Appendix A: A Practical Guide to Effective Discharge Calculations
Figure 11
Bed Material Load Histogram: Lower Mississippi River at Vicksburg
(adapted from Biedenharn and Thorne, 1994).
effective discharge should be the same at all three sites. This was in fact the case, illustrating consistency
in the effective discharge analysis using three separate flow duration and sediment transport records.
Comparison of the water surface profile at the effective discharge (30,000 m3s-1) to the long-
channel distribution of bank top elevations is illustrated in Figure 12. The graph shows that bank top
elevations are highly variable and can differ by 3 m or more between adjacent cross-sections. This makes
it difficult to assign a value to bankfull discharge for the reach. However, comparison of the water surface
profile for the effective discharge to the bank top data indicates that the effective discharge forms a very
good lower bound to the scatter, indicating that the capacity of the channel is adjusted just to contain flows
up to and including the effective flow. As discharge increases beyond the effective flow, water begins to
spill over the bank tops at more and more locations. The return period for the effective discharge (equal
to or just less than one year) is consistent with the usual range of 1 to 3 years, and its flow duration
(equalled or exceeded 13% of the time) is as expected for a river with a drainage area of approximately
3 million km2 (see Figure 8). These checks indicate that the calculated effective discharge is very
reasonable and they support the accuracy of the analysis, including the necessary assumptions concerning
the use of measured sand load to represent the bed material load.
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