Transcript Coasts – The essentials

Coasts – The Essentials
How do land processes shape the coast?
WEATHERING
This is the disintegration and decay of rocks in their original place or close
to the ground surface, largely caused by the weather e.g.
rainfall/temperature change.
There are 3 main types:
MECHANICAL/PHYSICAL – This is the disintegration of rock without any
chemical changes taking place in the rock. This would result in piles of
rock fragments (scree) accumulating at the base of a cliff.
CHEMICAL – This involves a chemical change e.g. rainwater, which is
slightly acidic, can slowly dissolve certain rocks.
BIOLOGICAL – This involves the actions of animals (fauna) and plants
(flora). Plant roots are very effective at growing and expanding in rock
cracks. Animals such as rabbits and sea birds will burrow in weak raocks
such as sandstone.
TYPES OF MECHANICAL/PHYSICAL
WEATHERING
FREEZE-THAW (frost shattering)
EXFOLIATION (onion skin weathering)
Exfoliation happens in areas which
experience large changes in temperature.
During the day when the rock heats up, it
expands but at night when it cools, it
contracts. When this is constantly repeated,
the outer layer will often peel away from the
rest of the rock.
TYPES OF CHEMICAL WEATHERING
SOLUTION
In the same way in which sugar dissolves in tea, some minerals and rocks
dissolve in rainwater. This process is called solution.
CARBONATION
Carbonation is very similar to
solution. It affects rocks that are
made up of calcium carbonate e.g.
chalk and limestone. Rainwater
picks up carbon dioxide (CO2) in
the air, so becomes a weak
carbonic acid. This then reacts with
calcium carbonate (found in rocks
such as chalk/limestone) to form
calcium bicarbonate which
dissolves weak points in the rock,
forming cracks (see photo on left)
MASS MOVEMENT
Mass movement is the downhill movement of material under the influence
of gravity. This will once again have an effect on the shape of a coastline.
Common types of movement are:
1. Rockfall
2. Landslide
3. Mudflow
4. Rotational slip
Rockfall – fragments of rock
break away from the cliff face,
often due to freeze-thaw
weathering, leaving screes at the
base of the cliff
Mudflow – where a layer of soil on
top of a layer of rock becomes
saturated (cannot soak up anymore
water), it will flow down a slope.
Where layers of rock sit on one
another at an angle, there is always
the possibility that they will create a
landslide, and slide downhill.
A rotational slip is a slump of
saturated soil and weak rock
along a curved surface.
Types of waves which shape the coastline
Destructive waves destroy beaches. The waves are usually
very high and very frequent. The back wash has less time to
soak into the sand. As waves continue to hit the beach there
is more running water to transport the material out to sea.
These waves are most common in winter.
Constructive waves build beaches. Each wave is low.
As the wave breaks it carries material up the beach in its
swash. The beach material will then be deposited as the
backwash soaks into the sand or slowly drains away.
These waves are most common in summer.
Processes of erosion – which shape the coastline
Attrition
Material carried by the waves bump into each other and so are
smoothed and broken down into smaller particles.
Hydraulic action
This process involves the force of water against the coast. The
waves enter cracks (faults) in the coastline and compress the air
within the crack. When the wave retreats, the air in the crack
expands quickly causing a minor explosion. This process is
repeated continuously.
Corrosion
This is the chemical action of sea water. The acids in the salt
water slowly dissolve rocks on the coast. Limestone and chalk are
particularly prone to this process.
Abrasion/Corrasion
This is the process by which the coast is worn down by material
carried by the waves. Waves throw these particles against the
rock, sometimes at high velocity.
Types of coastal transportation
Material eroded by the sea is carried within the water in a number of
ways:
1. Minerals dissolved from rocks are carried in solution
2. Small rock fragments, light enough to be held within the water, float in
suspension.
3. 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.
4. 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 loses its energy.
Processes of transportation – LONGSHORE DRIFT
This movement of sediment along the
coastline is called longshore drift.
Landforms formed by erosion
You need to know the formation of:
1. Cliffs
2. Wave cut platforms
3. Caves
4. Arches
5. Stacks
6. Headlands and bays
Cliff formation
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 as it becomes unstable- this process continues
and the cliff gradually retreats and becomes steeper.
Formation of wave cut platform
The base of the cliff is attacked by destructive waves, especially at
high tide when they have more power. These erode the cliffs by
hydraulic power and corrasion forming a wave-cut notch. This gets
larger until the cliff collapses. As the process is repeated many times
the cliff retreats leaving a wave-cut platform below (a sloping
platform of sand, pebbles and rocks) exposed at low tide.
Caves, arches and stack formation
Headlands and Bays
Sections of coastline which are
particularly resistant to erosion (hard
rock) stick out to form headlands.
Weaker sections of coastline which are
more easily eroded form bays.
The waves are less powerful in bays as
they are more sheltered, so deposition
tends to dominate and explains why
sandy beaches are common features.
Formation of depositional features
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.
Landforms formed by deposition - Spits
Winds from
NW
coastline changes
direction
A spit is an area of sand or shingle which either extends at a gentle angle out to
sea or which grows across a river estuary e.g. Dawlish Warren, Devon.
