Processes in the Coastal Environment
The coastal environment can be viewed as a system, with inputs, processes and outputs. Here, the processes associated with the shaping of the coast are looked at in more detail and the reasons for the constant change is explained. These processes can be split into five separate areas: Erosion, Transportation, Deposition, Sub-Aerial Processes and Mass Movement.
Coastal Erosion
The sea shapes the coastal landscape through the process of erosion, wearing away and breaking up the rock along the coast. Destructive waves, which are most associated with erosion due to their energy, do this in a number of ways.
- Hydraulic Action: As waves approach the cliff, air may become trapped in joints and cracks along a cliff face. When the wave breaks, the trapped air is compressed which weakens the rock over time. Loose rocks are dislodged and transported out to sea with the waves.
- Abrasion: Bits of rock and sand that have been picked up by the wave crash into the shore and begin to grind down the cliff surfaces, in a similar action to sandpaper.
- Attrition: When a wave breaks upon the shore, rocks and pebbles that are being carried collide wirh each other breaking them and eventually making them smaller and smoother.
- Corrosion: (Also known as solution) Salts and acids contained within the sea water will gradually dissolve some types of rock found along the coast. Examples of susceptible rock types include chalk or limestone. This process happens over thousands of years.
Transportation
- There are various sources of material in the sea which has potentially been eroded from cliffs, transported along the coastline via longshore drift, brought inland by constructive waves or carried to the coast along the rivers that flow to the sea.
- Waves can approach the coast at any angle because of the changing direction of the prevailing wind.
- The swash (the movement of water rushing up the beach) of the waves carries material to the shore at an angle , then flows in the backwash (the movement of water flowing back down the beach towards the sea) straight at a 90° angle to the beach back to the ocean.
- Continual swash and backwash transports material sideways along the coast, moving material in a zig-zag fashion. This is called longshore drift.
Transportation Methods
There are four primary ways that waves and tides transport sediment, which in turn contributes to the longshore drift. These include:
> Solution: Minerals that are eroded and dissolved through salt and acids in the sea water are then carried in solution. As the material has dissolved it has effectively vanished and cannot be seen within the water. One example of solution is when chalk or limestone is eroded leading calcium carbonate to be transported through solution.
> Suspension: This is when small particles of sediment and other material is light enough to be carried by the water for most of the time. The type of material that is likely to be carried are silts and clays, often making the water discoloured or cloudy. During storm conditions, due to stronger winds and higher energy waves, more material can be carried in suspension.
> Saltation: Saltation involves the lifting of material and smaller sediments along the seabed. Small pieces of shingle or rocks and large sand grains are transported like this, but as they can be relatively heavy the currents cannot keep them afloat for long periods of time. This results in the 'bouncing' of the sediment being lifted and dropped in the waves.
> Traction: Larger and heavier sediment is transported through traction, whereby the material is rolled along the seabed rather than being lifted by the waves and currents.
> Solution: Minerals that are eroded and dissolved through salt and acids in the sea water are then carried in solution. As the material has dissolved it has effectively vanished and cannot be seen within the water. One example of solution is when chalk or limestone is eroded leading calcium carbonate to be transported through solution.
> Suspension: This is when small particles of sediment and other material is light enough to be carried by the water for most of the time. The type of material that is likely to be carried are silts and clays, often making the water discoloured or cloudy. During storm conditions, due to stronger winds and higher energy waves, more material can be carried in suspension.
> Saltation: Saltation involves the lifting of material and smaller sediments along the seabed. Small pieces of shingle or rocks and large sand grains are transported like this, but as they can be relatively heavy the currents cannot keep them afloat for long periods of time. This results in the 'bouncing' of the sediment being lifted and dropped in the waves.
> Traction: Larger and heavier sediment is transported through traction, whereby the material is rolled along the seabed rather than being lifted by the waves and currents.
Deposition
- When the sea loses energy, it drops the sand, rock particles, pebbles and any other material it was carrying back down on the beach.
- Deposition happens when the swash is stronger than the backwash and is associated with constructive waves.
- Deposition is more likely to occur when:
> Waves enter a sheltered area, for example a cove or a bay,
> There is little wind to create the energy for larger waves,
> There is a good supply of material.
Sub-Aerial Processes
Sub- Aerial processes are land based. They are a combination of weathering (mechanical and chemical) and mass movement.
Mechanical Weathering
Freeze-Thaw
- Water gets into the rock and freezes. When frozen, the water expands by 10% which puts more pressure on the rock. - Repeated action leads to cracks and bits of rock breaking away. - Freeze-thaw usually occurs in areas where the temperature oscillates around freezing point. - The shattered fragments called at the bottom of the cliff and this is then called scree. |
Salt Crystallisation
- Salt crystals from the sea water are deposited within the small cracks and crevices within the cliff face. - Over time, the salt accumulates and applies pressure to the surrounding rock, resulting in cracks widening and rock falling away. This is similar to freeze-thaw. |
Biological Weathering
- Growing plants find small cracks whose roots will begin to make them larger. - Leverage needed by the roots of bushes and small trees create larger cracks as they sway in strong winds. - Burrowing animals and nesting birds will excavate material from the cliff, making the rock there more vulnerable to other types of weathering. |
Chemical Weathering
- Involves the decomposition of rocks or the changing of the minerals.
- Chemical weathering is a result of a weak chemical reaction between water and the rock.
- Rain water mixed with carbon dioxide that is present in the atmosphere forms carbonic acid.
- This reacts with the calcium carbonate in some rocks, for example limestone, which then forms calcium bicarbonate.
- Bicarbonate is soluble in water which is carried away when the caves crash into the cliffs.
Mass Movement
Mass movement is the movement of material down slope as a result of gravity . It can be a slow process, for example soil creep, or it can be a fast process, such as with rockfalls. Water places a big role in most forms of mass movement, acting as a lubricant to add to the movement.
Soil Creep
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Rockfalls and Landslides
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Earth flows and Mudslides
- Earth flows happen faster than soil creep but occur less sporadically.
- They form a characteristic 'hour-glass' shape as slope material liquefies and runs out.
- Earth flows usually occur on slopes made of fine-grained material or clay-bearing rocks when heavily saturated.
- Mudslides or mud flows are earth flows that consist of at least 50% sand, silt and clay-sized particles. The material is so wet and fully saturated that it can flow rapidly.
- The slopes where both earth flows and mudslides occur have very little vegetation growing , leaving nothing to bind the soil together.
- The soil becomes saturated, heavy and lubricated leading to the rapid movement of mud, sand and other materials.
Rotational Slumps
> Marine processes erode and undermine the base of the cliff. > Rainwater infiltrates the cliff, acting as a lubricant beneath loose rocks and soil and becoming a slip plane. > This saturated soil and rock becomes heavier. Under this weight, the material causes the rock to slump. |
Run-Off
- Run-off has the ability to move the finest material down slope, especially where flow of the water is a continuous layer.
- Run-off moves small silt and clay sized particles.