips/living-with-coastal-erosion-in-
happisburgh-east-anglia-pt-1-2/7361.html
Case study on Happisburgh - http://www.bbc.co.uk/learningzone/clips/living-with-coastal-erosion-in-
happisburgh-east-anglia-pt-2-2/7364.html
Various factors can contribute to cliff collapse. These include:


                                              weathering processes - such as heavy rainfall that can
                                               saturate the land and make it unstable
                                              mass movement such as sliding and slumping - which
                                               is more likely if the land is made of soft weak rock
                                               types
                                              the power of the waves - crashing against the cliffs and
                                               undercutting them from below


Barton-on-Sea, Hampshire

Barton-on-Sea is a small village in Christchurch Bay and has long been affected by coastal erosion
and cliff collapse.

Over the years a number of buildings and most recently a cafe have been lost to the sea.

Sea defences have been built to prevent coastal erosion but in 2008 there was a fresh landslip. The
older houses in the settlement are now only 20m from the sea. It is expected that the houses will be
lost to the sea in the next 10 to 20 years.

Why are the cliffs at Barton-on-Sea so prone to collapse?


                                              the rocks are weak sands and clays. They are easily
                                               eroded by the sea and have little strength to resist
                                               collapse.
                                              the arrangement of the rocks causes water to 'pond-up'
                                               within the cliffs. This increases the weight of the cliffs.
                                               The increase in water pressure within the cliffs which
                                               encourages collapse.
                                              the coastline is exposed to the direct force of the
                                               prevailing south-westerly winds. This means a very
                                               long fetch and as a result the waves are very powerful
                                               leading to rates of erosion as much as 2m a year in
                                               places.



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   small streams increase the amount of water entering
    the cliffs which increases the weight of the cliff and
    then leading to collapse
   buildings on the cliff top have increased the weight on
    the cliffs, making them more vulnerable to collapse




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Sea Defences – See Website for excellent videos and
animations
http://onlinegeography.wikispaces.com/C+-+Sea+defences
Salt Marshes
Salt marshes are areas of periodically flooded low-lying coastal wetlands. They are often rich in
plants, birds and animals.

A salt marsh begins life as an accumulation of mud and silt in a sheltered part of the coastline for
example in lee of a spit or bar. As more deposition takes place, the mud begins to break the surface
to form mudflats. Salt-tolerant plants such as cordgrass soon start to colonise the mudflats. These
early colonisers are called pioneer plants. Cordgrass is tolerant of the saltwater and its long roots
prevent it from being swept away by the waves and the tides. Its tangle of roots also helps to trap
sediment and stabilise the mud.

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 the fertility
of the newly forming soil. New plant species such as sea asters start to colonise the area and
gradually, over hundreds f years, a succession of plants develops. This is known as a vegetation
succession.

Salt Marsh Vegetation Succession




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Case study: Keyhaven Marshes
Make sure you can describe the threats to the ecosystem and management options there.




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Glossary
Fetch: the distance of open water over which the wind can blow

Beach: a deposit of sand or shingle at the coast, often found at the head of a bay

Crest: the top of a wave

Swash: the forward movement of a wave up a beach

Backwash: the backward movement of water down a beach when a wave has broken

Constructive wave: a powerful wave with a strong swash that surges up a beach

Destructive wave: a wave formed by a local storm that crashes down onto a beach and has a
powerful backwash

Rockfall: the collapse of a cliff face or the fall of individual rocks from a cliff

Hydraulic power: the sheer power of the waves

Corrasion: the effect of rocks being flung at the cliff by powerful waves

Solution: the dissolving of rocks, such as limestone and chalk

Attrition: the knocking together of pebbles, making them gradually smaller and smoother

Traction: heavy particles rolled along the seabed

Solution: the transport of dissolved chemicals

Saltation: a hopping movement of pebbles along the seabed

Suspension: lighter particles carried (suspended) within the water

Longshore drift: the transport of sediment along a stretch of coastline caused by waves approaching
the beach at an angle

Headland: a promontory of land jutting out into the sea

Bay: a board coastal inlet often with a beach

Wave-cut platform: a wide, gently sloping rocky surface at the foot of a cliff

Cave: a hollowed-out feature at the base of an eroding cliff

Arch: a headland that has been partly broken through by the sea to form a thin-roofed arch

Stack: an isolated pinnacle of rock sticking out of the sea

Spit: a finger of new land made of sand or shingle jutting out into the sea from the coast

Salt marsh: low-lying coastal wetland mostly extending between high and low tide

Bar: a spit that has grown across a bay

Shoreline Management Plan: an integrated coastal management plan for a stretch of coastline in
England and Wales

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Hard engineering: building artificial structures such as sea walls aimed at controlling natural
processes

Soft engineering: a sustainable approach to managing the coast without using artificial structures

Managed retreat: allowing controlled flooding of low-lying coastal areas or cliff collapse in areas
where the value of the land is low

Pioneer plant: the first plant species to colonise an area that is well adapted to living in a harsh
environment

Vegetation succession: a sequence of vegetation species colonising an environment




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