3 Theoretical Basis for SAM.sed
Calculations
Purpose
Sediment transport functions can be used to calculate the bed material portion
of the sediment discharge rating curve. This rating curve can then be used in the
Flow-Duration Sediment-Discharge Rating Curve Method, programmed in
SAM.yld, to calculate the bed-material sediment yield. Various sediment
transport equations have been programmed into SAM.sed.
EM 1110-2-4000, "Sediment Investigations of Rivers and Reservoirs,"
provides a good description of the mechanics of sediment transport (USACE
1989).
General
SAM.sed will calculate a sediment discharge rating curve based on hydraulic
conditions and the bed gradation. The hydraulic input data for SAM.sed can be
calculated using SAM.hyd or or it can be determined external to the program and
input directly (SAM.m95 can be sued only in DOS mode). SAM.sed will create
a partial input data set for SAM.yld. There is also a module, SAM.aid, which
provides guidance on the selection of transport functions.
Sediment transport functions in SAM must be used with care. There is no
allowance for variability in the size class distribution over time or space. In
natural rivers the size class distribution of bed material varies with discharge,
reach, time of year, and other temporal factors. SAM's use of a fixed, average
size class distribution for all calculations presents the possibility that the
calculated transport rates are not truly representative of the natural river. The
procedure in HEC-6, which integrates processes over several cross sections and
provides a continuity equation for sediment movement, will consequently
produce a more reliable result. SAM provides reasonable time-averaged results
in cases where the river is in general equilibrium; that is, it is neither aggrading
or degrading. It is very important that the user correctly prescribe a
representative bed material gradation.
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Chapter 3
Theoretical Basis for SAM.sed Calculations
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