chapter 3 fundamentals of fluvial geomorphology and stream ...

continent (Inglis 1941, 1947, 1949) and North America (Blench1952, 1957). Later, flume experiments extended the regimeapproach to streams with meandering planforms (Ackers andCharlton 1970a,b). However, for widely varying flows emanatingfrom a natural watershed, the problem of identifying the singlechannel-forming discharge is both challenging and critical.Soar (2000) reviewed the literature pertaining to the concepta channel-forming flow. This concept is closely related to thetheory of dynamic-equilibrium, which is characterized byfluctuations of channel form around an average condition thatpersists through time. In perennial rivers, recovery of equilibriumfollowing a major event occurs relatively quickly, partly becauserapid vegetation growth encourages sedimentation (Hack andGoodlett 1960; Gupta and Fox 1974). Hence, the long-term timeaveragedcondition is a valid representation of channel form.Recovery in the ephemeral streams of semi-arid regions tends totake longer, reflecting the influence of relatively wet and dryperiods on vegetation growth (Schumm and Lichty 1965; Burkham1972). In arid areas, infrequent floods impart long-lasting imprintson the stream because more frequent flows do not have the powerto restore a regime condition (Schick 1974). It has been concludedthat the channel-forming flow concept may be inapplicable toephemeral rivers that exhibit highly variable flow regimes, becausethere may not be a single discharge that can explain channel form(Stevens et al. 1975; Baker 1977). This is the case, becausestream morphology is likely to be perpetually in disequilibrium withthe prevailing flows rather than fluctuating around an averagestate.The channel-forming flow or dominant discharge is actuallya geomorphological concept and not strictly a measurableparameter. However, there are a number of discharges that maybe taken to represent the channel-forming flow and that can bedefined and calculated using prescribed methodologies. The firstapproach is to identify a candidate flow based on streammorphological, such as the bankfull discharge. A second approachis to select a discharge based on a specified recurrence intervaldischarge, typically between the one- and three-year event in theannual maximum series. The third approach is analytical andinvolves calculating the effective discharge.3.5.1 Bankfull DischargeBased on both theoretical and empirical arguments, bankfulldischarge is generally recognized as being the moderate flow thatbest fits Wolman and Miller’s (1960) dominant discharge conceptfor rivers in dynamic equilibrium. Leopold et al. (1964) proposedthat the bankfull discharge was responsible for streammaintenance and form, and, therefore, implied that it is equivalent34 Fundamentals of Fluvial Geomorphology and Stream Processes