Sediment transport

Sediment transport occurs in natural systems where the particles are clastic rocks (sand, gravel, boulders, etc.

), mud, or clay; the fluid is air, water, or ice; and the force of gravity acts to move the particles along the sloping surface on which they are resting.

Typically, the size of the transported sediment is fine sand (<1 mm) and smaller, because air is a fluid with low density and viscosity, and can therefore not exert very much shear on its bed.

Aeolian sediment transport is common on beaches and in the arid regions of the world, because it is in these environments that vegetation does not prevent the presence and motion of fields of sand.

Glaciers also pulverize rock into "glacial flour", which is so fine that it is often carried away by winds to create loess deposits thousands of kilometres afield.

Sediment entrained in glaciers often moves approximately along the glacial flowlines, causing it to appear at the surface in the ablation zone.

[10] Large masses of material are moved in debris flows, hyperconcentrated mixtures of mud, clasts that range up to boulder-size, and water.

Therefore, managers of highly regulated rivers, which are often sediment-starved due to dams, are often advised to stage short floods to refresh the bed material and rebuild bars.

Geologists can use inverse solutions of transport relationships to understand flow depth, velocity, and direction, from sedimentary rocks and young deposits of alluvial materials.

Therefore, good knowledge of the mechanics of sediment transport in a built environment are important for civil and hydraulic engineers.

When suspended sediment transport is increased due to human activities, causing environmental problems including the filling of channels, it is called siltation after the grain-size fraction dominating the process.

For a fluid to begin transporting sediment that is currently at rest on a surface, the boundary (or bed) shear stress

This basic criterion for the initiation of motion can be written as: This is typically represented by a comparison between a dimensionless shear stress

They do not work for clays and muds because these types of floccular sediments do not fit the geometric simplifications in these equations, and also interact thorough electrostatic forces.

The equations were also designed for fluvial sediment transport of particles carried along in a liquid flow, such as that in a river, canal, or other open channel.

This allows the criterion for the initiation of motion to be rewritten in terms of a solution for a specific version of the particle Reynolds number, called

For a river undergoing approximately steady, uniform equilibrium flow, of approximately constant depth h and slope angle θ over the reach of interest, and whose width is much greater than its depth, the bed shear stress is given by some momentum considerations stating that the gravity force component in the flow direction equals exactly the friction force.

[16] For a wide channel, it yields: For shallow slope angles, which are found in almost all natural lowland streams, the small-angle formula shows that

[21] Ferguson and Church (2006) analytically combined the expressions for Stokes flow and a turbulent drag law into a single equation that works for all sizes of sediment, and successfully tested it against the data of Dietrich.

Bed load transport rates are usually expressed as being related to excess dimensionless shear stress raised to some power.

("breadth"): Due to the difficulty of estimating bed load transport rates, these equations are typically only suitable for the situations for which they were designed.

Because of its broad use, some revisions to the formula have taken place over the years that show that the coefficient on the left ("8" above) is a function of the transport stage:[19][26][27][28] The variations in the coefficient were later generalized as a function of dimensionless shear stress:[19][29] In 2003, Peter Wilcock and Joanna Crowe (now Joanna Curran) published a sediment transport formula that works with multiple grain sizes across the sand and gravel range.

[31] As sand is added to the system, it moves away from the "equal mobility" portion of the hiding function to one in which grain size again matters.

[31] As sand is added to the system, it moves away from the "equal mobility" portion of the hiding function to one in which grain size again matters.

of sand and gravel, respectively in the surface layer, the submerged specific gravity of the sediment R and shear velocity associated with skin friction

is the critical shear stress for incipient motion of the sand fraction, which was calculated graphically using the updated Shields-type relation of Miller et al.(1977)

That all of these formulae cover the sand-size range and two of them are exclusively for sand is that the sediment in sand-bed rivers is commonly moved simultaneously as bed and suspended load.

The bed material load formula of Engelund and Hansen is the only one to not include some kind of critical value for the initiation of sediment transport.

Wash load is carried within the water column as part of the flow, and therefore moves with the mean velocity of main stream.

Structures that modify local near-field secondary currents are useful to mitigate these effects and limit or prevent bed load sediment entry.

Dust blowing from the Sahara Desert over the Atlantic Ocean towards the Canary Islands
Sand blowing off a crest in the Kelso Dunes of the Mojave Desert , California
Deep, eroding glaciofluvial deposits alongside the Matanuska River , Alaska
Sand ripples , Laysan Beach , Hawaii . Coastal sediment transport results in these evenly spaced ripples along the shore. Monk seal for scale.
A glacier joining the Gorner Glacier , Zermatt, Switzerland . These glaciers transport sediment and leave behind lateral moraines .
Suspended sediment from a stream emptying into a fjord ( Isfjorden , Svalbard, Norway)
Mechanisms of movement in sediment transport. (a) Roll: sediment particle is rotated by shear stress along the ground (b) Lift: Sediment particle is lifted by the shear stress into the bulk (c) Pluck: Sediment particle is extracted from crevices in the ground by shear stress
Original Shields diagram, 1936
Streamlines around a sphere falling through a fluid. This illustration is accurate for laminar flow , in which the particle Reynolds number is small. This is typical for small particles falling through a viscous fluid; larger particles would result in the creation of a turbulent wake.
For particles with a small settling velocity, diffusion will increase the complexity of the particle's path to the bottom and the time it takes to settle compared to particles with high settling velocities.
A plot showing the relationship between sediment particle diameter and the Stokes settling velocity
The logarithmic Hjulström curve
A schematic diagram of where the different types of sediment load are carried in the flow. Dissolved load is not sediment: it is composed of disassociated ions moving along with the flow. It may, however, constitute a significant proportion (often several percent, but occasionally greater than half) of the total amount of material being transported by the stream.