Appendix A: A Practical Guide to Effective Discharge Calculations
Figure 9
Flow Duration Curve for Mean Daily Discharge: Lower Mississippi River at
Vicksburg, 1950-1982 (adapted from Biedenharn and Thorne, 1994)
description of the sediment measurement program on the lower Mississippi. The period of sediment
records includes both low runoff years and several events of high magnitude and long duration, so that the
full range of sediment transporting flows is represented in the measured data.
The measured sediment loads were divided into two components: 1) silt load consisting of particles
less than 0.062 mm, and; 2) sand load consisting of particles coarser than 0.062 mm. The bed of the lower
Mississippi River is formed in sand and so the sand fraction of the measured load was taken to represent
the bed material load. The silt load was taken to represent "wash load" for the lower Mississippi and was
excluded from the analysis.
There are no measurements of bed load for the lower Mississippi River, but according to the
calculations of Toffaleti (1968) the bed load comprises less than 5% of the total sand load. Hence, it was
deemed to be acceptable to ignore the bed load and to take the measured sand load as indicative of the
bed material load. The measured sand load data were used to construct a sand load rating curve for the
study site (Figure 10). Regression analysis of sand load as a function of discharge produced a coefficient
of determination, r2, of 0.82 and defined the bed material load rating curve as:
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