chapter 3 fundamentals of fluvial geomorphology and stream ...

to the channel-forming discharge. Dury (1961) also suggested thatthe channel-forming discharge is approximately equal to thebankfull discharge and Dunne and Leopold (1978) concluded thattheir maintenance discharge corresponded to the bankfull stage.Field identification of bankfull discharge is, however, problematic(Williams 1978). It is usually based on identification of theminimum width to depth ratio (Wolman 1955; Pickup and Warner1976), together with the recognition of some discontinuity in thenature of the stream such as a change in sedimentary orvegetative characteristics. Nixon (1959) defined the bankfull stateas the highest flood of a river that can be contained within thestream without spilling water on the river floodplain. Wolman andLeopold (1957) defined bankfull stage as the elevation of the activefloodplain. Woodyer (1968) suggested that bankfull dischargecorresponds to the elevation of the middle bench of rivers havingseveral overflow surfaces. Schumm (1960) defined bankfull as theheight of the lower limit of perennial vegetation, primarily trees.Similarly, Leopold (1994) states that bankfull is indicated by achange in vegetation, such as herbs, grasses, and shrubs. Finally,the bankfull stage is also defined as the average elevation of thehighest surface of the stream bars (Wolman and Leopold 1957).Harrelson et al. (1994) provide explanations of field methods forfield-determining bankfull discharge using vegetation, gradation ofbank materials, and elevation of sedimentary features. Althoughseveral criteria have been identified to assist in field identificationof bankfull stage, ranging from vegetation boundaries tomorphological breaks in bank profiles, considerable experience isrequired to apply these in practice, especially on rivers that have,in the past, undergone aggradation or degradation.3.5.2 Specified RecurrenceInterval DischargeProblems and subjectivity in the field identification ofbankfull elevation and discharge make it attractive to use anobjectively defined discharge such as a specific recurrence intervalflow. This recurrence interval flow can, in turn, be related to thebankfull elevation (Table 3.2). Wolman and Leopold (1957)suggested that the bankfull frequency has a recurrence interval ofone to two years. The most often-quoted recurrence interval is 1.5years. Dury (1973) concluded that the bankfull discharge isapproximately 97% of the 1.58-year discharge, or the mostprobable annual flood. Hey (1975) demonstrated that the 1.5-yearflow (annual maximum series) passed through the scatter ofbankfull discharges along three British gravel-bed rivers. Richards(1982) suggests that, in a partial duration series, bankfulldischarge equals the most probable annual flood, which has a 1-year return period. Leopold (1994) concludes that mostinvestigations have found that the recurrence interval for bankfulldischarge ranges from 1.0 to 2.5 years. However, there are manyFundamentals of Fluvial Geomorphology and Stream Processes 35