4.5.4 Colby's Method of Estimating Bed Sediment Discharge
Colby's (1964) method is a graphical method for estimating total load, which can also be
used to check the reasonableness of other sediment transport calculations. One very
important feature of the Colby method is the inclusion of a correction factor for fine sediment
(washload) concentration effects. Colby's method illustrates that washload can have a
significant impact on bed material load transport capacity.
After investigating the effect of all the pertinent variables, Colby (1964) developed four
graphical relations shown in Figures 4.13 and 4.14 for determining the bed sediment
discharge. In arriving at his curves, Colby was guided by the Einstein bed-load function
(Einstein 1950) and a large amount of data from streams and flumes. However, it should be
understood that all curves for 100-foot depth, most curves of 10-foot depth and part of the
curves of 1.0 and 0.1 ft are extrapolated from limited data and theory. Extrapolated curves
are shown as dashed lines in Figure 4.13.
Figure 4.13. Relation of discharge of sands to mean velocity for six median sizes of bed
sands, four depths of flow, and a water temperature of 60F (Colby 1964).
In applying Figures 4.13 and 4.14 to compute the bed sediment discharge, the following
procedure is used:
1. Required data are the mean velocity V, the depth yo, the median size of bed material
D50, the water temperature F and the measured fine sediment concentration Cf.