Sedimentological and Geomorphological Effects of Reservoir Flushing: The Cachí Reservoir, Costa Rica, 1996

The Cachí Reservoir on the Reventazón River, Costa Rica, is flushed on an almost yearly basis. A field investigation of the 1996 flushing was carried out in order to elucidate from where in the reservoir the material originates, 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 reservoir survey was conducted by means of echo-sounding along certain cross-sections before and after the flushing. The material was found to both deposit in between flushings and to be eroded during flushing, mainly in the uppermost and lowermost parts of the old river channel. In the downstream reaches, the pulse of suspended sediment pertaining to the flushing was found to be clearly distinguishable 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 samplings of the hydrological stations along the river. A major factor in explaining the amounts and distribution of deposits was shown to be the phase lag between water discharge and suspended-sediment con-centration peaks. The influence of this phase lag on the deposition and erosion processes is illustrated by a schematic figure. A more detailed sediment budget is presented for the reach between the hydrological stations c . 10 and 30 km downstream from the dam. It was found that approximately 250 000 tonnes were deposited within the reach. Of these, 82% were channel-bed deposits while 18% were deposited on the river banks.