Coastal Processes

Wave processes

The extent to which the shape of a beach or coast is altered depends largely on the action of waves upon it. Waves can be gentle and infrequent or larger, more frequent and more powerful.

The formation of waves and their size and shape is a result of the exchange of energy from wind blowing over the sea. The longer the wind blows for, and the greater the distance it blows over, the larger the waves that result, and the greater their energy.

Other factors include:

  1. Wind strength.
  2. Time wind blows for.
  3. Distance (fetch).

In the UK, the direction of maximum fetch is from the South West (for example, if you stand at Lands End, your nearest land mass is the USA) this is why the Cornish Coastline can experience huge high-energy waves.

Constructive waves

These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists. They tend to have a low gradient, a larger swash than backwash, low energy and an elliptical orbit. The wave period is long, with 6-8 waves breaking in a minute.

Destructive waves

These act as agents of erosion, because backwash is greater than swash. They are most common where fetch is short, have a mainly circular orbit, a steep gradient, and 'plunge' onto the beach. The wave period is short, with 12-14 waves breaking per minute.

Wave fetch: The distance of open water over which a wave has passed. Maximum fetch is the distance from one coastline to the next landmass, it often coincides with prevailing wind direction (South West in the UK).

Wave crest: Highest point of a wave.

Wave trough: Lowest point of a wave.

Wave height: Distance between trough and crest.

Wave length: Distance between one crest/trough and the next.

Swash: Water movement up a beach.

Backwash: Water movement down a beach.

Wave definitions

It is very rare for waves to approach a regular uniform coastline, as most have a variety of bays, beaches and headlands.

Because of these features, the depth of water around a coast varies and as a wave approaches a coast its progress is modified due to friction from the seabed, halting the motion of waves.

As waves approach a coast they are refracted so that their energy is concentrated around headlands but reduced around bays. Waves then tend to approach coastline parallel to it, and their energy decreases as water depth decreases.

The process of refraction is outlined below:

Wave refraction

Coastal erosional processes

Abrasion/corrasion:

When waves approach the coastline they are carrying material such as sand, shingle, pebbles and boulders. Abrasion occurs when this material is hurled against cliffs as waves hit them, wearing the cliff away.

Hydraulic pressure:

Cliffs and rocks contain many lines of weakness in the form of joints and cracks. A parcel of air can become trapped/compressed in these cracks when water is thrown against it. The increase in pressure leads to a weakening/cracking of the rock.

Corrosion/solution:

Seawater contains carbonic acid, which is capable of dissolving limestone. The evaporation of salts in seawater produces crystals and their formation can lead to the disintegration of rocks.

Sub-aerial:

Coastal erosional processes that are not linked to the action of the sea. Erosion occurs via rain, weathering by wind and frost. Its impact is often seen in soil creep, slumping and landslides.

Human activity:

Much building and recreation occurs at the coast, and this increases pressure on cliff tops, making them more liable to erosion and subsidence. The building of sea defences upsets the dynamic equilibrium of the coastline.

The rate at which a stretch of coastline is eroded is related to the following factors:

  1. The point at which the wave breaks - (if at the foot of a cliff, the cliff is subject to maximum energy and most erosion).
  2. Steepness of the wave.
  3. Depth of sea, fetch, aspect.
  4. Amount of beach material - (a wide beach protects a cliff more than an arrow beach).
  5. Rock type and structure - (hard rock such as granite is far more resistant to erosion than soft rocks, such as clay).

Features of coastal erosion

It can be said that these are the most common and important erosional coastal landform, due to their number and the amount of pressure human activity places upon them.

They result from the interaction of a number of processes:

  1. Geological.
  2. Sub-aerial.
  3. Marine.
  4. Meteorological.
  5. Human activity.

Cliffs are steep if removal of material at its base is greater than supply.

Cliffs are shallow if the supply of material is greater than removal.

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A direct relationship exists between rock type, erosion rate and cliff morphology.

Hard rock cliffs:

Examples include granite and basalt cliffs. They exhibit a slow rate of erosion and tend to be stable.

