- Sediment transport
Sediment transport is the movement of solid particles ("sediment ") due to the movement of thefluid in which they are immersed. This is typically studied in natural systems, where the particles areclastic rocks (sand, gravel, boulders, etc.) orclay , and the fluid is air, water, or ice.Sediment transport is important in the fields of

sedimentary geology ,geomorphology ,civil engineering andenvironmental engineering (see [applications] , below). Our understanding of it is most often used to know whethererosion ordeposition will occur, and in what magnitude it will occur.=Mechanisms for Sediment Transport=

**Eolean**Eolean is the term for sediment transport bywind . This process results in the formation ofripples andsand dunes . Typically, the size of the transported sediment is finesand (<1 mm) and smaller, becauseair is is a fluid with lowdensity andviscosity , and can therefore not exert very muchshear on its bed.Aeolean sediment transport is common on beaches and in the arid regions of the world, because it is in these enviornments that vegetation does not prevent the presence and motion of fields of sand.

Wind-blown very fine-grained

dust is capable of entering the upper atmosphere and moving across the globe. Dust from theSahara deposits on theCanary Islands and islands in theCarribbean Fact|date=September 2008, and dust from theGobi desert has deposited on thewestern United States Fact|date=September 2008. This sand is important to the soil budget and ecology of several islands; the soil formed by this wind-blown sediment is calledloess .**Fluvial**In

geology ,physical geography , andsediment transport ,fluvial processes relate to flowingwater in natural systems. This encompasses rivers, streams,periglacial processes,flash floods and Glacial lake outburst floods. Sediment moved by water can be larger than sediment moved by air because water has both a higherdensity andviscosity . In typical rivers the maximum size of this sediment is of sand and gravel (<32 mm), but larger floods can carryboulders .**Glacial**Glaciers can carry the largest sediment, and areas of glacial deposition often contain a large number of

glacial erratics , many of which are several meters in diameter.=Modes of Entrainment=

Sediment entrained in a flow can be Sediment can be transported along the bed as

bed load , in suspension assuspended load , or along the top (air-water) surface of the flow aswash-load .The location in the flow in which a particle is entrained is determined by the

Rouse number , which is determined by the density $ho\_s$ and diameter$d$ of the sediment particle, and the density $ho$ and kinematic viscosity $u$ of the fluid, determine in which part of the flow the sediment particle will be carried.$extbf\{Rouse\}=frac\{w\_s\}\{kappa\; u\_*\}$

The term in the numerator is the (downwards) sediment the sendiment

settling velocity $w\_s$, which is discussed below. The upwards velocity on the grain is given as a product ofvon Kárman's constant ,$kappa\; =\; \{0.407\}$

and the

shear velocity $u\_*=sqrt\{(frac\{tau\_b\}\{\; ho\_w\})\}=kappa\; z\; frac\{partial\; u\}\{partial\; z\}$

The following table gives the required Rouse numbers for transport as

bed load ,suspended load , andwash load .**ettling velocity**The settling velocity (also called the "fall velocity") can be calculated with

Stokes' Law for small particles and with theDrag Law for large particles. Ferguson and Church (2006) [*Ferguson, R. I., and M. Church (2006), A Simple Universal Equation for Grain Settling Velocity, Journal of Sedimentary Research, 74(6) 933-937, doi: 10.1306/051204740933*] analytically combined these two expressions into a single equation that works for all sizes of sediment.$w\_s=frac\{RgD^2\}\{C\_1\; u\; +\; (0.75\; C\_2\; R\; g\; D^3)^(0.5)\}$

In this equation "w_s" is the sediment settling velocity, "g" is acceleration due to gravity, and "D" is mean sediment diameter. R is the

submerged specific gravity of the sediment, which is given by:$R=frac\{\; ho\_s-\; ho\_w\}\{\; ho\_w\}$

where "ρ" is density and the subscripts "s" and "w" indicate sediment and water, respectively. For quartz grains in water (a typical situation),

$ho\_s\; =\; 2650\; kg/m^3$

$ho\_w\; =\; 1000\; kg/m^3$

$R=1.65$

u is the

kinematic viscosity of water , which is approximately 1.0 x 10^{-6}m^{2}/s for water at 20ºC.C_1 and C_2 are constants related to the shape and smoothness of the grains.

The expression for fall velocity can be simplified so that it can be solved only in terms of "D". We use the sieve diameters for natural grains, $g=9.8$, $u=1.0$, and $R=1.65$. From these parameters, the fall velocity is given by the expression:

$w\_s=frac\{16.17D^2\}\{18\; +\; (12.1275D^3)^(0.5)\}$

Below are explanations of some standard equations that relate to sediment transport. These equations describe the initiation of sediment motion and the vertical location within the flow in a

channel , such as a river, that sediment will occupy. These equations are designed for the transport of sediment inwater orair . They only work forclastic ,granular sediment: floccular sediment (includingclays andmuds ) do not fit the geometric simplifications in these equations, and also interact thoroughelectrostatic forces.=Initiation of motion=

For a fluid to begin transporting sediment in rest on a surface, the boundary, or bed, shear stress $au\_b$ exerted by the fluid must exceed the critical shear stress $au\_c$ for the motion of grains at the bed.

**Bed shear stress****Depth-slope product**For a river of approximately constant depth "h" and slope &theta over the reach of interest, and not involving a

backwater , the bed shear stress is given by the depth-slope project:$au\_b=\; ho\; g\; h\; sin(\; heta)$

For shallow slopes, the

small-angle formula shows that $sin(\; heta)$ is approximately equal to $an(\; heta)$, which is given by "S", the slope. Rewritten with this:$au\_b=\; ho\; g\; h\; S$

**Other methods of calculating bed shear stress**For all flows that cannot be simplified as a single-slope infinate channel (above), the bed shear stress can be found by

**Critical shear stress**The critical shear stress that a particle must overcome to enter motion can be given by a variety of formulas.

Shield's Diagram =Applications of Sediment Transport=

**References**http://ocw.mit.edu/OcwWeb/Earth--Atmospheric--and-Planetary-Sciences/12-090Fall-2006/CourseHome/index.htm

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