Sediment Transport Modes

Once sediment has been eroded and put into motion, the transport path it takes during further sustained downcurrent movement is a function of the settling velocity of the particle and the magnitude of the current velocity and turbulence. ^[1] Under a given set of conditions, the sediment load may consist entirely of very coarse particles, entirely of very fine particles, or of mixtures of coarse and fine particles. ^[1] The distinction between transport modes is not merely academic - each mode leaves a different sedimentary record. Bedload deposits preserve cross-beds and ripples; suspended load deposits form fine laminations; wash load contributes to clay drapes and floodplain mudstones.

Bedload

Coarse sediment such as sand and gravel moves on or very close to the bed during transport and constitutes the bedload. ^[1] If the shear velocity U* is greater than the settling velocity V, material will remain in suspension; below that threshold, it travels as bedload. ^[1]

Traction Transport

Particles larger than sand size are commonly transported as part of the bedload in essentially continuous contact with the bed - a type of transport called traction transport. ^[1] Traction transport may include rolling of large or elongated grains, sliding of grains over or past each other, and creep. ^[1] Creep results from grains being pushed a short distance along the bed in a downcurrent direction owing to impact of other moving grains. ^[1]

Saltation

Saltation is a type of bedload transport in which grains - particularly sand-size grains - tend to move in intermittent contact with the bed. ^[1] Saltating grains move by a series of jumps or hops, rising off the bed at a steep angle to a height of a few grain diameters and then falling back along a shallow descent path of about 10°. ^[1] This asymmetric saltation path may be interrupted by turbulence or by collision with another grain. ^[1]

Saltation transport may be thought of as intermediate between traction transport and suspension transport, but it is described as part of bedload transport because most saltating grains remain relatively close to the bed during movement. ^[1] The asymmetric path - steep takeoff, shallow landing - is a direct consequence of the flow dynamics: upward turbulent bursts lift grains steeply, but the current then accelerates the grain horizontally so that it descends at a much shallower angle.

Suspended Load

As the flow strength of a current increases, the intensity of turbulence increases close to the bed, and particle trajectories become longer, more irregular, and higher up from the bed than the trajectories of saltating grains. ^[1] Upward components of fluid motion resulting from turbulence increase to the point that they balance downward gravitational forces on the particles, allowing the particles to stay suspended above the bed far longer than could be predicted from their settling velocities in nonturbulent water. ^[1]

Intermittent Suspension

If the lift forces arising from turbulence are erratic and do not continuously maintain balance - a common occurrence during transport of fine-to-medium sand - the grains may drop back from time to time onto the bed. ^[1] This behaviour is called intermittent suspension. ^[1] Intermittent suspension differs from saltation because the suspended particles tend to be carried higher above the bed and remain off the bed for longer periods of time. ^[1]

Smaller particles have settling velocities that may be so low they remain in nearly continuous suspension and are carried along at almost the same velocity as the fluid flow. ^[1]

Wash Load

Much of the sediment load undergoing continuous suspension transport is composed of fine, clay-size particles with very low settling velocities. ^[1] In rivers, this sediment - called the wash load - is derived either from upstream source areas or by erosion of the bank, rather than from the streambed. ^[1] Rivers have the capacity to transport large wash loads even at very low velocities of flow. ^[1] Because the wash load travels in continuous suspension at about the same velocity as the water, it is transported rapidly through river systems. ^[1]

The distinction between wash load and suspended bed-material load is practically important in river management and sediment budgeting: the wash load represents material that is not stored in the channel but passes straight through to the delta or estuary, while bed-material suspended load reflects the capacity of the river to resuspend its own bed sediment.

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