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Soft rock cliffs:

Examples include cliffs comprised of glacial till and clay, such as those found at Fairlight Cove in Hastings.

These cliffs often erode rapidly. In these cliffs, sub-aerial processes can contribute more to erosion than marine processes, leading to mass movements such as sliding, slumping and falls.

The diagram below illustrates this:

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Reasons for cliff erosion at Holderness:

The cliffs at Holderness have an average speed of retreat of 2m per year.

  1. Cliffs are made of soft glacial till.
  2. Till is easily eroded at base by waves, resulting in instability.
  3. Rainwater from above enters the till easily, adding to its weight and instability.
  4. Massive slumps and slides occur.

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A similar situation exists at Baton on sea in Hampshire and Beachy Head.

Usually found where less resistant and more resistant rock alternates. The less resistant rock is attacked, first forming bays, and the stronger rock remains as headlands. As wave refraction later occurs, energy becomes concentrated on headlands, leaving them more liable to erosion.

These are gently sloping features, often found extending from the base of a cliff. They consist partly of material removed from the cliff (wave cut notch) as a result of continual undercutting by waves. (See diagram below):

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As undercutting increases, the cliff slowly retreats, leaving a platform with an angle of less than 4 degrees. The platform widens to a point, but due to the cliff being attacked less frequently by waves, it is thought that they can only reach a maximum of 0.5km.

All of the above are secondary features occurring during cliff formation. They originate due to lines of weakness such as joints or faults being attacked and made larger by marine erosion. Caves occur where the weakness is at the base of the cliff, and can become a blowhole if the crack extends all the way to the surface.

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  1. Caves formed on either side of a headland may form an arch if the 2 caves join together.
  2. Stacks are collapsed arches.
  3. Stumps are stacks that have been eroded and lost height.

Coastal transportation

Sediment is moved either up or down the beach by swash/backwash or along by long shore drift.

  1. Clastic sediment: Comes from weathering of rock and varies from very small clay particles to sand/pebbles/boulders.
  2. Biogenic sediment: Skeletons and sediments of marine organisms.
  3. Non-cohesive sediment: Larger particles (for example, sand) moved grain by grain.
  4. Cohesive sediment: Very small clay and mud particles that bond together.

Sources of sediment (load):

  • Rivers entering the sea.
  • Cliffs.
  • Wave erosion.
  • Mud, sand, shingle.

It has been found that the movement of sediment close to the coast around the UK occurs in 'cells'. The result is that the movement of sediment in one cell does not impact on beaches in another.

The coast

The process whereby material is moved along a stretch of coastline. Waves approach the shore at an angle (usually in line with prevailing wind direction) and swash moves material up the beach in this direction. Backwash pulls material straight down the beach.

The result is that material is transported in a zig-zag fashion.

It is important to remember that longshore drift can act on a beach in more than one direction, depending on the approach of waves and wind direction. For example, Newquay in Cornwall has a southwesterly prevailing wind direction and wave approach, but can also receive winds and waves from other directions, such as the North West.

Coastal deposition

Where sand/shingle is deposited on a beach rather than removed - inputs are greater than outputs.

The most common form of coastal deposition that occur as a result of sediment being deposited, that may have come from rivers, and cliff erosion. Human impact may increase the supply of material available.

Spits

Narrow, long stretches of sand/shingle that extend out to sea, or partway across a river estuary. One end is more protected than the other, and mud flats/salt marshes may develop in sheltered areas behind them. One of the most famous examples is Chesil Beach in Dorset:

  • Sandy spits form as a result of dominant constructive swell waves.
  • Shingle spits are a result of dominant destructive waves.

Why do spits develop hooked ends?

Two explanations are offered:

  1. A change from the prevailing wind direction, coinciding with the direction of second most dominant fetch and wave direction.
  2. Wave refraction occurs at the end of the spit which carries some material into more sheltered areas.

Tombolos

This is where a spit or bar connects the mainland to an island.

