Sedimentological and geomorphological effects of reservoir flushing: The Cachi Reservoir, Costa Rica, 1996

The Cachi Reservoir on the Reventazon River. Costa Rica, is flushed on an a lmost yearly basis. A field investigation of the 1996 flushing was carried out in order to elucidate from where in the reservoir the material originat es, where and to what extent the released sediments will deposit along the river from the dam to the sea. and finally how this can be explained. A res ervoir survey was conducted by means of echo-sounding along certain cross-s ections before and after the flushing. The material was found to both depos it in between flushings and to be eroded during flushing, mainly in the upp ermost and lowermost parts of the old river channel. In the downstream reac hes, the pulse of suspended sediment pertaining to the flushing was found d istinguishable by means of grain-size distribution, even in the lower parts . Surveys were conducted at eight sites along the river downstream from the dam both before and after the flushing such that the volumes of deposited or eroded material were found. By use of the dry bulk density, the results could be calculated in tonnes, thus facilitating comparison to the sampling s of the hydrological stations along the river. A major factor in explainin g the amounts and distribution of deposits was shown to be the phase lag be tween water discharge and suspended-sediment concentration peaks. The influ ence of this phase lag on the deposition and erosion processes is illustrat ed by a schematic figure. A more detailed sediment budget is presented for the reach between the hydrological stations c. 10 and 30 lan downstream fro m the dam. It was found that approximately 250 000 tonnes were deposited wi thin the reach. Of these, 82% were channel-bed deposits while 18% were depo sited on the river banks.