5 Theoretical Basis for SAM.aid
-- Guidance in Sediment
Transport Function Selection
Purpose
SAM.aid is a module of the SAM package that provides guidance in the
selection of the most applicable sediment transport function(s) to use for given
hydraulic conditions for a specified river or stream. The traditional approach for
selecting a function has involved collecting field data, including both suspended
sediment measurements and bed material gradations; processing and testing that
data with a number of sediment transport functions; and then selecting the
function that best matched the field measurements. Because many of today's
projects are on small ungaged streams and because field data are often too limited
for this approach to be satisfactorily applied, SAM.aid was developed to provide
an alternative in which only bed-material gradations and hydraulic parameters are
required.
General
Different functions may give widely differing results for a specified channel.
Therefore it is important to test the predictive capability of a sediment transport
function against measured data in the project stream or in a similar stream before
its adoption for use in a sediment study. Also, different functions were
developed from different sets of field and laboratory data and are better suited to
some applications than others.
Most sediment transport functions predict a rate of sediment transport for a
given set of steady-state hydraulic and bed material conditions. Typically,
hydraulic variables are laterally averaged. Some sediment transport functions
were developed for calculation of bed-load only, and others were developed for
calculation of total bed-material load. This distinction can be critical in sand-bed
streams, where the suspended bed-material load may be orders of magnitude
greater than the bed-load. Another important difference in sediment transport
functions is the manner in which grain size is treated. Most sediment transport
functions were developed as single-grain-size functions, usually using the median
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Chapter 5
Theoretical Basis for SAM.aid
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