Sediments loads

Lecture 6Jyoti anischit

Roll 10

Sediment load

Bed load transport

• Bed load. The heavier, coarser grained earth ‐material that travels along the bottom of the stream is the bed load. Traction occurs when these fragments move along by rolling and sliding. Turbulent or eddying currents can temporarily lift these larger grains into the overlying flow of water—the grains advance by short jumps or skips until the surge diminishes and then fall back to the bottom because of their greater weight. This process is called saltation.

Incipient motion

• Incipient motion is an important critical condition, for which sediment starts to move under the action of flow. In 1753, Brahms suggested that the velocity for incipient motion is proportional to the grain weight raised to the one-sixth power.

Boundary condition: Bed shear or tractive force

tractive force• Traction, or tractive force, is the force used to

generate motion between a body and a tangential surface, through the use of dry friction, though the use of shearforce of the surface is also commonly used.

• For a fluid to begin transporting sediment that is currently at rest on a surface, the boundary (or bed) shear stress exerted by the fluid must exceed the critical shear stress for the initiation of motion of grains at the bed

Tractive force

Empirical equation

• an empirical relationship is a relationship or correlation based solely on observation rather than theory. An empirical relationship requires only confirmatory data irrespective of theoretical basis.

Hjulstrom approach

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.

• In 1935, Filip Hjulström created the Hjulström curve, a graph which shows the relationship between the size of sediment and the velocity required to erode (lift it), transport it, or deposit it.[18] The graph is logarithmic.

• Åke Sundborg later modified the Hjulström curve to show separate curves for the movement threshold corresponding to several water depths, as is necessary if the flow velocity rather than the boundary shear stress (as in the Shields diagram) is used for the flow strength.[19]

• This curve has no more than a historical value nowadays, although its simplicity is still attractive. Among the drawbacks of this curve are that it does not take the water depth into account and more importantly, that it does not show that sedimentation is caused by flow velocity deceleration and erosion is caused by flow acceleration. The dimensionless Shields diagram is now unanimously accepted for initiation of sediment motion in rivers. Much work was done on river sediment transport formulae in the second half of the 20th century and that work should be used preferably to Hjulström's curve, e.g. Meyer-Peter & Müller (1948), Engelund-Hansen (1967), Lefort (1991), Belleudy (2012).

Modes of entrainmentThe sediments entrained in a flow can be transported along the bed as bed load in the form of sliding and rolling grains, or in suspension as suspended load advected by the main flow. Some sediment materials may also come from the

upstream reaches and be carried downstream in the form of wash load.Rouse number The location in the flow in which a particle is entrained is determined by the Rouse

number, which is determined by the density ρs and diameter d of the sediment particle, and the density ρ and kinematic viscosity ν of the fluid, determine in which part of the flow the sediment particle will be carried.

Here, the Rouse number is given by P. The term in the numerator is the (downwards) sediment the sediment settling velocity ws, which is discussed below. The upwards velocity on the grain is given as a product of the von Kármán constant, κ = 0.4, and the shear velocity, u .∗

The following table gives the approximate required Rouse numbers for transport as bed load, suspended load, and wash load

sediments concentration profile

total sediment load and transport capacity

• Mode of Transport Rouse Number• Bed load >2.5• Suspended load: 50% Suspended >1.2, <2.5• Suspended load: 100% Suspended >0.8, <1.2• Wash load <0.8

It is a characteristic scale parameter in the Rouse Profile of suspended sediment concentration with depth in a flowing fluid. The concentration of suspended sediment with depth goes as the power of the negative Rouse number. It also is used to determine how the particles will move in the fluid. The required Rouse numbers for transport as bed load, suspended load, and wash load, are given below.

The Rouse number (P or Z) is a non-dimensional number in fluid dynamics which is used to define a concentration profile of suspended sediment and which also determines how sediment will be transported in a flowing fluid

Sediment Transport and Transport Capacity

Sediment transport capacity, Tc, defined as the maximum amount of sediment that a flow can carry is the basic concept in determining detachment and deposition processes in current process-based erosion models..Sediment transport is the movement of solid particles (sediment), typically due to a combination of gravity acting on the sediment, and/or the movement of the fluid in which the sediment is entrained.In physical geography, entrainment is the process by which surface sediment is incorporated into a fluid flow (such as air, water or even ice ) as part of the operation of erosion.