Download Coastal Erosion Landforms

Coastal Processes: Erosion, Transport and Deposition
Remember, there are 3 main processes that cause a coastline to change:
1. Erosion
2. Transport
3. Deposition.
There are number of factors which affect each of these processes - we are going to start by
exploring erosion processes and the factors that can affect the amount of erosion that may
take place along a coastline.
COASTAL EROSION
Erosion Processes:
Erosion is the wearing away of rocks, at the coast there are 6 main types of erosion
processes in action (see animations here):
1. ABRASION (this is also known as corrasion) - this is where rock fragments are hurled at
cliffs by breaking waves, gradually scraping away at the cliff face;
2. HYDRAULIC ACTION - as waves break against the cliff face, the pressure of the breaking
wave can compress air in cracks. This compressed air gradually forces open the crack in the
rock - as this process continues, the rock becomes increasingly weakened.
3. SOLUTION (this is also known as corrosion) - this occurs where the salt water is able to
dissolve some of the chemicals in rocks - for example, limestone cliffs are gradually
weakened as the salt water dissolves the calcium carbonate in the limestone.
4. SCOURING - this occurs at the base of the cliff as the waves break and swirl around,
gradually removing loose rock.
5. ATTRITION - this is where rock fragments carried by the waves hit against each other and
gradually wear down to form sand and silt
6. WAVE POUNDING - the sheer force of waves hitting against the cliff face
These processes of erosion form a series of distinctive landforms at the coast.
Rates of Coastal
Erosion
So what are the factors that determine how much erosion can take place?
1. The Resistance of the Rocks - e.g. limestone, chalk and granite are resistant rocks
(often forming cliffs and headlands) and erode relatively slowly, whilst less resistant rocks
such as clay are easily eroded.
2. The Strength of the waves - affected by the wind strength and duration and its fetch
3. The shape of the coastline (which is dependent on its geology) - on concordant
coastlines, rocks are parallel to the wave front and therefore rates of erosion are similar
along the coastline. On discordant coastlines, differential erosion may occur, where bands of
hard and soft rock outcrop at right angles to the sea. Consequently headlands and bays form
along discordant coastlines and whilst headlands remain exposed to the force of the waves,
bays are sheltered.
COASTAL TRANSPORT
The second process operating at the coast is transport. Material eroded by the sea is carried
within the water in a number of ways, minerals dissolved from rocks are carried in solution,
whilst small rock fragments, light enough to be held within the water, float in suspension. The
largest rock fragments which are too heavy to be picked up by the waves, are transported by
the process of traction, this is where they roll along the bed when the waves pick up enough
energy. Finally, medium sized rock particles, which cannot be carried by the waves all the
time, are moved by saltation. This is where during times of higher wave energy the particles
are picked up and then dropped again as the wave looses its energy.
The main form of transport operating at the coast is that of LONGSHORE DRIFT.
Longshore drift is the process by which sand and pebbles are moved along a beach by the
movement of the waves.
COASTAL DEPOSITION
Material is moved up the beach by the swash at an angle which is controlled by the prevailing
wind. The backwash then carries material back down the beach at right angles to the
coastline under the influence of gravity. Gradually the material is moved along the coastline,
its direction being controlled by the prevailing wind direction.
The final process operating at the coast is that of deposition - this is where material that is
too heavy to be transported any more is left behind, building up the beach. Due to the
importance of energy in transporting sand and shingle, it is the largest material that is
deposited first. A number of distinctive features may form due to coastal deposition.
Follow up links:
Animations of Coastal Erosion Processes (BBC Bitesize)
Transport and Deposition (BBC Bitesize)
Key Terms Check:
Erosion - the wearing away and removal of material
Deposition - the dropping of material
Abrasion - the wearing of rock due to rock fragments being hurled against cliffs
Attrition - the breakdown of rocks as they hit against each other
Hydraulic Action - the force of waves causing rocks to split apart as waves compress air in
cracks in the rocks
Wave Pounding - sheer force of water hitting rocks
Solution - where minerals in rocks are dissolved by the action of sea water
Scouring - occurs where water and broken rock fragments swirl around at the base of cliffs
gradually wearing rock away.
Longshore Drift - the movement of material along a coastline
Coastal Erosion Landforms - Features and Formation
Coastal Erosion Features
There are 3 main groups of coastal features which result from coastal erosion:
1. Headlands and Bays
2. Caves, Arches, Stacks and Sumps
3. Cliffs and Wave-cut platforms
1. HEADLANDS AND BAYS
Headlands are resistant outcrops of rock sticking out into the sea, whilst bays are indents in
the coastline between two headlands.
So how do headlands form?
- Headlands form along discordant coastlines in which bands of soft and hard rock outcrop at
right angles to the coastline.
