where:
LT
=
Total load
=
Bed load which is defined as the transport of sediment particles that are
Lb
close to or maintain contact with the bed
Ls
=
Suspended load defined as the suspended sediment passing through a
stream cross-section above the bed layer
=
Measured sediment
Lm
=
Unmeasured sediment that is the sum of bed load and a fraction of
Lu
suspended load below the lowest sampling elevation
Lw
=
Wash load which is the fine particles not found in the bed material
(Ds < D10), and originates from available bank and upstream supply
Lbm
=
Capacity limited bed material load
Figure 4.1. Classification of sediment transport in streams (rivers).
4.3.3 Total Sediment Discharge
The total sediment discharge of a stream is the sum of the bed sediment discharge (bed
material load) and the fine sediment discharge (washload), or the sum of the contact
sediment discharge (bed load) and suspended sediment discharge. In the former sum, the
total sediment discharge is based on source of the sediments and the latter sum is based on
the mode of sediment transport. Whereas suspended sediment load consists of both bed
sediments and fine sediments (washload), only the bed sediment discharge can be
estimated by the various equations that have been developed. The fine sediment discharge
(washload) depends on its availability not on the transporting capacity of the flow and must
be measured (Figure 4.2). The presence of high concentrations of wash load affects the
apparent viscosity of the water-sediment mixture and reduces the fall velocity of silt and sand
grains.
The sediment load that is measured by suspended-sediment samplers consists of both the
washload (fine sediment load) and suspended-sediment load. The contact load is not
measured, and because suspended sediment samplers cannot travel the total distance in the
vertical to the bed, part of the suspended sediment in a vertical is not measured. Generally,
the amount of bed material moving in contact with the bed of a large sand-bed river is from 5
to 10 percent of the bed material moving in suspension. In general, the measured
suspended-sediment discharge is from 90 to 95 percent of the total sediment discharge.
However, in shallow sand-bed streams with little or no washload the measured
suspended-sediment load may be as small as 50 percent of the total load.
4.5
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