How do waves form?
Waves are caused by friction as the wind blows over the sea
strength affected by:
how long the wind has been blowing
strength of the wind
fetch - distance it has been travelling
formed more dramatically when earthquakes or volcanic eruptions shake the seabed
swash - when a wave washes up onto the shoreline
backwash - when the water retreats back into the sea
constructive waves
created in milder weather or from storms very far away
wave crests far apart and gently sloping wavefront
breaking wave spills forward
less energy than destructive waves
deposit their large load onto the shoreline
As you move higher up the beach, the rocks/ sediment is larger
Stronger swash than backwash
build the beach
destructive waves
usually made in storm like conditions that are closest
waves close together and steep wavefront
breaking wave plunges downwards
higher energy - larger and more powerful
stronger backwash than swash - net loss of beach material
created when fetch is large and a strong wind has been blowing for a long time
errode the coastline
weathering
cliffs collapse because of types of weathering - the weaking or decay of rocks in their original place on, or close to the ground surface
mechanical - the break up of rock without changing its chemical composition. When this happens, piles of rock fragments can be found at the foot of cliffs
An example is freeze thaw:
this occurs when rocks are porous or permeable
water enters the rock and freezes
The ice expands, putting pressure on the rock until it cracks
repeated freeze thaw can cause the rock to break up.
chemical - the breakdown of rock by changing its chemical composition.
An example is carbonation:
when slightly acidic rain or sea water comes into contact with sedimentary rock such as limestone or chalk, it is disolved
the acidic water and calcium carbonate react and the soluble solution is washed away
biological - the roots of vegetation causing the break up of rocks.
Roots enter a small crack in the bed rock
Growing roots put pressure on the rock, making the crack deeper
Rock breaks away
mass movement
The downhill movement of cliff material under the influence of gravity
Slumping/ rotational slip - The soft boulder clay holds rainwater and run off. Waves errode the cliff base, creating a wave cut notch. The clay becomes saturated and forms a slip plane. The weight of the cliff causes it to slump.
Landslides - in areas of more resistant cliff material. Erosion at the cliff base creates a notch which soon increases as it errodes. The weight of the cliffs is unable to be supported, leading to a landslide. This material provides temporary protection for the cliff behind.
Rockfall - involves fragments breaking away from the cliff face often due to freeze thawing.
Mudslide - saturated soil and weak rock flow down a slope. Typically occur where cliffs are made up of boulder clay.
coastal erosion processes
Abrasion - breaking waves which contain sand and larger fragments wear awat at the base of a cliff or headland (sandpaper effect)
Corasion - destructive waves pick up beach material and hurl them at the base of a cliff, loosening cliff material
Hydraulic action - waves hitting the base of a cliff causes are to be compressed in cracks, joints and folds - repeated changes in air pressure which weaken cliffs and chunks break off
Attrition - waves cause rocks and pebbles to bump into each other and break apart
Solution - sediment becomes dissolved in the water
coastal transportation
Traction - large pebbles and boulders being rolled along the sea bed. Carried out by high energy destructive waves
Saltation - small stones, pebbles and silt being bounced along the sea bed. Carried out by both types of waves.
Suspension - gine particles of clay and sediment are suspended in the sea and transported by both wave types
Solution - material is dissolved and carried by the sea
longshore drift
The zigzg movement of material along the coast by the sea
waves follow the direction of the prevailing wind
they usually approach the beach at an angle.
swash carries material up and along the beach in the same direction as the waves.
The backwash carries the material back down the beach at right angles, back towards the sea.
over time, the material zigzags along the coast
This process contributes to the formation of various depositional landforms
coastal deposition
When material that is being transported is dropped by constructive waves as the waves lose energy
likely to occur when:
waves enter an area of shallow water
waves enter a sheltered area such as a cove or bay
there is little wind
a river or estuary flows into the sea, reducing wave energy
There is a good suply of material and the amount of material is too much for a wave at that energy to transport
landforms
a feature of the landscape that has been formed through erosion, transportations or deposition
concordant and discordant coastline
concordant - layers of different types of rock run parallel to the coastline
discordant - bands of different types of rock run perpendicular to the coastline
bays and headlands
Headlands and bays can form along discordant coastlines
Headlands are formed as the harder rock is erroded less so it is left sticking out
Bays are formed where the softer rock is. Rapid hydraulic action, solution and abrasion errodes it further back.
once formed, the bay is less vulnerable to erosion as it is sheltered by headland
The headland is now more vulnerable to erosion as the energy of the waves is concentrated here
coves
coves form along a concordant coastline
these are formed when the outermost layer of harder rock is breeched which creates a small joint
This joint is eroded through hydraulic action, abrasion and solution
Eventually, it erodes enough to reach the softer rock layer behind.
