3.6.3 Suspended Sediment Discharge Measurement
The measurement of the suspended sediment discharge of a stream requires the time
dependent measurement of the water discharge (discharge hydrograph) and velocity
weighted measurement of the concentration of sediment particles moving past the cross-
section. In equation form:
Qs = K u Q w C
Suspended sediment discharge, metric or English tons per day
Water discharge, m3/s or cfs
Velocity-weighted mean concentration (by weight) of sediment, mg/l
Coefficient to convert to metric or English tons per day
The determination of water discharge was described in Chapter 2. The measurement of
suspended-sediment discharge is described in detail in Techniques of Water-Resources
Investigations of the United States Geological Survey (Guy 1970, 1977; Guy and Norman
1970; and Porterfield 1977). The essence of the procedure is as follows.
1. Time dependent measurement of the water discharge (discharge hydrograph). Standard
stream gaging procedure described in Chapter 2.
2. Measure the velocity weighted mean suspended-sediment concentration of the flow.
3. Develop a time suspended-sediment concentration graph similar to the stage hydrograph
at a gaging station (suspended sediment hydrograph).
4. Determine the daily suspended-sediment discharge in English or metric tons per day,
using Equation 3.53.
3.6.4 Velocity Weighted Mean Suspended-Sediment Concentration
As illustrated in Figure 3.21 the velocity decreases with depth and the sediment
concentration increases with depth. Very fine sediment or coarser sediment in a very
turbulent stream may not decrease in concentration with depth (uniformly distributed).
Whereas, very coarse sediment, or finer sediment in placid flow may have a very large
increase in concentration with depth. In addition to the velocity and concentration varying
with depth, they vary across the stream. The Federal Government through the Interagency
Subcommittee on Sedimentation (U.S. Interagency Subcommittee 1940a,b, 1941a,b,c, 1943,
1948, and 1952; Vanoni 1977; Richardson 1994) developed samplers that take a velocity
averaged suspended-sediment concentration in the vertical to 0.09 m (0.3 ft) above the bed
(Figure 3.22). These samplers are used to take samples across the stream or to obtain a
coefficient to be applied to a sample at a single vertical to obtain the mean discharge
weighted concentration in the cross-section.