Coast

CoastsKEY THEMES – SECTION 2

WeatheringMECHANICAL WEATHERING

The breakdown of rock without changing its chemical composition.

Freeze thaw is an example of this.

1. It happens when the temperature alternates above and below ) degrees C.

2. Water gets into rock that has cracks.

3. When the water freezes it expands which puts pressure on the rock.

4. When the water thaws it contracts which releases the pressure on the rock.

5. Repeated freezing and thawing widens the crack and causes the rock break up.

CHEMICAL WEATHERING

The breakdown of rock by it chemical composition. Carbonation weathering is a type if chemical weathering that happens in warm and wet conditions.

1. Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic acid.

2. Carbonic acid reacts with rock that contains calcium carbonate so the rocks are dissolved by the rain water.

Erosion Hydraulic action Waves crash against rock and compress the air in the cracks. This puts pressure on the rock. Repeated compression widens the crack and makes bits of rock break off.

Corrasion Eroded particles in the water scrape and rub against rock, removing small pieces.

Attrition Eroded particles in the water smash into each other and break into smaller fragments. Their edges also get rounded off as they rub together.

Corrosion Weak carbonic acid in seawater dissolves rock like chalk and limestone.

Mass movement

Shifting of rock and loss of material down a slope.

Waves Waves are the result of the wind blowing over the sea. As they approach land they break. The bottom of the wave touches the sand and slows down due to increased friction. The top of the wave becomes higher and steeper until it topples down. The waves energy depends on the fetch (the distance a wave travels by the wind), the strength of the wind and the length of time over which the wind blown.

There are two types of waves:

Destructive waves High frequency (10-14 waves per minute)

They’re high and steep

Their backwash is more powerful than their swash which means material is removed from the coast.

Constructive waves Low frequency (6- 8 waves per minutes)

they’re low and long

The swash is more powerful and it carries material up the coast and the backwash is weaker and it doesn’t

take a lot of material back down the coast which builds up the beach.

These waves are made by weaker winds and have a shorter fetch than destructive waves.

Coastal Landforms – Cliff retreatWaves cause erosion at the foot of a cliff which forms a wave-cut notch and is enlarged as erosion continues making the cliff above the notch unstable.

The part of the cliff above sea level is also affected by mechanical and chemical weathering processes. This makes the cliff more unstable and it eventually collapses.

The collapsed material is washed away and a new wave cut-notch starts to form. Repeated collapsing results in the cliff retreating. A wave cut platform is the platform that’s left behind as the cliff retreats.

The rate of retreat depends on things like geology of the cliff for example cliffs formed from soft rock or loose material can retreat very quickly but cliffs formed by hard rock can be eroded over thousands of years. Also vegetation of the cliff affect the rate of retreat for example cliffs covered in vegetation are more stable so they’re eroded less easily and retreat more slowly.

Coastal Landforms – Headlands and Bays

Some types of rocks are more resistant to erosion than others.

Headland and bays form where there are alternating bands of resistant and less resistant rocks along a coast.

The less resistant rock like clay would erode more easily and this would form a bay which have gentle slopes.

The resistant rock like chalk would erode more slowly and it is left jutting out forming headland which are steep sides.

Coastal Landforms – Caves, Arches, Stacks and Stumps

Headlands are usually made of resistant rocks that have a weakness like cracks.

Waves crash into the headlands and enlarge the cracks mainly by hydraulic action and corrasion.

Repeated erosion and enlargement of the crack causes a cave to form.

Continued erosion deepens the cave until it breaks through the headland forming an arch.

Erosion continues to wear away the rock supporting the arch and weathering weakens the roof of the arch until it eventually collapses.

This forms a stack which is an isolated rock that’s separate from the headland.

The stack eventually is worn away to give a stump which is covered by the water at high tide.

Coastal Landforms - Coves A cove is a wide, circular bay with a narrow entrance.

They form where there’s a band of hard rock along a coast with a band of softer rock behind it.

Where there’s a weakness in the band of hard rock a narrow gap will be eroded. The softer rock behind will then be eroded much more to form the cove.

Transportation Material is transported along coasts by a process called long shore drift:

Waves follow the direction of the prevailing wind and they usually hit the coast at an oblique angle (any angle that isn’t a right angle).

The swash carries material up the beach in the same direction as the waves. The backwash then carries material down the beach at right angles, back towards the sea. Overtime, material zigzags along the coast.

There are also four other types of transportation: Traction,

Saltation

Suspension

Solution

Deposition When material being carried by the sea water is dropped on the coast. This causes the coast to build up when the amount of deposition is greater than the amount of erosion.

The amount of material that’s deposited on an area of coast is increased when there is lots of erosion elsewhere on the coast so there’s lots of material available and there’s lots of transportation of material into the area.

Low energy waves carry material to the coast but they’re not strong enough to take a lot of material away which means there’s lots of deposition and very little erosion.

Beaches Beaches are formed by deposition by constructive waves which deposit material like sand and shingle. They are found between the high water mark and the low water mark.