Such features are uncommon in the UK, but are the most common feature of coastal deposition in the world, shown by their presence on the Eastern Seaboard of the USA from New Jersey south to Florida. They are a number of sandy beaches that are totally separate to the main land, but run parallel to it, meaning that lagoons may develop behind them.

Can be described as triangular beaches. Their origin is due to longshore drift operating on a coastline from two different directions. The two sets of storm waves build up a series of ridges, each protecting the material behind it, creating the triangular feature.

Not strictly a feature resulting directly from marine action, but the blowing of sand from a beach inland.

Conditions for formation:

  1. Strong on-shore winds.
  2. Large expanses of dry sand (spits, cuspate forlands, bays).
  3. Obstacles to limit sand movement.

Sand movement (saltation):

Is helped or hindered by:

  1. Wind velocity.
  2. Grain size and shape.
  3. Dampness of sand.
  4. An obstacle present around which deposition of sand occurs and vegetation grows.

Sand dunes

Sand dune characteristics:

These apply to the diagram above.

Name: Characteristics:
Embryo dune The first part of the dune to develop. Stabilisation occurs via marram and lyme grass, which act as traps for sand. Conditions are dry and plants adapt to this via long roots, or thorny leaves to reduce evapotranspiration.
Yellow dune Colour is due to a lack of humus, but with distance inland they become increasingly grey due to greater amounts of humus. Heights can reach 5m and plants include sand sedge, sea holly, and red fescue.
Fixed grey dunes Limited growth due to distance from beach. Far more stable as shown by existence of thistle, evening primrose, bracken, bramble and heather.
Dune slacks Depressions between dune ridges, which will be damp in summer and water-filled in winter. Species include water mint, rushes, and weeping-willow.
Blow outs Often evidence of over use by humans. Large 'holes' that appear in the dunes.

The most important component for their development is shelter, usually provided by estuaries, barriers, and spits. This is followed by fine sediment in the form of silt and clay grains that is the main input into the system. Over time, sediment is deposited and is not easily removed, especially as flow velocities are low, and the length of time the area is not covered by water increases.

Common vegetation includes algae and Salicornia due to their ability to withstand both being underwater and high levels of salinity. Eventually, Spartina grass may dominate.

Multiple Choice Questions

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Exam-style Questions

  1. a) Describe the main differences between constructive and destructive waves.

    (6 marks)

    b) Referring to a named stretch of coastline, explain how marine, physical and human processes interact to produce a variety of landforms.

    (12 marks)

    (Marks available: 18)

    Answer

    Answer outline and marking scheme for question: 1

    a) Once again, a diagram should be used to support your answer without it the maximum marks available will be 4. For maximum marks you should include:

    • Constructive Waves:

    These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists. They tend to have a low gradient, a larger swash than backwash, low energy, elliptical orbit and a low frequency of 6 - 8 per minute.

    • Wave period is high in contrast:

    Destructive waves act as agents of erosion, because backwash is greater than swash. They are most common where fetch is short, have a mainly circular orbit, a steep gradient, and "plunge" onto the beach. Frequency is height 10 - 12 per minute, but wave period is low, on average 16 -18 seconds between crests.

    So, the question requires 2 definitions (2 marks each and a diagram 2 marks).

    b) Important Points!

    • Name only 1 stretch of coastline it is what the question asks for!
    • 10 is the maximum mark if no human processes are included
    • If the account is general and the coast not located maximum marks will be 5

    The examiner will expect you to have written about:

    • Marine processes combining with sub-Ariel processes
    • Marine processes combining with human processes
    • Erosional features
    • Depositional features

    You gain credit in your answer for:

    • Naming and locating specific features along your stretch of coastline, for example, Spurn head
    • Explaining in depth how marine processes interact with human and other physical processes (a diagram would be a good idea)
    • Using a variety of features

    An answer that includes only a few limited statements, imprecise locations, is very descriptive and only relates to 1 or 2 landforms would not be getting above 5 marks.

    A top level answer would have good structure, reference to a range of physical and human interactions, show a sound understanding of the formation of a number of precisely located features and have accurate well labelled maps / diagrams.

