THE FLUSHING FLOW PROBLEM - DEFINING AND EVALUATING OBJECTIVES

Reservoir releases may be specified for the purpose of maintaining or improving the downstream channel and habitat, A wide variety of ecolog ical or management objectives may be defined for such flushing flows ( which may be broadly divided into sediment maintenance and channel mai ntenance flows). To specify a particular discharge and water volume fo r a flushing flow requires that the ecological or management objective s be translated into specific physical objectives for which flows can be specified. Flushing objectives that cannot be translated into defin able flows are of little practical use, regardless of their intrinsic importance. Once defined, flushing flow objectives may be shown to con flict in some cases. For example, no flushing flow can satisfy the typ ical sediment maintenance objectives of maximizing sand removal and mi nimizing gravel loss. A discharge that mobilizes sediment throughout t he channel cross section for channel maintenance purposes will often p roduce comparable transport rates of sand and gravel, thereby eliminat ing the selective transport of sand needed to reduce the sand content in the bed. Some nonflushing alternatives, such as artificial gravel r eplenishment and pool dredging, can be used to improve the performance of flushing flows. Selection among these alternatives and specificati on of a flushing flow discharge and volume depend directly on quantita tive estimates of sand and gravel transport as a function of flow rate and volume.