Appendix A: A Practical Guide to Effective Discharge Calculations
3)
Plot Bed Material Load Data
The bed material load (y-axis) is plotted as a function of discharge (x-axis) in a scattergraph, with
both axes on logarithmic scales, carefully matching the units of sediment load to the time base for the
discharge frequency distribution.
4) Determine Sediment Rating Curve
A power function best-fit regression line should be fitted to the data in the scatter graph to produce
a bed material load function of the form:
Qs = aQb
(1)
where, Qs = bed material load discharge, Q = water discharge, a = regression constant, and b = regression
coefficient.
Step 3 - Bed Material Load Histogram
1)
Calculate Representative Discharges
The discharges used to generate the bed material load histogram are the mean discharges in each
class of the flow frequency distribution. If discharge class intervals are arithmetic, the representative
discharges are the arithmetic means of each class. If the discharge class intervals are logarithmic, the
representative discharges are the geometric means of each class.
2)
Construct the Bed Material Load Histogram
The bed material transport rate for each discharge class is found from the rating curve equation.
This load is multiplied by the frequency of occurrence of that discharge class to find the total amount of bed
material transported by that discharge class during the period of record. Care should be taken to ensure
that the time units in the bed material load rating equation are consistent with the frequency units for the
distribution of flows. The results are plotted as a histogram.
3)
Find the Effective Discharge
The bed material load histogram should display a continuous distribution with a single mode (peak).
If this is the case, the effective discharge corresponds to the mean discharge for the modal class (that is the
peak of the histogram). If the modal class cannot be readily identified, the effective discharge can be
estimated by drawing a smooth curve through the tops of the histogram bars and interpolating the effective
discharge from the peak of the curve.
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