- Due to the presence of soft and hard rock, differential erosion occurs, with the soft, less
resistant rock (e.g. shale), eroding quicker than the hard, resistant rock (e.g. chalk)
- Where the erosion of the soft rock is rapid, bays are formed
- Where there is more resistant rock, erosion is slower and the hard rock is left sticking out
into the sea as a headland.
- The exposed headland now becomes vulnerable to the force of destructive waves but
shelters the adjacent bays from further erosion.
Named Examples of Headlands and Bays: (LEARN!)
The Dorset coast has excellent examples of Headlands and Bays
e.g. Swanage Bay and the Foreland (a headland)
2. CAVES, ARCHES, STACKS and STUMPS
Once a headland has formed it is then exposed to the full force of destructive waves and it
gradually begins to erode. you need to be able to describe the erosion of a headland and the
features that form.
So how does a headland erode and caves, arches, stacks and stumps form?
- Firstly, the sea attacks the foot of the cliff and begins to erode areas of weakness such as
joints and cracks, through processes of erosion such as hydraulic action, wave pounding,
abrasion and solution;
- Gradually these cracks get larger, developing into small caves;
- Further erosion widens the cave and where the fault lines runs through the headland, two
caves will eventually erode into the back of each other forming an arch, passing right through
the headland.
- A combination of wave attack at the base of the arch, and weathering of the roof of the arch
(by frost, wind and rain), weakens the structure until eventually the roof of the arch collapses
inwards leaving a stack, a stack is a column of rock which stands separate from the rest of
the headland.
- The stack will continue to erode, eventually collapsing to form a stump which will be
covered by water at high tide.
Named Examples:
The Foreland (Dorset Coastline) is a great example of a headland which shows these
features - there is a distinctive stack called Old Harry and a stump known as Old Harry's
Wife.
A good example of a distinctive arch, also found on the Dorset Coast is Durdle Door.
3. CLIFFS AND WAVE-CUT PLATFORMS
Cliffs are steep rock faces along the coastline, they tend form along concordant coastlines
with resistant rocks parallel to the coast.
So how do cliffs and wave-cut platforms form?
- The erosion of a cliff is greatest at its base where large waves break - here hydraulic action,
scouring and wave pounding actively undercut the foot of the cliff forming an indent called a
wave-cut notch whilst the cliff face is also affected by abrasion as rock fragments are hurled
against the cliff by the breaking waves.
- This undercutting continues and eventually the overhanging cliff collapses downwards - this
process continues and the cliff gradually retreats and becomes steeper.
- As the cliff retreats, a gently-sloping rocky platform is left at the base, this is known as a
wave-cut platform which is exposed at low tide.
Named Examples:
Good examples of cliffs and wave-cut platforms can be found at Hunstanton (North Norfolk)
and Flamborough Head (Yorkshire)
------------------------------------------------------------------------------REVISING COASTAL EROSION FEATURES
Remember - for each
erosion feature try and learn a labelled diagram to show its formation, make sure that
you also mention examples of erosion processes when describing how the features
are actually formed. Finally to access the highest marks remember to name and locate
examples of each feature.
- Swanage Bay (Dorset Coast)
- The Foreland (Headland) (Dorset Coast)
- Old Harry (Stack) (Dorset Coast - off of the Foreland)
- Old Harry's Wife (Stump) (Dorset Coast - off of the Foreland)
- Durdle Door (South Dorset Coast)
- Cliffs and Wave-cut platforms - Hunstanton (N Norfolk) and Flamborough Head (Yorkshire)
Coastal Deposition Landforms: Features and Formation
Material that is transported by the waves along a coastline is eventually deposited forming
distinctive deposition features. There are four main deposition features that you need to learn
the formation of. These are:
1. Beaches
2. Spits
3. Bars
4. Tombolos
Beaches Beaches are the main feature of deposition found at the coast, these consist of all
the material (sand, shingle etc.) that has built up between the high and low tide mark. There
are number of different sources of beach material - the main source being rivers, where fine
muds and gravels are deposited at the river mouth. Other sources of beach material include
longshore drift (bringing material from elsewhere along the coast); constructive waves
(bringing material up the beach from the sea) and from cliff erosion.
As constructive waves build up beaches, they often form ridges in the beach known as
berms. The berm highest up the beach represents the extent to which the water has reached
during high tide.
SPITS
Spits are long narrow ridges of sand and shingle which project from the coastline into the
sea.
The formation of a spit begins due to a change in the direction of a coastline - the main
source of material building up a spit is from longshore drift which brings material from further
down the coast.
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, projecting
outwards into the sea - this continues to grow by the process of longshore drift and the
deposition of material.
A change in prevailing wind direction often causes the end of spits to become hooked (also
known as a recurved lateral).