This is able to erode much faster and can erode laterally until it reaches the other layer of harder rock behind that requires much more energy to erode.
A cove is formed
cliffs and wave cut platforms
a wave cut notch is formed when waves break against a cliff base. erosion close to the high tide line will wear away the cliff
over a long period of time, the notch will get deeper and deeper, undercutting the cliff
eventually the overlying cliff cannot support its own weight and collapses
collapsed material is washed away
as this continues, the cliff will gradually retreat and in its place will be a wave cut platform
this is gently sloping and typically quite smooth due to abrasion
caves, arches and stacks
the resistant rock that makes up headlands often has lines of weakness like cracks which are more vulnerable to erosion
waves crash into the headlands and enlarge the cracks, mainly by hydraulic power and abrasion
repeated erosion and enlargement of the cracks causes a cave to form
the cave deepens from erosion until it breaks through the headland to form an arch
the rock is worn away at the base so that the arch is no longer supported and weathering weakens the roof until it collapses
a stack is formed - isolated pillar of rock
beaches
deposits of sand and shingle at the coast
sandy beaches are mainly found in sheltered bays
the waves entering the bay are low energy constructive
flat and wide
shingle beaches are created by high energy destructive waves
the pebbles come from nearby erroded cliffs or are deposiited on shore
steep and narrow
spits
the long fingers of sand sticking out into the sea from the land
form at sharp bends at the coastline where there is significant longshore drift
sand and shingle are trasnsported and deposited by longshore drift
as it builds up, it starts to form an extension from the land
strong winds and waves can curve the end of a spit - recurved end
in the sheltered area behind the spit, deposits of mud have built up - mud flats and saltmarshes
bars
longshore drift may cause a bar to grow right across a bay, joining two headlands together
the bay gets cut off from the sea, allowing a lagoon to form behind the bar
offshore bars can form if the coast has a gentle slope
friction from the sea bed causes waves to slow down and deposit sediment offshore, creating a bar thar is not connected to the coast
dunes
Sand dunes are hills of sand created at the back of a beach.
The wind blows deposited sand up the beach. Objects like wood, driftwood or human rubbish can block the wind, causing deposition of sand as small embryo dunes
embryo dunes are colonised by plants like marram grass. the roots of the vegetation stabilise the sand, encouraging more sand to build up there.
This forms foredunes and eventually mature dunes.
new embryo dunes form in front of stabilised dunes
dune slacks, small pools, can form in hollows between dunes
location of dorset
The Dorset Coast, located in the south of England on the English Channel, runs from Lyme Regis in the west, past Old Harry Rocks in Purbeck to Highcliffe in the east. Excluding the shoreline of Poole Harbour, the Dorset coastline is 142 kilometres (88 mi) long.
chesil beach
Barrier beach connecting the mainland to the isle of portland
18 miles long with a max height of 15m
pebble and shingle beach
rolled into a tombolo (spit that joins an island to the mainland)
stretches northwest from portland to west bay
formed through longshore drift
fleet lagoon - shallow salt water lagoon that seperates the beach from the mainland
weymouth spit
A spit formed as the mainland behind
changes direction sharply.
A recent change in the prevailing wind’s direction has caused the spit to curve.
A salt marsh has formed behind, as the spit offers protection from the sea’s currents.
swanage
Swanage lies between Swanage bay and Studland bay
Studland bay has 4 miles of sandy beaches within sheltered waters and backed by sand dunes
a bay is a body of water partially surrounded by land
the cliffs behind the bays are ereas of soft sandstone and clay
Between the bays is the foreland, a headland of harder chalk
erosion is the dominant process - depositional beach loses material
longshore drift carries material from south to north of the beach
durdle door
Arch tranaformation.
has formed parallel to the coastline
further erosion has lead to a cave which has opened an arch into the headland
softer rocks behind the limestone have been washed away, leaving chalk cliffs which continue to errode through weathering
lulworth cove
Small bay that was formed when a gap was eroded in a band of limstone (hard rock)
behind is a band of soft clay
entrance to the cove is narrow as the limstone is resistant
the clay has been erroded much more than the hard rock and has eroded laterally
the limestone cliffs at the back are vulnerable to mass movementlulworth cove
old harry rocks
Old Harry and his wife sit at the end of the Foreland
The chalk headland has erroded to form caves, arches and a stack
Further erosion has resulted in the formation of a stump
groynes
timber or rock structures constructed at right angles to the beach
trap sediment being moved by longshore drift and enlarge the beach
the wider beach acts as a buffe to reduce wave damage
cost: timber groynes £150,000 (at every 200m)
advantages
create a wider becach - popular with tourists
groynes act as wind breaks for people on the beach
groynes do not affect access to the beach
provide useful structures for people interested in fishing
not too expensive to install and repair
disadvantages
by interrupting longshore drift, they starve beaches further along the coast, often leading to increased rates of erosion elsewhere - problem shifted than solved
groynes are unnatural and rock groynes in particular can be unattractive
the beach on the downdrift side of the groyne can be much lower than the updrift side - dangerous, especially for young children
ineffective in stormy conditions
sea wall
concrete or rock barrier against the sea
placed at the foot of cliffs or at the top of a beach
has a curved face to reflect the waves back into the sea
cost: £5000-£10000 per metre
advantages
effective at stopping the sea
often has a walkway or promenade for people to walk along
disadvantages
can look obtrusive and unnnatural
very expensive and high maintenance costs
rock armour
piles of large boulders dumped at the foot of a cliff
the rocks force waves to break, absorbing their energy and protecting the cliffs
the rocks are usually bbrought by barge to the coast
cost: £2000000 per 100m
advantages
relatively cheap vand easy to maintain
can often provide interest to the coast
often used for fishing
disadvantages
rocks are usually from other parts of the coastline or from abroad
can be expensive to transport
do not fit in with the local geology
can be obtrusive
gabions
wire mesh cages filled with rocks or pebbbles placed at the back of sandy beaches
can be built up to support a cliff or provide a buffer against the sea
cost: up to £50000 per 100m
advantages
does not impede natural sediment movement
cheap to produce and flexible in the final design
can improve drainage of cliffs
will eventually become vegetated and merge into landscape
disadvantages
for a while they look unattractive
cages only last 5-10 years before they rust
damaged gabions are very dangerous and unsightly - sea birds
beach nourishment
the addition of sand or shingle to an existing beach to make it higher or wider
the sediment is usually obtained offshore locally so that it blends in with the existing beach material
usually transported onshore by barge
cost: up to £500,000 per 100m
advantages
relatively cheap and easy to maintain
blends in with the existing beach
increases tourist potential by creating a bigger beach
disadvantages
needs constant maintenance unless structures are built to retain the beach
Offshore dredging of sand and shingle increases erosion in other areas
could affect marine ecosystem
people may be prevented from using the beach during maintenance
dune regeneration
sand dunes are effective buffers to the sea
marram grass can be planted to stabilise dunes and help them develop
cost: £200-£2000 per 100m
advantages
relatively cheap
maintains a natural coastal environment that is popular with people and widlife - biodiversity
disadvantages
time consuming to plant the marram grass and fence areas off
land must be carefully managed to avoid newly planted vegetation being trampled on - temporary fencing off or building wooden walkways
people don’t always respond well to being prohibited from accessing planted areas
can be damaged by storms
dune fencing
fences are constructed on a sandy beach along the seaward face of existing dunes to encourage new dune formation
the new dunes help to protect the existing dunes
cost: £400-£2000 per 100m
advantages
minimal impact on natural systems
can control public access to protect other ecosystems
disadvantages
can be unsightly, especially if fences become broken
regular maintenance needed, especially after storms
managed retreat
a deliberate policy of allowing the sea to flood or errode an area of relatively low value land
a form of soft engneering as it allows natural processes to take place
in the long term, allowing managed retreat is a more sustainable option than spending large sums of money trying to protect the coast with sea walls or groynes
as sea levels continue, managed retreat seems likely to become an increasingly popular choice for managing the coastline
coastal monitoring and adaptation
areas that are low value farmland, forest or moorland in coastal zones do not require expensive intervention and can be left alone
people living or working in these areas have to adapt by relocating further inland
scientists conduct monitoring here - reduces the possibilties of conflict between managing the coast and the views of local people whose lives are affected
this monitoring involves studying marine procsses, mass movement and human activity
if conditions change, a new approach might be adopted
lyme regis