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Scheme of Work

The global hydrological cycle

The drainage basin hydrological cycle

The storm hydrograph

Fluvial erosion and transportation

Sediment deposition and the Hjulstrom curve

Valley Long Profile

Valley Cross Profile

Channel efficiency

Landforms of fluvial erosion

Landforms of fluvial deposition

Flood management

Hard and soft solutions

 

Sediment transportation

A river transports its sediment load in a variety of ways. The methods of transport are also used to describe the various loads of a river, i.e. the bed or traction load, suspension load, dissolved load, and suspended load).
The sediment load varies from river to river, along the course of one river or in the same place at different times.
This is because the velocity of the water is crucial in determining the way that sediment is transported. The relationship between erosion, transport and deposition of sediment is complex and can be shown by the Hjulstrom diagram.

 

This is based on experimental work rather than natural channels, but it shows the principles involved. Entrainment is the process of starting the particles of sediment moving - the opposite of settling.

  • High velocities result in sediments being transported in the river flow, while low velocities result in sediment being deposited.

  •  Medium sand (0.25-0.5 mm in diameter) is moved at the lowest velocities.

  •  Larger, heavier sediments need higher velocities to start movement.

  •  Silt and clay need higher velocities than their size would suggest because they are cohesive (they stick together) and so, in fact, are bigger than they should be.

  •  Once set in motion, fine particles can be transported even if the velocity falls.

  •  Larger coarse particles are deposited rapidly as velocity falls. In channels with mainly boulders and gravel, transport only occurs at high flows. 

  • In natural channels the situation will be more complex. For example, small particles may be sheltered by larger particles and therefore they are not moved.

  •  Velocity of flow is variable across and vertically in a natural channel, and this will affect the processes. 

  • Sediment transported under lower-flow velocities, as bed load, may become suspended load under high velocities.

A river must have energy available to perform the work of transporting sediment. A river's power is the energy available to overcome friction and to move sediment. This will be greatest in rivers with high discharge, high downstream gradient, and an efficient channel. If the river has just enough energy in a particular section (reach) to transport the sediment available, then it is in equilibrium. If it has more power available than needed to transport its sediment load, the river will have excess energy and will erode its channel. If there is less energy available than is needed to transport its load, then deposition of the sediment will result. Two further terms are useful in describing sediment transport. The competence is the maximum sediment particle size that can be carried at a particular velocity. The capacity is the total load of sediment that the river can carry. These will vary with both discharge and velocity.

Deposition
The velocity at which a sediment particle drops to the channel bed is called the settling velocity . This depends upon the size and shape and density of the sediment particle. Deposition may be temporary on the channel bed and the sediment may be moved again at a time of higher flow. In other situations there is a net deposition of sediment, and a deposition landform results, e.g. floodplains and point bars on the inside of meander bends.