On the spit itself, sand dunes often form and vegetation colonises (for example Blakeney
Point - North Norfolk)
Water is trapped behind the spit, creating a low energy zone, as the water begins to
stagnate, mud and marshland begins to develop behind the spit;
Spits may continue to grow until deposition can no longer occur, for example due to
increased depth, or the spit begins to cross the mouth of a river and the water removes the
material faster than it can deposited - preventing further build up.
Examples of Spits
- Spurn Head - Holderness Coast
- Orford Ness - Suffolk
BARS
These form in the same way as a spit initially but bars are created where a spit grows across
a bay, joining two headlands. Behind the bar, a lagoon is created, where water has been
trapped and the lagoon may gradually be infilled as a salt marsh develops due to it being a
low energy zone, which encourages deposition.
Example of a Bar: Slapton Sands - Devon.
TOMBOLOS
Tombolos are formed where a spit continues to grow outwards joining land to an offshore
island.
Example of a Tombolo: - Chesil Beach - which joins the South Dorset coast to the Isle of
Portland.
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REVISING COASTAL DEPOSITION FEATURES
Remember - as well as being able to describe the formation of each feature of coastal
deposition, you should be able to give a named and located example e.g. a spit Spurn Head (Holderness Coast). You should also try and learn a labelled diagram to
show the formation of each feature.
- Beach - Dawlish Warren (Devon)
- Spit - Orford Ness (Suffolk) or Spurn Head (Holderness Coast)
- Bar - Slapton Sands (Devon)
- Tombolo - Chesil Beach (joining S Dorset Coast to Isle of Portland)
Coastal Defence
When managing the coastline there are two main options:
1. HARD ENGINEERING
- this is where man made coastal defence structures are used to reflect large amounts of
wave energy and hence protect the coastline.
2. SOFT ENGINEERING
- this is where beaches or naturally formed materials are used to control / re-direct erosion
processes.
You need to know examples of coastal management techniques and their advantages and
disadvantages:
Hard Engineering Techniques:
1. Re-Curved Sea Wall
- concrete wall which is curved on the underside to deflect the power of the waves
- these can be very expensive (up to £1-2 million per km) and the deflected waves can
scour material at the base of the wall causing them to become undermined
- these are however a very effective means of preventing erosion and they reflect
rather than absorb wave energy.
2. Rip Rap
- large boulders on the beach absorb wave energy and break
the power of the waves
- although movement of the boulders is expensive this can be a much cheaper method
than some other solutions
- the boulders can however be undermined easily by waves washing away sand and
shingle beneath them. They also can be quite ugly, changing the appearance of a
coastline.
3. Groynes
- these structures (usually either wooden or steel) are designed to top longshore drift and
therefore act to build up and anchor beach material, protecting the base of cliffs.
- they are effective at reducing erosion in the area they are constructed in by causing
significant build up of beach material
- groynes may however starve areas further down the coast of material by stopping
longshore drift, resulting in an increase in erosion in these areas
4. Gabions
- these cages of boulders are built into cliff faces to protect the cliff from the force of the
waves;
- they are cheaper than sea walls and can be very effective where severe erosion is a
problem
- they are however visually intrusive
5. Revetments - these wooden structures break the force of
waves and beach material builds up behind them
- they are cheap and effective at breaking waves
- as well as being visually intrusive however they do need replacing more frequently than
most other defence methods.
Soft Engineering Techniques
Soft engineering includes beach replenishment in which beach
material is added to provide a "natural solution". Environmentally this is a preferred option
as it maintains the beauty of the landscape and avoids visual intrusion, however it can be
expensive to maintain as longshore drift continues to move beach material down the
coast and therefore regular replenishment is required.
Sand Dunes and salt marshes can also be encouraged to act as natural barriers to the
waves.
Case Study of Coastal Defence: Aldeburgh (Suffolk)
Aldeburgh is just south of Dunwich, here a large scale coastal defence scheme is in place
to control erosion. Aldeburgh is a busy town and tourism is very important to the local
economy in this area. It is therefore seen as cost effective to have a coastal defence
scheme in place to protect the economically valuable land.
Hard engineering is in place with a combination of sea wall, rip rap and groynes. There is
also an area of salt marsh to absorb some of the waves energy should it breach the sea
wall.
Another area severely affected by coastal erosion and where significant coastal defences
are in place is Overstrand on the North Norfolk Coast, pictures of the coastal defences in
this location can be seen here.
Further north, the small village of Dunwich is also severely affected by coastal erosion,
yet very little, apart from some small soft defences and a recent limited and unsuccessful
attempt at hard engineering is in place - here is a very useful account of the reasons
for the differences in the extent of coastal defence between Dunwich and
Aldeburgh.
Wooden Revetments and Gabions: