1C- Coasts

How do waves form?

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