The Flushing Flow Problem: Defining and Evaluating Objectives

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Abstract
Reservoir releases may be specified for the purpose of maintaining or improving the downstream channel and habitat. A wide variety of ecological or management objectives may be defined for such flushing flows (which may be broadly divided into sediment maintenance and channel maintenance flows). To specify a particular discharge and water volume for a flushing flow requires that the ecological or management objectives be translated into specific physical objectives for which flows can be specified. Flushing objectives that cannot be translated into definable flows are of little practical use, regardless of their intrinsic importance. Once defined, flushing flow objectives may be shown to conflict in some cases. For example, no flushing flow can satisfy the typical sediment maintenance objectives of maximizing sand removal and minimizing gravel loss. A discharge that mobilizes sediment throughout the channel cross section for channel maintenance purposes will often produce comparable transport rates of sand and gravel, thereby eliminating the selective transport of sand needed to reduce the sand content in the bed. Some nonflushing alternatives, such as artificial gravel replenishment and pool dredging, can be used to improve the performance of flushing flows. Selection among these alternatives and specification of a flushing flow discharge and volume depend directly on quantitative estimates of sand and gravel transport as a function of flow rate and volume.