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:
- Wind strength.
- Time wind blows for.
- 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.
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.
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.
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 reducedaround 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:
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:
- The point at which the wave breaks - (if at the foot of a cliff, the cliff is subject to maximum energy and most erosion).
- Steepness of the wave.
- Depth of sea, fetch, aspect.
- Amount of beach material - (a wide beach protects a cliff more than an arrow beach).
- 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
Cliff
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:
- Geological.
- Sub-aerial.
- Marine.
- Meteorological.
- 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.
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.
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:
Reasons for cliff erosion at Holderness:
The cliffs at Holderness have an average speed of retreat of 2m per year.
- Cliffs are made of soft glacial till.
- Till is easily eroded at base by waves, resulting in instability.
- Rainwater from above enters the till easily, adding to its weight and instability.
- Massive slumps and slides occur.
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):
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.
- Caves formed on either side of a headland may form an arch if the 2 caves join together.
- Stacks are collapsed arches.
- 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.
- 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.
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 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.
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:
- A change from the prevailing wind direction, coinciding with the direction of second most dominant fetch and wave direction.
- Wave refraction occurs at the end of the spit which carries some material into more sheltered areas.
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:
- Strong on-shore winds.
- Large expanses of dry sand (spits, cuspate forlands, bays).
- Obstacles to limit sand movement.
Sand movement (saltation):
Is helped or hindered by:
- Wind velocity.
- Grain size and shape.
- Dampness of sand.
- An obstacle present around which deposition of sand occurs and vegetation grows.
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.
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