Effects of hydrologic regime on magnitude and frequency of dominant discharge

Abstract

Estimates of three flows categorised as dominant discharge are derived for Cumberland Basin streams: the most effective discharge with regard to bed-load transport, the 1.58-year flood and the natural bankfull discharge. The most effective discharge is found to have a return period of 1.15–1.40 years on the annual series but is much smaller than the 1.58-year flood. Bankfull discharge return periods lie mainly within the range 4–10 years on the annual series. Observations of channel changes in Crawfords Creek indicate that the characteristics of the bed are related to flows similar to the most effective discharge but these flows do not erode the banks. Instead, bank erosion seems related to much larger floods. It is suggested that many Cumberland Basin streams are not incised but that their capacity is determined by large floods. Two groups of flows, therefore, seem to determine channel characteristics: the small flows which transport most of the bed load and the large flows which determine channel capacity. The reason for this lies in the hydrologic and sedimentary characteristics of Cumberland Basin streams. Streamflow appears to be dominated by two populations of runoff events, while channel bed material contains little fine to medium sand. Together, these two factors modify the typical process balance involved in channel and floodplain formation.