Appendix A: A Practical Guide to Effective Discharge Calculations
regarding the return period of the effective discharge for stable rivers. Experience indicates that it lies usually
within the range of 1.01 and 3 years, with a preponderance between 1.01 - 1.2 years, irrespective of the
type of river (Hey, 1994, 1997). On this basis, effective discharges with return periods outside the range
of approximately 1 to 3 years should be queried.
Basin Area - Flow Duration Curve
The percentage of the time that the effective discharge is equalled or exceeded should be compared
to the expected range of values reported in the literature. For example, Figure 8 presents a log-log plot
of the flow duration of effective discharge as a function of drainage area for several U.S. rivers (Andrews,
1980, 1984; Biedenharn et al., 1987). The graph can be used to assess whether the duration of the
effective discharge computed using the method described in this paper is consistent with the results of other
studies. It is not intended that this graph be used to predict effective discharge as a function of drainage
area, as large errors are likely to result from this application.
Check Effective Discharge against Bankfull Discharge
The effective discharge should be compared to the bankfull discharge. This can be accomplished
by identifying the bankfull stage during stream reconnaissance and calculating the corresponding discharge
using an available stage-discharge curve or the slope-area method. For naturally stable channels that are
in regime, the effective and bankfull discharges should coincide. If the effective discharge is substantially
different to the bankfull discharge, the results should be queried.
EXAMPLE: EFFECTIVE DISCHARGE CALCULATION FOR THE MISSISSIPPI RIVER
Discharge data were obtained from the Vicksburg gage for the period 1950 to 1982. This period
of record was selected as it encompasses the period when sediment loads were routinely measured at the
gaging station. The record contains a wide distribution of flows including both low and high runoff years
and with discharges ranging from about 4,200 to just over 56,600 m3s-1. On this very large river, mean daily
discharges do not differ significantly from instantaneous discharges and so the use of mean daily values was
acceptable in the production of the flow frequency distribution (Figure 9).
Bed Material Load Rating Curve
Sediment transport data were also obtained from the Vicksburg gaging station. The period of
record is 1969 to 1979, as this was the only period for which measured sediment transport records were
available. On average, sediment load was measured weekly. Robbins (1977) provides a detailed