# Coastal processes

this is coastal processes

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## intro

The coast is a narrow zone where the land and the sea meet and interact in various ways.

The coast is a complex zone shaped by its geology, tectonic processes, marine processes and atmospheric processes.

Humans now play a big part in altering, protecting and destroying the coast.

Most work along coasts is done by waves.

Waves are created by the transfer of energy from the wind blowing across the surface of the sea.

The size and strength of individual waves depends on:

- the velocity or speed of the wind

- the period of time that the wind has been blowing

- the maximum distance over the sea that the wind can blow (the fetch)

Local or sea waves travel only short distances and are created by local winds.

Swell waves travel huge distances and are created by large storms in the middle of the oceans

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## Properties of a Wave

1 = wave crest

2 = wave trough

3 = wave length

4 = wave height

5 = circular movement of wave particles

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## continued

Wave period = time taken for a wave to travel one wave length

Wave velocity = speed of movement of wave crest in a given period of time

Wave steepness = the ratio of the wave height to the wave length

Wave energy is proportional to the wave length times the square of the wave height

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## Waves

In deep water, wave particles follow a circular motion.

In shallow water (once the depth becomes less than a quarter of the wave length), the wave encounters friction with the sea bed and the circular motion changes to an elliptical motion.

The top of the wave continues to move forward faster than the base of the wave causing the wave to break.

The position of the plunge line will vary according to changing conditions.

As a wave breaks, water rushes up the beach (swash) and is then carried back down the beach by gravity (backwash).

The amount of percolation of the swash depends on the porosity of the beach material.

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## waves

When waves approach a coast with headlands and bays, the waves are refracted.

This means that the wave crests are bent to become increasingly parallel to the coast.

As the wave crest approaches the shallow water around a headland, friction slows the wave but that part of the wave crest in the deeper water of the bay, continues to move forward at a faster speed, so turning the wave crest.

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## continued

Lines drawn at right-angles to the wave crests (known as orthogonals) shown the bending of the wave crests by refraction.

The effect of refraction is to concentrate wave energy on the protruding headlands.

Longshore currents carry the eroded headland material and deposit it in the bays.

In time, the coastland becomes less irregular as headlands are eroded and bays filled in.

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## Constructive & Destructive Waves

1. constructive waves – these are usually low waves with a long wave length and along wave period (6 – 8 waves per minute). They are often swell waves.

Constructive waves steepen slowly as they approach a beach.

The wave breaks gently, the swash moves up the beach slowly and water percolates quickly into the sand.

The backwash is usually weak.

These waves slowly push material up the beach creating sandy ridges or berms (high sandy ridges), and ridges and runnels (small ridges and

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## destructive

2. destructive waves are high with a short wave length and a short wave period ( 10 – 14 waves per minute). They are often local waves.

Destructive waves steepen quickly as they approach a beach.

The waves plunge with greater force on to the beach.

The swash is short and there is a more effective backwash which drags material down the beach.

The overall effect is that whilst some large storm waves may throw shingle to the top of the beach and form a storm ridge, most material is dragged downwards to form a breakpoint bar.

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## Coastal erosion

Processes of coastal erosion can operate in isolation but often work in combination along a coast.

Subaerial processes cause a great deal of cliff recession along some coasts, particularly in areas of soft, sedimentary rocks.

Cliffs can become saturated with water by throughflow.

Cliffs become weakened by weathering processes, particularly freeze-thaw.

As a result mass movement processes, whether slow soil creep or more dramatic slumps and landslides, can cause rapid retreat of the coastline in combination with marine processes of erosion and transport.

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## processes

Processes of coastal erosion can operate in isolation but often work in combination along a coast.

Subaerial processes cause a great deal of cliff recession along some coasts, particularly in areas of soft, sedimentary rocks.

Cliffs can become saturated with water by throughflow.

Cliffs become weakened by weathering processes, particularly freeze-thaw.

As a result mass movement processes, whether slow soil creep or more dramatic slumps and landslides, can cause rapid retreat of the coastline in combination with marine processes of erosion and transport.

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## continued

On most coasts, storm waves are able to pick up large quantities of beach material and hurl it at the cliffs. In this way, the process of abrasion (corrasion) becomes a powerful force which erodes mainly the lower part of the cliffs.

Where the geology is suitable, corrosion (solution) can be an effective erosive force. In areas of limestone or chalk rocks, the sea is often a milky colour where the rock material has become dissolved in the seawater. The salt in seawater is also capable of corroding some rock types.

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## continued

Rates of erosion along the coast are determined by:

- supply of beach material

- beach morphology

- rock resistance, dip and structure of the rock layers

- type of waves

- depth of sea

- direction that coastline faces

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## coastal deposition

Waves are not only capable of transporting material up or down beaches but also along the coastline by the process of longshore drift.

