PLANNING AND EVALUATION METHOD FOR “FLUSHING FLOW” UNDER THE TRIAL OF FLEXIBLE OPERATION OF MANAGAWA DAM

Abstract

For the purpose of recovering river dynamics to maintain fish habitats by artificially increasing flow discharge from dams, flushing flows have been introduced in downstream rivers under the flexible dam operation. This paper discusses planning and evaluation method of the operation through results of the field test on Managawa Dam from 2003 through 2005.

Water levels, flow velocities, water quality such as temperature, turbidity, suspended sediment concentration, movement of river bed materials and quantity of detached algae on river bed gravels have been measured at several downstream points during the various magnitudes of flushing flows. In these field tests, effects of sediment replenishment which supply sediment to downstream rivers by damping some excavated sediment alongside of downstream river have been also investigated.

Through these studies, a peak discharge of 50m3/s in more than two hours at least once in each season is recommended for future operation. Sediment replenishment on downstream river reaches where there is lack of movable sediments is also effective to enhance these flushing flows.