3.6.7 Total Sediment Discharge
The suspended-sediment discharge determination described above is the measured
sediment discharge. There is the unmeasured sediment discharge composed of the
sediment moving in contact with the bed (contact sediment discharge or load) and the
suspended sediment discharge that the sampler doesn't sample. This unmeasured sediment
discharge can be as low as 10 percent of the total sediment discharge (or total load) to as
high as 50 percent or more of the total sediment discharge. The percent of the total sediment
discharge that is measured depends on the turbulence of the stream, bed material size, and
concentration of wash load. Streams with very large concentrations of wash load or very
large turbulence and fine bed material will have very low percent of unmeasured sediment
discharge.
In some stream reaches the turbulence is so large that the total sediment discharge is in
suspension and each size fraction of the total sediment discharge is uniformly distributed in
the vertical. Here, the suspended-sediment discharge is the total sediment discharge. Many
of these streams and in some cases streams where the turbulence was artificially created
were used by Colby et al. (1955, 1956, 1962) to develop the modified Einstein procedure to
determine the total sediment discharge of a stream as described in Chapter 4. Burkham and
Dawdy (1980) and others have further modified Colby's procedure.
As discussed in Chapter 4, the total sediment discharge has three classifications. These are:
1. By source: The bed material discharge and the wash load discharge (Figure 3.20).
2. By mode of transport: The suspended-sediment discharge and the bed (contact)
sediment discharge (Figure 3.20).
3. By measurement: The measured sediment discharge and the unmeasured sediment
discharge.
To determine the total sediment discharge various methods are used. Some of these are:
1. Where there is a suspended-sediment sampling program, the unmeasured sediment
discharge is determined using methods described in Chapter 4 and added to the
measured suspended-sediment discharge.
2. A suspended-sediment sampling program is established and the unmeasured sediment
discharge determined as in 1 above and added to the measured suspended-sediment
discharge.
3. The total sediment discharge is determined by the modified Einstein method described in
Chapter 4 for a range of discharges and a sediment rating curve developed. The curve
and a record of daily water discharge are used to determine daily and yearly total
sediment discharge.
4. Computer programs are used to determine the total bed material discharge. The wash
load discharge may be ignored or estimated from periodic suspended sediment
measurements.
3.51
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