The angle of the swash on a beach is determined by the direction of the winds that created the waves.

The direction of the backwash is always roughly perpendicular to the coastline.

In this way, waves may move material up a beach at an oblique angle but then drag it back down at right angles.

If this process is repeated many times, the beach material moves in a zig-zag pattern along the coast and can be moved many miles.

If, however, wind directions change, it is possible that material may be moved back in the direction from which it came.

On days when the winds blow onshore at right-angles, the material will simply

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longshore drift

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## erosion landforms

In the UK, most coastlines are being eroded.

The rate of erosion is very slow in areas such as the granite cliffs of Cornwall but is very rapid in areas such as the glacial till of Holderness on the east coast.

Fortunately, the highest energy waves from the Atlantic Ocean usually strike the more resistant igneous and metamorphic rocks of western and northern Britain.

The less resistant sedimentary rocks of southern and eastern Britain receive more low energy waves.

High rates of coastal erosion can result in cliff recession of 10 – 20 metres per year.

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## landforms

In the UK, most coastlines are being eroded.

The rate of erosion is very slow in areas such as the granite cliffs of Cornwall but is very rapid in areas such as the glacial till of Holderness on the east coast.

Fortunately, the highest energy waves from the Atlantic Ocean usually strike the more resistant igneous and metamorphic rocks of western and northern Britain.

The less resistant sedimentary rocks of southern and eastern Britain receive more low energy waves.

High rates of coastal erosion can result in cliff recession of 10 – 20 metres per year.

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## cliffs

Many coastlines are backed by cliffs.

The cliffs vary in height, steepness and shape according to the geology of the area.

In general, cliffs increase in height over time as the coast is eroded back.

Chalk rocks tend to form vertical cliffs sometimes of great height (up to 50 metres).

The lower part of the chalk cliffs is relatively easily eroded and regular collapse of the well-jointed rocks keeps the steep profile.

Some well-jointed but more resistant limestone rocks develop overhangs.

If the rocks occur in layers, the dip of the rock also helps control steepness and form.

Beds dipping seaward can produce overhangs whilst beds dipping landward tend to produce more sloping cliffs.

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## Wave cut platform

As cliffs are eroded at the base, a wave-cut notch forms, eventually the cliff collapses and the cliff retreats.

It leaves behind at its base, a wave-cut platform.

This may be bare rock or more usually, it is masked by layers of sand and shingle.

In places, the platform may have depressions which form rock pools at low tide.

The wave-cut platforms have gently sloping angles of around 4 degrees and continue to be eroded (if exposed) by abrasion and corrosion processes.

The width of the platform is determined by:

- the rate of retreat of the cliffs

- the length of time that the sea has been at its present level

- the tidal range

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## stacks caves arches

Along irregular coastlines, joints and other weaknesses in headlands tend to be exploited by the waves to form caves of various shapes and sizes.

These caves may eventually be eroded back through the headland to form an arch.

Continued erosion and weathering will eventually weaken the arch so that it collapses to leave behind a pillar of rock or stack.

Ultimately the stack will be reduced to a stump and eventually disappear.

Sometimes the sea cuts inland along a joint or fault line to form a narrow, steep-sided inlet called a geo.

In well jointed rocks, the sea may erode vertically at the back of caves to form a blowhole.

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## depositional landforms - beaches

Deposition along a coast occurs in areas of low-energy waves where the input of material exceeds the output.

Beaches and other depositional landforms represent a store within the marine system.

Beaches tend to be composed of either sand or shingle but sometimes both occur at different places along the beach profile.

Shingle beaches tend to be steeper than sandy beaches.

At the upper end of the beach, storm ridges occur.

As these are often beyond the reach of waves except in the highest tides, they may become colonised by salt-tolerant plants.

Lower down the beach, there are ridges or berms which correspond to successively lower tides.

The profile of a beach is constantly changing but will show the greatest contrast between spring and neap tides and between summer and winter seasons.

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## spits

When longshore drift transports material along the coast, it sometimes comes across an estuary or a change in the direction of the coastline.

In either case, the transport process tends to carry on moving the material in the same direction.

Over time, a ridge of material will build up into the deeper water.

This will form a spit.

Eventually the spit may form a substantial feature, many miles long with sand dunes and plants, possibly even buildings on it.

Many spits show a series of recurved ends which relate to material moved by winds creating waves from a different direction.

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## continued

In the low-energy environment behind the spit, deposition of fine silt and mud will occur and salt marsh forms.

This helps to stabilise the landform.

