At present, only the Colby function contains a correction for fines in SAM. In
HEC-6, the Colby correction factor is applied to all sediment transport equations
except Toffaleti, when Toffaleti is not combined with other equations.
Importance of Bed-material Gradation Designation
In the calculation of sediment transport, the designated bed gradation controls
the calculated sediment discharge. The rate of transport increases exponentially
as the grain size decreases, as shown in Figure 3.1. Therefore, bed-material
gradations must be determined carefully. Techniques for selecting a repre-
sentative sample are discussed in EM 1110-2-4000 (USACE 1989). Due to the
sensitivity of transport calculations to the grain size, especially the finer sizes,
Einstein (1950) recommended excluding the finest 10 percent of the sampled bed
gradation from calculation of the total bed-material load.
Figure 3-1. Variation of sediment transport with grain size (from EM 1110-2-4000 (USACE 1989),
Figure 10-7)
56
Chapter 3
Theoretical Basis for SAM.sed Calculations
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