Coastal Features -erosion Processes

Coasts Page

EROSION – TRANSPORTATION - DEPOSITION

There are four main processes by which the sea can erode a coastline. These are:

•  Hydraulic pressure (sometimes called hydraulic action)  This is the sheer force of the waves especially when they trap and compress air in the cracks and holes in a cliff. • Corrasion / Abrasion This happens when the waves hurl particles, for example small pebbles, at cliff surfaces. • Attrition This occurs when the waves causes rocks and pebbles on the beach to smash into each other and break down in size, eventually forming sand. • Corrosion/ Solution This happens when certain types of cliff are slowly dissolved or decomposed by the acids in the sea water.

Pure energy……...

Factor in the development of coastlines WAVES • •

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These are really a transfer of energy not a movement of water until the wave breaks They are caused by the friction of air passing over the water. The stronger the wind the greater the friction the bigger the wave. FETCH ~ the distance an energy wave travels and the direction it comes from is called the FETCH. E.g. The fetch of the West coat of Britain is about 3500 km whereas the fetch of the east coat is only about 500 km. The greater the fetch, the larger the wave.

Factor in the development of coastlines

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WAVES

Water particles move in an orbital pattern. As the water comes in contact with sea bottom when it gets shallower the lower section of the circular motion slows down, but the top section continues at the same speed. The wave becomes elliptical, top-heavy and then over balances; the wave breaks and the energy is released SWASH ~ the wave travelling up the beach BACKWASH ~ the wave travelling back down the beach. Waves are responsible for most erosion and deposition

A wave cycle

Factor in the development of coastlines

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CONSTRUCTIVE WAVES

~ Usually in the Summer. These are low energy

waves. They deposit materials and build beaches.These are wave with a long wave length, low in height. The swash travels a long way up the beach, but much of the water soaks into the beach before it runs back. There is little backwash. Sediment getting pushed up the beach gets left behind.

Factor in the development of coastlines

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DESTRUCTIVE WAVES

~ Usually winter. These are high energy waves.

They have more power and can remove the sand from a beach very quickly. The most destructive waves occur during storms. These are waves with a short wave length, high with steep slopes. This prevents a good swash, but encourages a very strong backwash. Sediment gets dragged off the beach.

CURRENTS

Factor in the development of coastlines

• This is a flow of water. They are able to prevent a build up of deposition and can remove and carry sediment but do not erode rocks.

TIDES

•The tides themselves cannot erode, but the AMPLITUDE, the difference between the high and low tide points. •The AMPLITUDE controls the amount of shoreline exposed to erosion.

Factor in the development of coastlines

NATURE OF THE COASTLINE

• DISCORDANT coastline ~ the “grain” or geology of the coast is the opposite to the coast. But the geology runs West to East Chalk

The coastline runs South to North

This is the resulting headland of the “Foreland”, made of a band of more resistant chalk. Stacks ~ Old Harry

Stack and arch Wave cut platform

Chalk Ridge

Clay Vale

Limestone Ridge

Resistant Headland with Caves, arches, stacks and stumps

Hills results in cliffs Bay forms in less resistant clay Headland of resistant limestone

Factor in the development of coastlines

NATURE OF THE COASTLINE

•CONCORDANT coastline ~ the “grain” or geology of the land is running parallel to the coast.

•Upland or lowland ~ resulting in cliffs or coastal plain.

Lulworth Cove

Lulworth Cove Aerial Photo

Chalk

Limestone Lulworth Cove formed behind the breach in the Limestone

Sand & Clay

Factor in the development of coastlines

Geology

• Small scale ~ joints, cracks and intrusions • Medium scale ~ bedding planes – lines of separation between two types or layers of sedimentary rocks. (Common in chalk and limestone).

• Large scale ~ different rock types affect the speed of erosion because of their structure and resistance. • Bedding plane angle ~ this is important because the angle of the cliff relates to it.

Factor in the development of coastlines

Bedding plane angle

Vertical bedding plane = steep cliff and vertical collapse.

Horizontal plane = steep cliff with undercutting of the strata and vertical collapse.

Dipping plane toward the sea = gentle cliffs, often with stepped or rotational slumping.

Factor in the development of coastlines

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Changing Sea Levels ~ this causes erosion and deposition to occur at different heights eg. Abandoned stacks, raised beaches

Factor in the development of coastlines

• MAN ~ building sea walls, breakwaters, harbours, groynes, and reclaiming land will all affect the amount and rate of erosion and deposition.

Note wider beach this side of the groyne.

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