Over time, spits may move inland and many are regularly broken by storms and reform in subsequent years.

If longshore drift carries material across a bay, it may form a bay barrier or bar.

Such bars rarely form across estuaries as the power of the river erodes material from the ends of the spit if the gap becomes very narrow. If longshore drift transports material across to a nearby island, then it forms a tombolo linking the island to the mainland.

Barrier islands are common depositional landforms along many coast such as the east coast of the USA.

They are created from a series of sandy islands totally detached from, but running parallel to, the mainland.

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In Britain rapid rates of erosion occur in the less resistant sedimentary rocks of the south and east

Headlands and bays are characteristic of areas where the geology consists of alternating bands of hard and soft rock

Many coastlines are backed by cliffs. The cliffs vary in height, steepness and shape according to the geology of the area.

As cliffs are eroded at the base, a wave-cut notch forms, eventually the cliff collapses and retreats. It leaves behind at its base, a wave-cut platform.

In headlands, lines of weakness are exploited to form caves, arches, stacks and stumps. Geos and blowholes may also form.

Deposition creates beaches with varying profiles, spits jutting out into estuaries, tombolos joining islands to the mainland, bars and barrier islands.

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## sand dunes

Many coasts have developed sand dune formations.

The source of the sand may be ancient offshore deposits or material eroded from local cliffs.

In order for sand dunes to develop, you need an abundant supply of fine sand, a prevailing onshore wind and then something to trap and retain the sand.

Seaweed and beach debris may initially trigger a small embryo dune.

Once the pioneer plant species such as lyme grass take root, more sand accumulates and the dune grows.

Marram grass is the main coloniser and stabiliser of sand more mature yellow and grey dunes.

Eventually other plants, which are less salt tolerant will colonise the dunes, leading ultimately to a climatic climax woodland ecosystem.

In low-lying areas or slacks, marsh plants such as juncus will colonise.

If the dune vegetation cover is broken by human activity or rabbit burrowing, blow-outs may result with large amounts of sand being removed from the damaged dune.

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## sea level change

Apart from daily and seasonal changes in sea level due to the tides, the average sea level changes long term due to a variety of factors. During the last ice-age, water was held back from re-entering the oceans as it remained frozen on the land. This eustatic fall in sea level resulted in a drop of between 100- 150 metres compared to present sea level. At the same time, the great weight of ice that accumulated (many kilometres thick in places) was sufficient to depress parts of the earths crust leading to an isostatic change in sea level. After the ice age, there was a relatively rapid eustatic rise in sea level as the ice melted and water returned to the oceans. It has, however, already taken at least 10,000 years for the land to very slowly rise again as the weight of the ice was removed. Thus the isostatic readjustment is still going on in areas such as Norway and Scotland (4-20mm per year). Other causes of sea level change include: - tectonic uplift of new mountain ranges - tectonic tilting of the land - local earthquake and volcanic activity  Changes in sea level lead to distinctive coastal scenery and landforms. Submergence of the coastline leads to the drowning of many low-lying coastal areas. Many river estuaries have been formed or enlarged by submergence. Rias are the drowned lower portions of river valleys. Many have only small rivers or streams flowing into large, deep estuaries. These form excellent natural harbours and are a common landform in south-west England. Fjords are the drowned lower portions of glaciated valleys. They are broader and deeper than rias and have very steep sides. They are a common landform along the coast of Norway. A dalmation coastline is formed where the geology creates valleys parallel to the coast so that when sea level rises, a series of elongated islands remain offshore.

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## coral reefs

Coral reefs are among the most diverse and productive communities on Earth.

They are found in the warm, clear, shallow waters of tropical oceans worldwide.

Reefs are formed from calcium carbonate produced by tiny coral polyps.

Reefs have functions ranging from providing food and shelter to fish and invertebrates to protecting the shore from erosion.

Revenue from tourists attracted to the beauty of coral reefs can be a significant source of income for human communities in these areas.

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Corals and coral reefs are extremely sensitive.

Slight changes in the reef environment may have detrimental effects on the health of entire coral colonies.

These changes may be due to a variety of factors, but they generally fall within two categories: natural disturbances and anthropogenic disturbances.

Although natural disturbances may cause severe changes in coral communities, anthropogenic disturbances have been linked to the vast majority of decreases in coral cover and general colony health when coral reefs and humans occur together.

Discharge from power plants and sewage works alter the sea temperature and the amount of nutrients in the water.

Agriculture, particularly where soil erosion is a problem, decreases the amount of light reaching the coral due to increased sedimentation.

Overfishing by humans has altered the food chain.

The collection of coral for tourist souvenirs has damaged many reefs.

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