(Note that Cmg/l equals the ratio of the mass of sediment in milligrams to the volume of the

water-sediment mixture in liters.)

In these equations, Sg is the specific gravity of the sediment (γs/γ). For sediment

concentrations (Cppm) less than 100,000 there is less than a 7 percent difference between

Cppm and Cmg/l. The difference is less than 1 percent for concentrations less than 10,000

ppm or mg/l. For low concentrations, they are often considered equivalent. This is because

the weight of the sediment is small compared to the weight of the water. At these low

concentrations, an approximate rate of sediment transport can be estimated by

Qs = Cppm x 10-6 x Qwater / Sg

where Qs and Qwater are in m3/s or ft3/s.

The exact conversion is:

Q s = Cppm x 10 -6 x Q water /(S g (1 - Cppm x 10 -6 ))

(4.62)

When sediment is eroded or deposited, the volumetric transport rate must include the void

space between the sediment particles. The void space, or porosity, is the ratio of voids to

the total volume and often ranges from 35 to 45 percent. The volume of eroded or deposited

material for a time interval, ∆t, is

V = Qs x ∆t / (1 - η)

(4.63)

where Qs is the rate excess (or deficit) sediment transport and η is the porosity of the bed

material.

No single sediment transport equation can encompass all alluvial channel conditions.

Therefore, an equation should be selected based on the particular river bed material and flow

characteristics. When possible, the results of the sediment transport calculations should be

compared with measured sediment transport. The equations presented in this section have

broad application; however, other equations can, and should also be considered.

There are several sources for obtaining recommendations on the applicability of sediment

transport equations. Appendix B includes a review of ten sediment transport equations. The

review tested the equations with a large compilation of field data that encompasses a wide

range of bed material, from silts to gravel, and a wide range of river sizes, from 1 m width to

over 1000 meters in width. The BRI-STARS manual (Molinas 2000) presents a comparison

of several sediment transport equations with a variety of measured data. The HEC-6 manual

(U.S. Army Corps of Engineers) refers to Vanoni (1975) as a guide to the applicability of

sediment transport equations. Another source for recommendations on suitability of various

transport functions is the SAM Hydraulic Design Package for Channels (Thomas et al. 2000).

If more than one equation appears suitable, one should apply the equations and determine

which one provides the most reasonable results. This is most easily done using sediment

transport programs such as BRI-STARS, HEC-6 and SAM. Table 4.4 includes a list of

sediment transport relationships and the application of each equation.

4.32

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