Appendix A: A Practical Guide to Effective Discharge Calculations
EVALUATION AND TROUBLESHOOTING
Derivation of an effective discharge is not a routine exercise and it is vital that at the end of the
procedure, the output is evaluated to ensure that the calculated effective discharge is a reasonable value
for the project river at the study site. This section of the paper presents a series of evaluations that should
always be performed as part of quality assurance in deriving an effective discharge, together with guidance
on troubleshooting some of the more common problems.
Problems with Lowest Discharge Class
When a significant proportion of the recorded discharges fall within the first arithmetic class interval
of the flow frequency distribution, the range of discharges is inadequately represented and it is likely that
the computed effective discharge will be significantly underestimated. In practice, this is likely to be the
case for streams that display a highly skewed distribution of flow events, for example, rivers in semi-arid
environments, channelized streams, or incised channels. Under these circumstances, it is advisable to modify
the flow frequency distribution to better represent the range of low discharges, using one of the two
approaches outlined below.
Use Logarithmic-Scale Class Intervals
The first solution is to replace the arithmetic class intervals with logarithmic ones. For example,
Watson et al. (1997) demonstrated that this was the preferred solution for Hotopha Creek in the Yazoo
Basin, Mississippi. In this example, approximately 97% of the recorded discharges fell within the lowest,
arithmetic class interval. This resulted in a computed effective discharge of 8 m3/s, which was known from
experience to be too low a flow to have any morphological significance. When logarithmic class intervals
were adopted, the smaller flow events were distributed between several classes and the effective discharge
was found to be 234 m3s-1 which was identified as a morphologically significant flow.
Use of logarithmic class intervals solves problems at the low end of the discharge range, but care
should be exercised because large flow events are grouped into a few, very large class intervals, which may
result in inadequate precision if the effective discharge is found to be at the high end of the range.
Sub-divide the Lowest Class
An alternative solution is to subdivide the lowest discharge class into a number of equal sub-classes
to ensure that the discharge events within each sub-class are more normally distributed. This will also
address any problems relating to bed load transport, since flows below the threshold discharge for transport
will be discounted. After completing this exercise, the total bed material load transported within the lowest
discharge class should be found by summing the sediment loads for the subdivisions, to facilitate plotting
of the bed material load histogram.