erosion, weathering and mass movement

Erosion, Weathering and Mass Movement

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First of all, a few reminders

Swash is the movement up the beach and backwash is down.

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Coastal erosion• The force of the sea changes the coastal

landscape. Waves get their energy from the wind.• The size of the wave is determined by:

– the speed of the wind – the length of time the wind has been blowing – the distance of sea it has travelled over (the fetch)

• The stronger the wave, the more erosion it will cause.

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• Hydraulic action: the sheer force of waves crashing against the shore and cliffs. The power of the waves forces air into cracks, compresses it and blows the rock apart as the pressure is released, but only after continuous wave attack month after month.

• Attrition: rocks and pebbles carried by the waves smash into each other, wearing each other away and gradually becoming smaller, rounder and smoother.

• Abrasion: also called corrasion - the process of rocks and pebbles carried by the waves wearing away rocks as they are thrown against cliffs.

• Solution: also called corrosion - chemicals in the seawater dissolve minerals in the rocks, causing them to break up.

Which one is not strictly an example of erosion? Why?

The processes involved in erosion are:

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Waves are one of the most significant forces in shaping the coastline. There are two main types of

wave ...Constructive waves are low energy waves. They are small in height. They have a strong swash and a weak backwash. This means that constructive waves tend to deposit material and build up a beach.

Destructive waves have much higher energy. They are much larger in height, often having been caused by strong winds and a large fetch. They have a weak swash but a strong backwash so they erode the beach by pulling sand and shingle down as water returns to the sea. This means that less beach is left to absorb wave energy.

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Cave cut by abrasion & hydraulic action where there is a weakness in the rock

Caves either side of headland meet to form arch

Arch collapses to detach land from mainland

Wave cut notch - undercutting

StackUndercutting

will reduce the stack to a

stump

Headland

Just below the water line will be a wave cut platform

Original position of headland

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• Erosion necessitates movement – i.e. the waves crashing against the cliff face

• Weathering is in-situ and does not involve movement. Weathering can be mechanical (freeze-thaw activity) or chemical (corrosion, sometimes called solution).

• If weathering occurs on a slope the loose debris will move with gravity down hill. This is called mass movement.

• Erosion by waves tends to occur at the cliff foot but weathering and mass movement affect the cliff face and these two are collectively known as sub-aerial processes

• There are three main types of mass movement …

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Rockfall: rapid, free-fall of rock from a steep cliff face due to gravity. This is made worse by freeze-thaw action loosening the rock - water enters the joint (vertical crack), freezes and expands, breaking up the rock. A scree slope of fallen rock is formed at the bottom of the cliff. It is also possible for free falls to occur in very dry conditions – for example clay cracks as it dries out and then crumbles easily

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Mudflow: occurs on steep slopes over 10°. It's a rapid sudden movement which occurs after periods of heavy rain. When there is not enough vegetation to hold it in place, saturated clay flows out of the cliff face, almost like a river of mud.

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Landslips: also known as rotational slumps, are occasional rapid movements of a mass of earth or rock dropping down along a concave plane. Water percolating through sandstone gets into the clay beneath, saturating it. With the weight of the rock above forcing down on it the clay moves seawards as a mud flow. With the clay moving sideways the sandstone above slumps down. Undercutting of a steep slope by the sea weakens the rock above, making a slump more likely.

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The speed and nature of mass movements is influenced by …

• Angle of slope – the steeper the slope the faster the movement

• Rainfall – a lot of water will lubricate the cliff, especially if it is clay – and lead to mud flows

• Vegetation cover – with no vegetation there is nothing to impede the movement of debris. But if vegetation is preit will absorb some of the water and the roots will bind the soil making it more stable.

Sept 1990 July 1991

Early Aug 1991Oct 1991

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What has happened

here? What is this called?

Scree - that will be washed away by the sea at high tide

Free fall mass movement in dried out crumbling clay

Sandstone

Clay

Wet sand falling down the cliff

Why is there a pool of water

here?

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Describe what you see. Can

you explain what is going on?

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Slumping

Wet clay ‘flowing’ from cliff

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Do you know what form of

mass movement this

is?

Mudflow

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Waterlogged /saturated clay

Water flowing out of the clay

What is this?Where has the sand gone?

What’s the evidence that these cliffs are receding?

How do we know these

cliffs are receding?

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So what’s this here?

It’s not that long ago that I would be standing on the cliff top out here!

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What is it that has weakened the cliff face?

Why has water

collected down here?

Why is some of the cliff

face a darker orange?

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How long will it take for the sea to remove the

clay fallen from this cliff?

How far does the tide come in? How do you know?

Describe what this is.

What process has shaped the material?

Describe what has happened on this stretch

of the cliff

Which direction is the longshore drift? How do you know?

Why have they managed the area to the left (south) and not the right (north)?

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WHAT? WHEN? WHERE? WHY? WHO?

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Holbeck Hall Hotel, Scarborough – a massive slump (1993)

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Would you spend millions protecting this coast from

erosion? Why?

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Managing coasts• Approaches to managing coasts fall into two

types - hard engineering and soft engineering techniques.

• Coastal protection that uses engineered man made structures such as sea walls to defend the coast against erosion are examples of hard engineering

• A form of coastal protection that works with nature and does not change the character of the environment in striking ways such as beach nourishment are examples of soft engineering.

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Give TWO pieces of

evidence to suggest these groynes are

working

What’s the purpose of groynes? Are they

hard or soft engineered protection

techniques?

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Gabions are wire cages filled with hard rocks like flint. They act like a sea wall but are less expensive. The downside is that they do not last as long as a sea wall because the rocks will get worn down by abrasion and attrition. They are sometimes used to hold back the land on the cliff face helping to reduce the chance of mass movement

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This is a revetment. As the waves armed with sand and pebbles hit them the particles are able to move through the slats but as the wave retreats with the backwash there is insufficient energy to move them back out to sea. In this way a beach on the land side is able to accumulate to protect the land.

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Rock armour (rip-rap) is sometimes dumped at the foot of the cliff. As waves crash onto them the gaps between the boulders help dissipate the wave energy

Rip-rap is made from very hard rock, like granite. In soft rock areas like Walton-on-the-Naze they have to be imported and are therefore not sustainable.

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When the beach is depleted sometimes sand is dredged from the sea bed and deposited on the beach to make it more substantial and a better protection. When the beach is wide and deep waves gently wash onto it and do not attack the land behind it.

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How is the land being managed?

1. Re-shaping the land to make it less steep and therefore more stable2. Vegetated slopes to bind soil and take some water out of the land3. Groynes to help stop longshore drift and therefore build up the beach4. Footpaths to discourage clambering down the cliffs and eroding the land5. Sea wall to protect against wave erosion6. Various drainage channels (difficult to see amongst the vegetation) to

ensure the clay soils do not get waterlogged

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How many types of coastal

management can you find?