Figure 4.2. Sediment transport capacity and supply curves (Shen 1971; Simons and
Sentrk 1992; Julien 1995).
The magnitude of the suspended or bed sediment discharge can be very large.
Suspended-sediment concentrations as large as 600,000 ppm or 60 percent by weight have
been observed. Concentrations of this magnitude are largely fine sediments. By increasing
fluid properties (viscosity and density), the fine material in the flow increases the capacity of
the flow to transport bed material.
The sediment load of a stream at a cross section or through a reach of a stream can be
determined by measuring the suspended-sediment portion of the load using samplers and
estimating the unmeasured discharge or by using one of the many methods that have been
developed for computing the bed sediment load and estimating the washload. In many
problems, only the bed sediment load, both in suspension and in contact with the bed, is
important. In these cases, the washload can be eliminated from the measured
suspended-sediment load if the size distribution of the material is known.
Many equations have been developed for the estimation of bed sediment transport. The
variation between the magnitude of the bed sediment discharge predicted by different
equations under the same conditions can be significant. For the same water discharge, the
predicted sediment discharge can have a 100-fold difference between the smallest and the
largest value. This can be expected given the number of variables, the interrelationships
among them, the difficulty of measuring many of the variables and the statistical nature of
bed material transport. Nevertheless, with proper use, knowledge of the river, and
knowledge of the limitations of each method, useful bed material discharge information can
be obtained.
4.4 SUSPENDED BED SEDIMENT DISCHARGE
The sediment transport process is best described through a discussion of suspended bed
sediment discharge, which usually accounts for the majority of the total load. The amount of
material transported in suspension varies with depth, with the highest concentrations
occurring near the bed. At equilibrium transport, the vertical exchange of sediment is
balanced between particle settling (due to gravity) and turbulence (that mixes higher
concentration flow up into the water column). For very fine particles, settling is small in
comparison to turbulent mixing and the sediment concentration is vertically uniform. For
4.6
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