Download COASTAL EROSION

COASTAL EROSIONAL
AND DEPOSIONAL
FEATURES
• Coasts are shaped by the sea and the action of
waves. The processes that take place are erosion,
transportation and deposition.
The action of waves
• The power of waves is one of the most
significant forces of coastal change.
• Waves are created by wind blowing over the surface of
the sea. As the wind blows over the sea, friction is created
- producing a swell in the water. The energy of the wind
causes water particles to rotate inside the swell and this
moves the wave forward.
The size and energy of a wave is influenced by:
• how long the wind has been blowing
• the strength of the wind
• how far the wave has travelled (the fetch)
Waves can
be destructive or constructive.
• When a wave breaks, water is washed up the beach - this
is called the swash. Then the water runs back down the
beach - this is called the backwash. With a constructive
wave, the swash is stronger than the backwash. With a
destructive wave, the backwash is stronger than the
swash.
Constructive and destructive waves
• Destructive waves
• Destructive waves are created in storm conditions.
• They are created from big, strong waves when the wind is
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powerful and has been blowing for a long time.
They occur when wave energy is high and the wave has
travelled over a long fetch.
They tend to erode the coast.
They have a stronger backwash than swash.
They have a short wave length and are high and steep.
Constructive waves
• They are created in calm weather and are less powerful
than destructive waves.
• They break on the shore and deposit material, building up
beaches.
• They have a swash that is stronger than the backwash.
• They have a long wavelength, and are low in height.
Coastal erosion
• The sea shapes the coastal landscape. Coastal erosion is
the wearing away and breaking up of rock along the
coast. Destructive waves erode the coastline in a
number of ways:
• Hydraulic action. Air may become trapped in joints and
cracks on a cliff face. When a wave breaks, the trapped
air is compressed which weakens the cliff and causes
erosion.
• Abrasion. Bits of rock and sand in waves grind down cliff
surfaces like sandpaper.
• Attrition. Waves smash rocks and pebbles on the shore
into each other, and they break and become smoother.
• Solution. Acids contained in sea water will dissolve some
types of rock such as chalk or limestone.
Transport
• There are various sources of the material in the sea. The
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material has been:
eroded from cliffs
transported by longshore drift along the coastline
brought inland from offshore by constructive waves
carried to the coastline by rivers
• Coastal transport
• There are four ways that waves and tidal currents
transport sediment. These can then contribute to the
movement of sediment by longshore drift.
Deposition
• When the sea loses energy, it drops the sand, rock
particles and pebbles it has been carrying. This is
called deposition. Deposition happens when the swash
is stronger than the backwash and is associated with
constructive waves.
• Deposition is likely to occur when:
• waves enter an area of shallow water.
• waves enter a sheltered area, eg a cove or bay.
• there is little wind.
• there is a good supply of material.
Longshore drift
• Longshore Drift: This is the process of waves moving
(transporting) material (load) along a coastline.
Longshore drift only happens when the waves hit the
beach at an angle. It is the process of the swash
transporting material up the beach at an angle and the
backwash returning directly under the force of gravity that
causes material to be transported along the beach.
Erosional landforms
• Along a coastline there are features created by
erosion. These include cliffs, wave-cut platforms and
wave-cut notches. There are also headlands and
bays, caves, arches, stacks and stumps.
Cliffs, wave-cut platforms and notches
• One of the most common features of a coastline is a cliff.
Cliffs are shaped through a combination of erosion and
weathering - the breakdown of rocks caused by weather
conditions.
• Soft rock, eg sand and clay, erodes easily to create
gently sloping cliffs. Hard rock, eg chalk, is more resistant
and erodes slowly to create steep cliffs.
• The process of cliff erosion
• Weather weakens the top of the cliff.
• The sea attacks the base of the cliff forming a wave-cut
notch.
• The notch increases in size causing the cliff to collapse.
• The backwash carries the rubble towards the sea forming
a wave-cut platform.
• The process repeats and the cliff continues to retreat.
Headlands and bays
• 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.
Caves, arches, stacks and stumps
• Weathering and erosion can create caves, arches, stacks
and stumps along a headland.
• Caves occur when waves force their way into cracks in
the cliff face. The water contains sand and other materials
that grind away at the rock until the cracks become a
cave. Hydraulic action is the predominant process.
• If the cave is formed in a headland, it may eventually
break through to the other side forming an arch.
• The arch will gradually become bigger until it can no
longer support the top of the arch. When the
arch collapses, it leaves the headland on one side and
a stack (a tall column of rock) on the other.
• The stack will be attacked at the base in the same way
that a wave-cut notch is formed. This weakens the
structure and it will eventually collapseto form a stump.
• One of the best examples in Britain is Old Harry Rocks, a
stack found off a headland in the Isle of Purbeck.
Depositional landforms
• Along a coastline you find features created
by deposition. These include beaches, spits and
tombolos.
• Beaches
Beaches are a common feature of a coastline. Beaches are
made up of eroded material that has
been transported from elsewhere and deposited by the
sea.
Spits
• Spits are also created by deposition. A spit is an
extended stretch of beach material that projects out to sea
and is joined to the mainland at one end.
• Spits are formed where the prevailing wind blows at an
angle to the coastline, resulting in longshore drift. An
example of a spit is Spurn Head, found along the
Holderness coast in Humberside.
Tombolos
• A tombolo is a spit connecting an island to the mainland.
An example of a tombolo is Chesil Beach, which
connects the Isle of Portland to the mainland of the Dorset
coast.