Many spits are characterised by a hooked or curved end. Spits develop where
longshore drift moves large amounts of material along the beach and where there
is a sudden change in the direction of the coastline. Where the coastline
changes direction, sand and shingle are deposited, which builds upwards and
outwards to form a spit. Occasionally when winds blow from a different direction,
material will be pushed inland causing the spit to curve. Spits cannot grow across
estuaries because of the speed of rivers flowing out to sea. Behind the sheltered
water of the spits, salt marsh and mud creeks develops.
Leisure facilities along spits
•
•
•
•
Golf course
Fishing
Walking
Nature reserves e.g. SSSI
(Sites of Special Scientific
Interest)
• Bird/animal watching e.g.
grey seals
Dawlish
Warren
• Beaches
• Holiday parks
Bars
Chesil
Beach
Occasionally, longshore drift may cause a spit to grow across a bay, trapping a
freshwater lake or lagoon behind it. This feature is called a bar e.g. Slapton, South
Devon.
In the UK, following ice melt at the end of the last ice age and sea level rise, some
offshore bars have been driven onshore. This type of bar is called a barrier beach
e.g. Chesil Beach, Dorset.
Sea level rise
Background
Over the last 15 years, global sea levels have risen on average by 3mm a year.
Latest estimates suggest that by the end of the century (2000), global sea levels
will rise by between 28 and 42 cm. Since more than 70% of the world’s population
live near the coast, the effects could be devastating.
What are the causes?
The main cause of sea-level rise is thermal expansion of seawater, as it absorbs
more heat from the atmosphere (as a result of global warming). Incredibly,
scientists believe that the melting of ice on land (e.g. Greenland) and at sea (e.g.
Arctic) will have no direct effect on sea levels but it is the thermal expansion of sea
water which will lead to sea level rises.
What would be the impact of sea level rises?
The impacts of sea level rising can affect countries/communities in 4 main areas:
A.ECONOMIC (money)
B.SOCIAL (people)
C.ENVIRONMENTAL (natural world)
D.POLITICAL (decision making)
Case study of sea level rise Maldives
80% of the islands are only 1m above seal level. It is thought that sea levels are
rising there about 9mm/year and that the entire population may have to abandon
their homes by the end of the century.
So, what would the impacts be?
1. ECONOMICALLY – fishing is the biggest industry but tourism comes a close
second, thanks to its crystal clear water, beautiful beaches and tropical climate.
Many people rely on these industries for jobs and income for their families.
2. SOCIALLY – the people fear that they are literally being washed away, which
will force them to lose their home, jobs, culture and separate families.
3. ENVIRONMENTALLY – with the majority of the islands protected by only a
barrier of coral, there is little to stop the advancing sea level rise and the
flooding and loss of the islands forever..
4. POLITICALLY – most decisions are taken locally by local government, police
and fire services. An evacuation may be needed of many islands within 50
years. The Maldives government is appealing to countries to cut their carbon
emissions and therefore reduce global warming and the consequential sea
level rises associated with this.
Case study of recent cliff collapse –
Holderness, E.Yorkshire
Where is it?
The Holderness Coast is on the east coast of the UK,
facing the North Sea.
What is the problem there?
•The coastline is mainly made up of cliffs (20-30m high),
consisting of soft, easily eroded boulder clay.
• The cliff line is retreating at an alarming rate - greater
than 1m / yr (fastest rate in Europe) - 4km of land have
been lost since Roman Times, including many villages
and farm buildings.
Why is cliff erosion such a problem here?
1. The cliffs are made up of soft glacial material (Boulder Clay - made up of
sands and gravels). This is easily eroded by the waves and the cliffs are easily
undermined.
2. The waves are mainly destructive - eroding the base of the cliffs (hydraulic
action etc.)
3. Most of the material eroded from the cliffs is washed out to sea, the rest is
moved by longshore drift - the beaches are therefore narrow and do little to
protect the coastline. (If the beaches were wider, the waves would break on the
beaches reducing their erosive power).
What have the effects been on people and the environment?
 By 1998, the main road running through the village of Mappleton was only 500m
from the cliff top and in places it is now only 50m
 29 villages have been claimed by the sea in 1000 years
 Farmers are losing valuable farmland
 Holiday caravans have collapsed into the sea
 In June 1993 the Holbeck Hall Hotel slumped into the sea
 Easington Gas Station (a North Sea Gas Terminal) on top of the cliffs is under
threat
How can coastlines be managed?
Hard Engineering
Hard engineering involves using man made structures to control the actions of the
sea and protect property from flooding and erosion. However, they are less
commonly used today, as they are expensive to build and maintain, as well as being
visually obtrusive and unnatural.
They tend to interfere with natural coastal processes and can cause problems
further down a coastline e.g. erosion and cliff collapse.
Soft Engineering
Soft engineering approaches try to fit in and work with the natural coastal
processes. They are often involve low maintenance costs and are environmentally
more friendly and sustainable. They are usually the preferred option of coastal
management.
