The concept of transportation in rivers and coastal environments

Transportation refers to the movement of eroded material from one location to another within natural water systems. This process is primarily carried out by rivers and the sea, where water acts as a medium to shift sediment.

The processes involved in transporting eroded material

Transportation occurs through distinct mechanisms depending on the size of the particles and the energy of the water.

Mechanisms of transportation in water systems

• Traction - Large particles, such as rocks and boulders, are rolled or pushed along the river bed or sea floor by the sheer force of the moving water.
• Saltation - Medium-sized particles, like pebbles, are lifted briefly by the water flow and bounce along the river bed or sea floor in a hopping motion.
• Suspension - Smaller particles, including silt and clay, remain afloat and are carried within the water column due to turbulence.
• Solution - Soluble materials, such as certain minerals, dissolve completely in the water and are transported as a dissolved load.

The process of deposition and its causes

Deposition is the process where transported material is dropped or settled in a new location. This happens when the water carrying the sediment loses its energy and can no longer support the weight of the particles.

Conditions leading to deposition

• Loss of velocity - When water slows down, it lacks the energy to carry sediment, causing particles to settle out.
• Reduced carrying capacity - As the flow weakens, the amount of material the water can transport decreases.

Coastal deposition and the role of constructive waves

Coastal deposition shapes shorelines by building up landforms through the accumulation of sediment.

Characteristics of constructive waves

• Wave frequency - These waves have a low frequency, typically around 6-8 waves per minute.
• Wave shape - They are long and low in height.
• Swash strength - The swash, or the forward movement of water up the beach, is strong and carries a significant amount of material onto the coast.
• Backwash weakness - The backwash, or the return flow down the beach, is weaker, meaning less material is pulled back into the sea.
• Net effect - The result is a greater amount of deposition compared to erosion.

Factors increasing coastal deposition

• High erosion rates nearby - Significant erosion along other parts of the coast produces abundant sediment that can be transported and deposited in the area.
• Effective transportation - Large volumes of material moved into the region by currents or waves increase the likelihood of deposition.

Factors leading to river deposition

River deposition occurs under specific conditions where the river's ability to transport sediment is reduced.

Conditions causing deposition in rivers

• Decrease in water volume - A reduction in the amount of water flowing in the river lowers its energy, causing sediment to settle.
• Increase in sediment load - When the river carries more eroded material than it can sustain, it deposits the excess.
• Shallower water - In areas where the river becomes less deep, such as on the inner side of a bend, friction increases, slowing the flow and prompting deposition.
• Reaching a larger body of water - At the river's mouth, where it meets the sea or a lake, the sudden reduction in velocity leads to significant sediment deposition.