    (Total = 18 marks)

  2. a) Outline 2 of the processes by which coastlines are eroded.

    (4 marks)

    b) Briefly outline the key stages in the formation of a stump.

    (4 marks)

    c) Explain how urbanisation at the coast impacts on coastal processes.

    (8 marks)

    (Marks available: 16)

    Answer

    Answer outline and marking scheme for question: 2

    a) Again, really another question requiring 2 good full definitions of the processes you choose.

    For example:

    • Hydraulic Pressure:

    Cliffs and rocks contain many lines of weakness in the form of joints and cracks. A parcel of air can become trapped / compressed in these cracks when water is thrown against it. The increase in pressure leads to a weakening / cracking of the rock.

    (2 marks)

    b) A fully labelled diagram would help you obtain good marks. In order to have a high level answer be sure to include:

    • Mention of joints or faults being attacked and made larger by marine erosion. Caves occur where the weakness is at the base of the cliff, and can become a blowhole if the crack extends all the way to the surface.

    The followng sequence:

    Cliffs, caves, arch, collapsed arch, stack stump.

    c) This question cannot be answered properly unless you show some understanding / appreciation of both coastal and river processes interacting at the coast.

    For example, "urbanisation and artificial channelisation of rivers speeds up river flow. It can also reduce sediment available to beaches, starving coasts of sediment. Rates of beach erosion are increased as at Barton on Sea that may require human intervention in the form of concrete coastal management strategies. This can then cause problems further down the coast within the same sediment cell."

    Marks: What to look for in your answer:
    7 - 8 A diagram to illustrate the points you have made that is accurately labelled. Clear demonstration of the impact on coastal and river processes. Good use of examples.
    4 - 6 Poor diagram, less detailed explanation and fewer references to examples. More concentration on either rivers or coasts.
    1 - 3 No diagram, very little evidence of understanding of the links between processes. No examples used (or vague).

    (Total = 16 marks)

  3. a) Draw a fully labelled diagram to show the process of long shore drift.

    (4 marks)

    b) Explain why many spits develop a curved end.

    (4 marks)

    c) For any named stretch of coastline evaluate the success of the coastal management strategies adopted.

    (12 marks)

    (Marks available: 20)

    Answer

    Answer outline and marking scheme for question: 3

    a) Make sure that your diagram is annotated as fully as possible in order to gain full marks. In addition you should illustrate:

    • That you know what long shore drift is
    • That it can operate in more than one direction
    • It tends to correspond with direction of maximum fetch and prevailing wind direction
    • It can be limited by the use of groynes

    b) Although a brief answer it is important that you relate to a named spit in this answer. For example, Spurn Head. = 3 marks maximum without it.

    Other marks will come from mentioning:

    • Usual direction of dominant fetch / prevailing wind
    • 2nd most dominant fetch / prevailing wind
    • Frequency of 2nd most dominant fetch and wind
    • Importance of presence of a river or any other area of shelter
    • A quick sketch of the processes acting on the feature

    c) It is vital that you name a stretch of coastline to gain maximum marks. You will need to examine the success and failure of the strategy you have chosen to look at.

    Marks will be awarded in your answer for the following points:

    • Description of area you are looking at, for example, location, pressures on the area past history, rates of erosion, etc.

    (2 marks)

    • Why there was a need for coastal management - aims

    (2 marks)

    • A sketch map

    (2 marks if well annotated)

    • Outline of the measures adopted and how they work

    (2 marks)

    • Evidence of success

    (2 marks)

    • Evidence of failure (perhaps at other coastal locations)

    (2 marks)

    • Conclusion

    (2 marks)

    This means not all the points are needed but instead a good explanation of the points you are making.

    Remember to include facts and figures to support point you make. An example of a good part of this answer is illustrated below:

    "The need for coastal management at Chesil Beach stems from its importance for coastal protection, and the wide variety of flora and fauna that is found on its stony habitat. The plan was devised in 1989 with its main aims being, to manage conflicts between recreational users and limit their impact on ecosystems, educate visitors about the area, reduce soil erosion and produce a sustainable development plan for all interested parties."

