A Laboratory Exercise Introducing the Concept of Effective Discharge in Fluvial Geomorphology

Abstract

Students generally leave their introductory geoscience classes with the notion that streams behave like conveyor belts wherein all flow regimes contribute more or less equally to the work done in the transportation of sediment. This view ignores both the frequency distribution of flow regimes and the exponential increase in stream capacity with increasing discharge. The reality is that some flows transport much more sediment than others. Low flows occur frequently but lack the power to move much material. Extreme flows can move large quantities of sediment in short periods of time but occur so infrequently that they do not transport a great deal of sediment in the long term. It follows that there is an intermediate flow, referred to as the effective discharge, that is frequent enough and powerful enough to move the most sediment in the long term. This exercise introduces upper-level geoscience students to this concept by asking them to calculate the effective discharge of a stream using statistically and empirically based solution techniques. The expectation is that the exercise will foster a more realistic view of the relative importance of various flow regimes in the transportation of sediment.