Factors that affect the rate of erosion

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1. Wave steepness and the breaking point of a wave

Steep waves are destructive - are high energy and so have the power to erode.

Waves that break at the base of a cliff will release more energy on the foot of a cliff and so cause more damage than waves which break some distance away on a wider beach.


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2. The fetch

Waves which have travelled large distances will have more energy to erode.


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3. Sea depth

Coastline with a steeply shelving sea beds are those which will create the highest energy and steeper waves because waves will not break at earlier opportunites.

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4. Configuration of the coastline (shape/alignment

Head lands will experience greater wave attack than bays.


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5. Beach presence and width

Beaches absorb wave energy. Wide, gently shelving beaches are good at dissipating (breaking) wave energy while narrow, steeply shelving beaches provide less opportunity to reduce the impact of wave power upon the cliff foot.


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6. Human activity

Beach width, gradient and type of sediment can be altered by activities such as dredging, sand and gravel extraction and coastal defence schemes.

Coastal defences such as sea walls and groynes may reduce erosion in one location, but because the coastal systems has been altered, this action may lead to increased rates of erosion downdrift.


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7. Geology (rock type)

This is a very important factor. Variation in the rates at which rocks are worn away is known as differntial erosion.

How easily rocks will be eroded will depend on:

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7a. Lithology

Rocks which have few cracks / joints / weaknesses will be more resistant to erosion and take longer to wear away (e.g. granite).

Rocks which have few weaknesses are called massive.

Well jointed rocks such as limestone have lots of weaknesses which may be exposed to wave attack.


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7b. The dip of the rock

The steepest cliffs tend to form where the rocks lie in horizontal layers, or where the rocks dip slightly inland. (Purbeck coastline at Lulworth cove.).

Cliffs that have rock layers or strata that dip towards the sea tend to experience more mass movement and are more gently sloping. Sub - aerial processes are more likely to be important.

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7c. Differences in the variation of resistance to

A concordant coastline occurs when rocks lie parralel to the coast. There is little variation in landform because there is little or no variation in the rock which is experiencing erosion. (The south purbeck coastline is concordant - dominated by tall limestone cliffs.

A Discordant coastline occurs when bands of rock of alternating resistance to erosion are aligned at 90 degrees to the sea. In these circumstances, headlands and bays will form.

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7c. Continued


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Section 2. Sub - Aerial processes (land based)

These are processes operating on the land but which affect the shape of the coastline. Examples include, weathering, mass-movement and run-off.

Sub-aerial processes are actions that happen on the cliff face above high tide.

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  • Frost shattering (freeze-thaw action)

This occurs when water repeatedly freezes and melts with cracks / joints in rocks. As water expands when it turns to ice, pressure is exerted onto the surrounding rock. This pressure is released when the ice thaws. This process when repeated will cause fragments of rock to break off (frost shattering).


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Weathering cont.

  • Biological weathering - weathering resulting from organic agents

Some marine creatures (shellfish) have a specially adapted shells that enable them to "drill" into solid rock, especially chalk. Seaweed attaches itself to rock and wind and wave action prise rock away. Some algae secrete chemicals which slowly dissolve rock. On the cliff face, the roots of vegetation, nesting birds etc can weaken and dislodge rock and soil. (e.g. sandmartins at Hengistbury head)


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Still weathering cont.

  • Chemical weathering

This tends to occur where there is alternate wetting and drying of rocks and towards the base of the cliffs where moisture can accumulate.

A. Oxidation - some rocks can change chemical composition when exposed to water and oxygen. E.g., iron in rocks can turn a rusty colour.

B. Hydration - Some rocks which contain salts may absorb water and swell.

C. Hydrolysis - Granite can be weathered and broken down into china clay because the mineral feldspar is decomposed by water.

D. Carbonation - Carbon dioxide in solution found in rainwater produced carbonic acid. This attacks attacks the calcium carbonates found in limestones for example.

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More chemical weathering..

E. Solution - this is dissolving action on the rocks.

F. Organic weathering - The chemicals released by plants when they decay can break up the land.

G. Acid rain - Limestones and sandstones can be eroded by acidic rainwater which has occured when human activites have released large amounts of carbon dioxide, sulphur dioxide and nitrous oxide into the atmosphere.

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Mass movement

Mass movement is a very common process along the coastline, especially where there are steep, unstable cliffs.

Mass movement takes many forms:

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A. Landslides - These occur where slopes are steeper than 40 degrees. They are rapid events and can happen with little warning. There material removed will accumulate at the base of the cliff (cliff foot) and may be further eroded, or transported by wave action over time. Rock falls are common when cliffs are nearly vertical. Material collapses into the sea from above wave cut notches when cliffs are undercut by the sea. 


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Mudflows & slides, slumps/rotational slips

B. Mudflows - These are fast moving (5 - 15km per year) and occur on steep slopes made of loose, unconsolidated material. Flows are most likely after very heavy rainfall.

C. Slides, slumping/rotational slips - A slide is the movement of a large mass of material. Slumps occur where the movement appears to be rotational in nature. For example, cliffs formed of relatively weak and/or impermeable rocks such as clays are prone to rotational slumping after prolonged rainfall. The cliffs at Barton-on-sea, Hampshire are an example.


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Run - off

D. Run-off - Water which freely flows down the cliff face also has the ability to move fine material such as silt and clay sized particles downslope. This can be significant where there is little vegetation cover to anchor loose soil particles or slow down the flow.

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