    (Total = 20 marks)

  4. a) What is a coastal sediment budget?

    (2 marks)

    b) Explain the difference between isostatic and eustatic sea level change.

    (4 marks)

    c) For a named feature of coastal submergence, explain its formation and usefulness to man.

    (18 marks)

    (Marks available: 24)

    Answer

    Answer outline and marking scheme for question: 4

    a) No need for detail here just a straight definition will gain 2 marks.

    Movement of sediment in one cell does not impact on beaches in another. It is the division of the UK coastline into separate cells, where sediment from one cell does not venture into another. It is very important for coastal management strategies.

    b) 2 marks per definition one on isostatic change the other on eustatic.

    • Isostatic Change:

    Local changes in sea level, due to ice weight depressing Earth's crust lying beneath it.

    • Eustatic Change:

    Large scale / worldwide changes. In times of maximum glaciations three to four times more water was stored on land than it is today. Global change in the hydrological cycle resulted and a worldwide eustatic fall in sea level of up to 150m.

    c) Several choices of features are available although the best / easiest example is that of a Ria. Marks will be awarded for the following:

    • Definition of a Ria: drowned river valleys. In the ice age when rivers still flowed they cut down to the new lower sea level. With the rise in temperatures and release of water from land they became flooded
    • Example of a Ria (Milford Haven in South Wales)
    • Brief diagram of formation
    • Sequence of events in ice age leading to its formation (- Ice sheets and glaciers form, eustatic fall in sea level, and negative change in base level
    • Ice sheets continue to grow. Land is depressed by weight, isostatic change, and positive change in base level

    Ice sheets begin to melt, eustatic sea level rise, positive change in base level.

    • Continued decline of glaciers, isostatic uplift of land, positive change in base level.)
    • What it is used for today (deep water port for the docking of oil tankers)
    • Why it is so suitable for this.
    • Examples of economic and recreational activities that take place in the named ria.

    Each statement will receive at least one mark. Up to 2 with more detailed explanation.

    (Total = 24 marks)

Beach Terminology

Of all the earth's natural environments, the coast is the most rapidly changing and dynamic. It can be defined in several ways, but at its simplest, it is the place where land and sea meet and interact. The coast is under numerous pressures due to its pull for economic activity, settlement, recreation and wildlife. Its form is greatly influenced by:

  1. Terrestrial: Weathering, erosion, deposition, rock type/structure.
  2. Human: Pollution, recreation, settlement, defences.
  3. Atmospheric: Climate.
  4. Marine: Waves, tides, salt spray.

All of the above interact to produce the enormous variety of coastlines found worldwide (from beaches and cliffs to coral reefs).

A beach can be divided into different zones, and the activities that occuron a beach can be placed within particular areas (as shown on the diagram below). The most important function of a beach is to act as a barrier between waves and the coast.

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Backshore:

This area is not usually encroached upon by waves - unless storm conditions arise. Sand dunes and cliffs may be found as well as human activities such as golf courses, nature reserves, conservation and buildings.

Foreshore and nearshore:

The foreshore is located closest to the backshore and it is here, due to the breaking of waves that sediment transport may take place. Much of the energy of waves is reduced (dissipated). The nearshore performs similar functions, but usually only at low tide. Activities in this area include, recreation, quarrying, and coastal defences.

Offshore:

There is limited direct sediment movement here as tidal currents are more important than wave action. Sewage outfalls, oil extraction and fishing may occur.

On the beach itself, different features arise as a result of the action and strength of waves on sand and shingle.

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Berms are formed as sand/shingleis slowly moved up a beach by successive incoming tides - they are more commonon shingle beaches, whilst ridges and runnels are found more on sandy beaches. Both form 'crests'.

These are associated with large spring tides, where, due to the time of greatest gravitational pull tides are highest (see diagram). Some material is thrown up and beyond the usual high water mark because of the large waves. The material remains at the top of the beach if it is not pulled back down the beach by 'swash'. Storm beaches are more common on steep shingle beaches that are affected by destructive waves.