Managed retreat
Managed retreat or coastal realignment is when some of the coastline is allowed to
retreat. It is a viable option when there is a high risk of flooding or cliff collapse and
when the land is of relatively low value e.g. poor quality grazing land.
MANAGEMENT TECHNIQUES FOR PREVENTING COASTAL EROSION
Rip Rap (rock
armour)
Sea wall
Wooden groynes
Beach
replenishment
Dune regeneration
Marsh creation
Methods of coastal protection (management)
A rip rap is rocks laid
out as a ramp, with the
largest and heaviest
ones on the outside
These large rocks
reduce the energy of
the waves and are
relatively cheap
Can be very expensive
if you can’t use local
rocks
A sea wall is made of
concrete and sometimes
has a curved lip
This structure reflects
wave energy (they
bounce off) and helps
prevent flooding
It is very expensive to
replace a section of
this after storms
A groyne is a wooden
barrier of planks at
right angles to the
shore
This helps to stop
material moving down
the coast and helps
build up a small beach
This will rot after
about 20 years and will
need replacing
Beach replenishment is
when sand is brought
from elsewhere to build
up a beach
This is a cheap way of
building up the level of
a beach
The sea will eventually
erode and transport
the sand somewhere
else
A concrete breakwater
is a platform made of
concrete or rocks
extending out to sea
This helps to stop
material moving down
the coast and helps
build up a large beach
Places down the coast
suffer from reduced
material being
transported and
deposited
Revetments are wooden
slats laid out like a
ramp underneath a cliff
These help break up
wave energy, so helping
to protect a cliff
These are not as strong
as sea walls. Wood can
be damaged in strong
waves
Gabions are cube
shaped wire cages filled
with rocks and large
pebbles
These are relatively
cheap and can be easily
fixed, as they are
individually caged
These are unattractive
and have a relatively
short life span (about 4
years)
Advantages and disadvantages of coastal
protection schemes
Advantages
 Wave energy is broken/absorbed/reflected
 Flooding is stopped e.g. sea wall – less chance of loss of money to businesses
along beach e.g. cafes, restaurants, shops, amusements etc.
 Longshore drift is stopped e.g. by groynes, breakwaters resulting in the
building up beaches, making more sand available to visitors/tourists/locals
 Residential housing and farmland is protected from cliff collapse
Disadvantages
 Schemes are often ugly
 Noise pollution from construction
 High cost of construction e.g. sea wall
 Stopping longshore drift results in erosion somewhere else
 More tourists means more litter, more cars (congestion)
Managed retreat (costs and benefits)
Case study: Wallasea Island, Essex
Location:
Wallasea Island is a low-lying coastal island formed at the confluence of the
Rivers Crouch and Roach in Essex.
What has happened?
With the coastal defences in the north of the island crumbling away, the
government decided to realign (change the shape of) the northern part of the
island by constructing a new embankment (wall) inland (further south) and allow
the sea to further break through (breach) the old sea defences.
Benefits:
 The new mudflats and salt marsh (created where the defences are breached –
see photo) will help protect the new sea wall
 They will offer additional protection to the property to the south
 The scheme aims (still too early to assess) to bring back bird habitats which were
lost to previous development, improve flood defences on the island and create new
leisure opportunities
Costs:
 The scheme cost taxpayers £7.5 million
 Sediment (mud) and vegetation (salt marsh) has not been immediate
For a coastal environment that you have studied,
describe the environment and explain why it
provides a suitable habitat for the species living
there (Foundation)
Dawlish Warren is a coastal spit, a narrow finger of land. It
is an area of grassland, sand dunes, mud flats and beach
that lies at the mouth of the River Exe. Behind the sand
dunes is a nature reserve and it has been designated a
SSSI (Site of Special Scientific Interest).
It is important for wildfowl e.g. pheasants and wading birds
e.g. geese, oystercatchers, as the birds arrive to escape the
extreme cold winter of Northern Europe and also because
the spit provides roosting (resting) beyond high tide levels.
There are also many very special plants e.g. Sand Crocus,
which can only grow as a result of the warm, sunny climate
and low rainfall.
For a coastal environment that you have studied,
describe the ways in which it is managed to
ensure it is conserved, but used sustainably
(Higher)
Firstly, at Dawlish Warren, there is an education centre
which provides information to students and the public about
what they can do to care for the area. The centre will also
cordon areas off e.g. at times of animal breeding and put
signs up to ensure that habitats are protected when
necessary e.g. damaged (trampled) sand dunes. Secondly,
boardwalks have been put down to allow public access and
use but also to try and stop people from tramping the dunes
and vegetation in the area, therefore protecting them.
Thirdly, the sand dunes have been planted with marram
grass (has very long roots), so stabilising them for the
future.
Useful websites for animations
http://www.bbc.co.uk/schools/gcsebitesize/geography/coastal/coastalprocesses
rev4.shtml (processes of coastal erosion)
http://www.rgs.edu.sg/events/geotrip/stack.html (formation of erosional
landforms)
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21605 (spit
formation)
Geography at the movies http://www.geographyatthemovies.co.uk/ (Coasts)