Characteristics of a sand beach – Flat and wide with sand particles which are small and the weak backwash can move them back down the beach creating long and gentle slope.

Characteristics of a shingle beach – Narrow and steep with shingle particles are large and the weak backwash can’t move them back down the beach creating steep slopes.

Spit Spits are beaches that stick out into the sea and they are joined to the coast at one end. If it stick out far enough and connect with another bit of the mainland it will form a bar. Spits and bars are formed by the process of longshore drift.

Spits:

Sharp bend in the coastline for example at s river mouth. Longshore drift transports sand and shingle past the bend and deposits it in the sea. Strong winds and waves can curve the end of the spit (forming a recurved end).

The sheltered area behind the spit is protected from waves as lots of material accumulates in this area which means plants can grow there. Overtime, the sheltered area can become a mud flat or a salt marsh.

Bar and tombolo A bar is formed when a spit joins two headlands together.

The bar cuts off the bay between the headlands from the sea. This means a lagoon can form behind the bar.

A bar that connects to the shore to an island often a stack is called a tombolo.

Coastal landforms on maps Erosional landforms:

Waves and arches can’t be seen on a map because of the rock above them. But stacks look like little blobs in the sea.

Cliffs and other steep slopes are shown on maps as little black line.

Wave-cut platforms are shown as bumpy edges along the coast.

Depositional landforms:

Sand beaches are shown on maps as pale yellow.

Shingle beaches are shown as white or yellow with speckles.

Spits are shown by a beach that carries on out to sea, but is still attached to the land at one end. There might also be a sharp bend in the coast that caused it to form.

Economic factors to protect the coastline

1. Loss of tourism because many coastal areas are popular tourist destination. And flooding and erosion can put people off visiting. Fewer tourists means businesses that only rely of tourism may close.

2. Businesses near cliffs that are eroding are at risk of collapsing into the sea and coastal flooding can damage or destroy business premises. Businesses have to relocate, make costly repairs or can be forced to shut down.

3. Coastal flooding damages agricultural land because sea water has a high salt content. Salt reduces soil fertility and so crop production can be affected for years after a flood. Farmland is also lost to coastal erosion which has a huge effect on farmers’ livelihoods.

4. Property prices go down if houses are affected by flooding or are at risk from erosion. People may also be unable to get their properties insured.

Social factors to protect the coastlines

1. Deaths may occur due to coastal floods and they have killed thousands of people on the past.

2. Water supplies can be affected as floodwater can pollute drinking water with salt and sewage.

3. Loss of housing due to homes near the cliffs being affected by erosion and causing them to be at a risk of collapsing into the sea. Over time whole villages can be lost due to erosion. Many people are also made homeless because of floods.

4. Loss of jobs as coastal industries may be shut down because of damage to equipment and buildings for example fishing boats can be destroyed.

5. Damage to infrastructure like roads near cliffs can be affected by erosion and are at risk of collapsing into the sea.

Environmental factors to protect coastlines

1. Ecosystems affected as seawater has a high salt content and increased salt levels due to coastal flooding can damage or kill organisms in an ecosystem. The force of floodwater also uproots trees and plants and standing floodwater drowns some trees and plants.

2. Some Sites Of Special Scientific Interest (SSSIs) are threatened by coastal erosion.

Hard engineering costal defences - ManagementStrategy What it is Benefits Disadvantages

Sea wall A wall made out of a hard material like concrete that reflects waves back to sea.

It prevents erosion of the coast and it also acts as a barrier to prevent flooding.

It creates a strong backwash which erodes under the wall. Sea walls are very expensive to build and to maintain.

Rip rap Boulders that are piled up along the coast. The boulders absorb wave energy and so reduce erosion and flooding.

Boulders can be moved around by strong waves so they need to be replaced.

Groynes Wooden or stone fences that are built at right angles to the coast which trap transported material by longshore drift.

They create wider beaches which slow the waves and this gives greater protection from flooding and erosion.

They starve beaches further down the coast and sand making them narrower. Narrow beaches don’t protect the coast as well leading to greater erosion and floods.

Revetments Slanted structure made of concrete, wood or rocks built at the foot of the cliff.

They absorb wave energy and so reduce erosion.

Expensive to build and they create a strong backwash that erodes under the barrier.

Gabions Rock-filled cages, built at the foot of cliffs. They absorb wave energy so reduce erosion.

They look ugly.

Breakwaters Concrete blocks or boulders deposited on the sea bed off the coast.

They force waves to break offshore so their erosive power is reduced before they reach the shore.

They are expensive and can be damaged by storms.

Soft engineering coastal defences - Management

Strategy What it is Benefits Disadvantages

Beach replenishment Sand and shingle from elsewhere that’s added to the beach

Creates a wider beach which slows the waves down. This gives greater protection from flooding and erosion.

Taking material from the seabed can kill organisms like sponges and corals. It’s very expensive defence and has to be repeated.

Managed retreat Removing existing defences and allowing the land behind it to flood.

Overtime the land will become marshland creating new habitats. Flooding and erosion are reduced behind the marshland. It’s a fairly cheap defence.

People may disagree over what land is allowed to flood.