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S-Cool Revision Summary

Backshore This area is not usually encroached upon by waves - unless storm conditions arise.
Foreshore and nearshore The foreshore is located closest to the backshore and it is here, due to the breaking of waves that sediment transport may take place.
Offshore There is limited direct sediment movement here as tidal currents are more important than wave action.
Constructive waves These are depositional waves as they lead to sediment build up, and are most common where a large fetch exists.
Destructive waves These act as agents of erosion, because backwash is greater than swash.
Wave fetch The distance of open water over which a wave has passed.
Wave crest Highest point of a wave.
Wave trough Lowest point of a wave.
Wave height Distance between trough and crest.
Wave length Distance between one crest/trough and the next.
Swash Water movement up a beach.
Backwash Water movement down a beach.
Abrasion/corrasion Occurs when material, such as sand, shingle, pebbles and boulders is hurled against cliffs as waves hit them, wearing the cliff away.
Hydraulic pressure When water is thrown against rock, a parcel of air can become trapped/compressed in a joint or crack. The increase in pressure leads to a weakening/cracking of the rock.
Corrosion/solution Seawater contains carbonic acid, which is capable of dissolving limestone. The evaporation of salts in seawater produces crystals and their formation can lead to the disintegration of rocks.

Sub-aerial

Coastal erosional processes that are not linked to the action of the sea. Erosion occurs via rain, weathering by wind and frost. Its impact is often seen in soil creep, slumping and landslides.
Human activity Much building and recreation occurs at the coast, and this increases pressure on cliff "top"s, making them more liable to erosion and subsidence.
Hard rock cliffs Examples include granite and basalt cliffs. They exhibit a slow rate of erosion and tend to be stable.
Soft rock cliffs Examples include cliffs comprised of glacial till and clay. These cliffs often erode rapidly.
Headlands and bays Hard rock, which resists erosion followed by soft rock that has been eroded to form a bay.
Wave cut platforms These are gently sloping features, often found extending from the base of a cliff.
Caves, arches, stacks, blowholes Secondary features occurring during cliff formation. Originate due to lines of weakness (joints or faults) being attacked and made larger by marine erosion.
Arch When two caves formed on either side of a headland join together.
Stacks Collapsed arches.
Stumps Stacks that have been eroded and lost height.
Clastic sediment Comes from weathering of rock and varies from very small clay particles to sand/pebbles/boulders.
Biogenic sediment Skeletons and sediments of marine organisms.
Non-cohesive sediment Larger particles (for example, sand) moved grain by grain.
Cohesive sediment Very small clay and mud particles that bond together.
Longshore drift Process whereby material is moved along a stretch of coastline. Material is transported in a zig-zag fashion.
Beaches Commonest form of coastal deposition - occurs as a result of sediment being deposited, that may have come from rivers, and cliff erosion.
Spits Narrow, long stretches of sand/shingle that extend out to sea, or partway across a river estuary.
Sandy spits Form as a result of dominant constructive swell waves.
Shingle spits Form as a result of dominant destructive waves.
Tombolos Where a spit or bar connects the mainland to an island.
Barrier beaches and islands A number of sandy beaches that are totally separate to the main land, but run parallel to it.
Cuspate forelands Triangular beaches formed by longshore drift operating on a coastline from two different directions.
Sand dunes Not strictly a feature resulting directly from marine action, but the blowing of sand from a beach inland.
Embryo dune The first part of the dune to develop.
Yellow dune Colour is due to a lack of humus, but with distance inland they become increasingly grey due to greater amounts of humus.
Fixed grey dunes Limited growth due to distance from beach.
Dune slacks Depressions between dune ridges, which will be damp in summer and water-filled in winter.
Blow outs Large ‘holes’ that appear in the dunes. Often evidence of over use by humans.
Mud flat A level area of fine silt along a shore as in a sheltered estuary. Alternatively covered and uncovered by the tide, or covered by shallow water.
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