Appendix A: A Practical Guide to Effective Discharge Calculations
generating the flow frequency distribution should be set equal to the critical discharge for the initiation of
bed load transport.
6)
Calculate Flow Frequency Distribution
The frequency of occurrence for each discharge class is determined from the record of observed
flows. The frequency units should reflect the time base in the flow record. For example, if mean daily flows
have been used then the frequency is expressed in days. If a regional flow duration curve has been
developed for an ungaged site, then the frequency for each discharge class must be calculated using the
equation for the curve, which is usually a power function. This can be achieved by calculating the geometric
mean discharge of each discharge class and deriving the frequency from the equation of the curve.
7)
Check For Extreme Flow Events
It is recommended that all discharge classes display flow frequencies greater than zero and that
there are no isolated peaks in individual classes at the high end of the range of observed discharges. If this
is not the case, it is likely that either the class interval is too small for the discharge range, or the period of
record is too short. Both zero frequencies and extreme flow events (outliers), can be eradicated by
reducing the number of classes or using logarithmic class intervals, as described in the later section on
Evaluation and Troubleshooting, but noting the cautions in each case. In either case, Steps 5 and 6 are
repeated to generate the flow frequency distribution for the new class intervals.
Step 2 - Bed Material Load Rating Curve
1)
Determine Sediment Data Availability
Sediment transport data are required to generate the bed material load rating curve. These data
may be obtained from measurements at a gaging station if the gage is in close proximity to the study site and
the sediment record at the gage is representative of the sediment load there. Otherwise, sediment transport
data must be derived for the study site.
2)
Define Composition of Bed Material Load
It is recommended that wash load (generally defined as particles less than 0.063 mm) be excluded
from the data set used to develop the sediment rating curve. If the bed material load moves both as bed
load and suspended load, then bed load and suspended load measurements are required to determine the
bed material load. If measured load data are insufficient, appropriate equations in a hydraulic design
package, for example SAM (Thomas et al., 1994), can be used to generate bed material loads for selected
discharges.
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