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COASTAL ENVIRONMENTS 1 background waves coastal erosion coastal deposition

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slide1

COASTAL ENVIRONMENTS 1

  • background
  • waves
  • coastal erosion
  • coastal deposition
slide2

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.
slide3

Diagram adapted from: http://www.geology.wmich.edu/kominz/ex3waves.jpg

1 = wave crest 2 = wave trough 3 = wave length

4 = wave height 5 = circular movement of wave particles

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

slide4

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.

Image source: http://www.geographypages.co.uk/coastless.htm

slide5

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.

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.

Diagram source: http://piru.alexandria.ucsb.edu/collections/geography3b/p-s/ps17-07.jpg

slide6

There are two types of waves that affect the coast:

  • 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.
  • destructive waves are high with a short wave length and a short wave period ( 10 – 14 waves per minute). They are often local 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 depressions on the lower beach).
  • 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.
slide7

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.

Wave pounding results from the weight of water in individual waves striking the coast. Typical winter storms may generate waves with an average pressure of 10 tonnes per square metre. This can rise to 30 tonnes per square metre in extreme conditions.

Hydraulic pressure results from air trapped in cracks by high-energy waves. As waves hit the cliffs, air is compressed and then suddenly released. This process weakens cliffs and enlarges cracks.

Photo source: http://www.visitshetland.com/images/dp/eshaness-cliffs-waves-enlarge.jpg

slide8

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.
  • 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
slide9

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 move up and down the beach.

Due to prevailing winds in the UK being from the south-west, the direction of longshore drift along the south coast is west to east. On the east coast, it is from north to south.

Diagram adapted from:http://www.cofc.edu/CGOInquiry/Graphics/longshore%20drift.gif

slide10

Summary of key points:

  • 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 and time that the wind has been blowing which in turn depends on the fetch
  • waves move up the beach (swash) and back down again (backwash)
  • waves can be divided into constructive waves and destructive waves
  • waves approaching irregular coastlines are refracted
  • coastal erosion occurs by wave pounding, hydraulic action, abrasion and corrosion
  • wave transport occurs along the coast by longshore drift which moves beach material in a zig-zag pattern in a direction determined by the prevailing winds