Appendix A: A Practical Guide to Effective Discharge Calculations
Alluvial rivers have the potential to adjust their shape and dimensions to all flows that transport
sediment, but Inglis (1941) suggested that, for rivers that are in regime, a single steady flow could be
identified which would produce the same bankfull dimensions as the natural sequence of events. He referred
to this flow as the dominant discharge. Based on field observations, Inglis (1947) took into consideration
that the dominant discharge was approximated by flows at or about bankfull stage. This finding has been
supported by subsequent research on the discharge controlling regime channel morphology (Nixon, 1959;
Simons and Albertson, 1960; Kellerhals, 1967; Hey and Thorne, 1986) and studies of shallow overbank
flows (Ackers, 1992; James and Brown, 1977).
To explain this phenomenon it is necessary to recognize that any local imbalance in the sediment
budget must generate change in the morphology of a river through either erosion or deposition. Hence, to
remain dynamically stable, the regime dimensions of the channel must be adjusted so that, over a period
of years, sediment input and output balance. Over this time scale, Wolman and Miller (1960) showed that
rivers adjust their bankfull capacity to the flow which, cumulatively, transports the most sediment and
Andrews (1980) named that flow the effective discharge.
Wolman and Miller (1960) found that the effective discharge corresponds to an intermediate flood
flow since very frequent minor floods transport too small a sediment load to have a marked impact on the
gross features of the channel. While catastrophic events, which individually transport large sediment loads,
occur too infrequently to be effective in forming the channel. The potential for large floods to disrupt the
regime condition and cause major channel changes is recognized by this concept. However, large floods
are not channel-forming provided that the return period of these extreme events is longer than the period
required for subsequent, lesser events to restore the long-term, average condition (Wolman and Gerson,
1978).
Perennial rivers usually recover their long-term, average morphology within 10 to 20 years
following a major event, principally because riparian and floodplain vegetation limits the impacts of major
floods while vegetation regrowth encourages the processes of siltation involved in morphological recovery
(Gupta and Fox, 1974; Hack and Goodlett, 1960). In semi-arid regions, the recovery period tends to be
longer, reflecting the reduced effectiveness of sparse vegetation in increasing the channel's resilience to
change and the sensitivity of the channel to the occurrence of relatively wet and dry periods (Schumm and
Lichty, 1963; Burkham, 1972). In arid areas, the largest floods leave very long lasting imprints on the
channel. Primarily because of the lack of vegetation and, secondly, because lesser events capable of
restoring a regime condition rarely occur (Schick, 1974). For this reason, the dominant discharge concept
is generally thought to be inapplicable to ephemeral streams in arid regions.
Equivalence between bankfull and effective discharges for natural alluvial channels that are stable
has been demonstrated for a range of river types in different hydrological environments provided that the
flow regime is adequately defined and the appropriate component of the sediment load is correctly identified
(Andrews, 1980; Carling, 1988; Hey, 1997).
The equivalence of bankfull and effective discharges for stable channels suggests that either one
could be used to define the channel-forming discharge. Also, in theory, this discharge could be determined
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