Processes of Transportation and Deposition (AQA A-Level Geography): Revision Notes

Processes of Transportation and Deposition

Marine transportation

Wave and tidal energy that isn't consumed during erosion or lost through friction with the seabed can be used to move material along the coast. The transportation of sediment by the sea is an important mechanism of coastal change. It drives both erosion and deposition processes and represents a significant transfer of material along coastlines.

The sea transports material through four main processes:

Traction

This process involves the largest sediment. Heavy stones and boulders are rolled and pushed along the seabed and beach by the force of moving seawater. Traction requires high energy conditions to move these large particles, so it typically occurs during storms or in areas with powerful waves and strong currents.

Saltation

Smaller stones and pebbles bounce or jump along the seabed and beach in a leapfrog motion. This process happens in relatively high energy conditions. The movement works through a chain reaction: small particles are thrust upward from the seabed, only to fall back down. As these particles land, they dislodge other particles upwards, creating continuous bouncing movements across the seabed.

Suspension

Very fine particles of sand and silt are carried along within the moving water itself. The material doesn't just get transported passively; it's also picked up from the seabed, mainly through turbulence in the water. Large amounts of suspended sediment, particularly near river mouths and estuaries, can give the sea a milky or cloudy appearance.

Solution

Dissolved minerals and chemicals are transported within the mass of moving water. This is an invisible form of transportation, as the materials are chemically dissolved rather than physically carried. The dissolved load can be substantial over time.

As waves rarely approach the coastline straight on, these transport processes combine to move material sideways along the coast.

Longshore drift

When waves approach the shore at an angle, material gets pushed up the beach by the swash in the same direction as the incoming waves. As the water runs back down the beach, the backwash drags material down the steepest gradient, which is usually perpendicular to the shoreline where gravity pulls the water most directly back to sea.

Obstacles such as groynes (wooden barriers built perpendicular to the beach) and piers can interrupt this drift. Sediment accumulates on the windward side of these structures, leading to entrapment of beach material. Deposition of trapped material occurs in sheltered locations, such as the head of a bay or where the coastline changes direction abruptly. In these areas, spits often develop.

Marine deposition

Deposition occurs when waves have low energy or where rapid coastal erosion provides an abundant supply of material. The sea lays down material when there is a reduction in energy. This happens due to a decrease in velocity or volume of water.

Several situations lead to deposition:

• Sand and shingle accumulate more quickly than they are removed
• Waves slow down after breaking, reducing their carrying capacity
• Water pauses at the top of the swash before backwash begins, causing temporary settling
• Water percolates down into the beach material as backwash occurs, reducing the volume of water returning to the sea

These conditions are most common in sheltered bays, behind coastal defences, and in areas protected from prevailing winds and waves.

Aeolian deposition

Wind is an almost constant feature of coastal areas, not just because of the general pattern of prevailing winds that drives the waves. During the day, wind on the coastal fringe usually blows from the sea onto the land. Air moves in response to small pressure differences created by the warmer land and colder sea. When there is a large tidal range, extensive areas of sand may be exposed at low tide. This provides an ample supply of sediment to be picked up and transported by the wind.

Sand-sized sediment is the most significant material for creating depositional features at the coast. Once picked up by the wind, sand typically travels close to the ground and over relatively short distances before being deposited.

Surface creep

This process works similarly to traction in water. Wind rolls or slides individual sand grains along the surface. The grains maintain contact with the ground as they move.

Saltation

When wind is strong enough, it temporarily lifts sand grains into the airflow. The grains can be carried to heights of up to one metre and transported for distances between 20 and 30 metres before falling back to the surface.

Sub-aerial weathering

Sub-aerial weathering encompasses processes that slowly break down the coastline. These processes weaken the underlying rocks, making them more vulnerable to sudden movements or erosion. Material is broken down in situ, meaning it remains in or near its original position rather than being transported away immediately.

Three main categories exist: mechanical/physical weathering, biological weathering, and chemical weathering.

Mechanical/physical weathering

These processes depend on the nature of the climate and physical forces acting on rocks.

Pressure release occurs where processes of erosion, weathering and mass movement remove overlying material from rock. The rock beneath experiences pressure release as the weight above it is unloaded. This unloading mechanism causes the rock to expand slightly, which develops weaknesses such as cracks and joints. These allow the rock to expand further, making it increasingly susceptible to other erosion and weathering processes.

Biological weathering

These are processes that lead to rock breakdown through the action of vegetation and coastal organisms. Biological weathering is particularly active on coastlines.

Some marine organisms have specially adapted features that enable them to bore into solid rock. The piddock, a type of shellfish, is especially effective at drilling into rock. These organisms are particularly active in areas with chalk geology, creating a distinctive sponge-like rock surface pitted with holes.

Seaweed attaches itself firmly to rocks. The action of the sea can be powerful enough to cause swaying seaweed to prise away loose rocks from the seafloor.

Chemical weathering

Chemical weathering occurs where rocks are exposed to air and moisture, allowing chemical processes to break down the rock material.

Solution is the main chemical process at coasts. It combines with erosion to produce many distinctive coastal features. When minerals dissolve in water, they are removed in solution.

Oxidation causes rocks to disintegrate when oxygen dissolved in water reacts with certain rock minerals. This particularly affects ferrous (iron-rich) rocks, producing distinctive brownish or yellowish staining of the rock surface.

Hydration makes rocks more susceptible to further chemical weathering. It involves the physical addition of water to minerals in the rock. This causes the rock to expand, creating stress which can itself cause the rock to disintegrate. The process weakens the rock and can create or widen cracks and joints, allowing further chemical weathering to occur.

Carbonation occurs where carbon dioxide dissolved in rainwater creates a weak carbonic acid. This reacts with calcium carbonate in rocks like limestone and chalk to create calcium bicarbonate, which then dissolves easily in water. Carbonation is more effective with cooler temperatures, as this increases the amount of carbon dioxide that can dissolve in the water. As levels of carbon dioxide in the atmosphere continue to rise due to industry and fossil fuel burning, there are increasing levels in rainwater too, making it mildly acidic.

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