The Cachí Reservoir

Investigation of Flushing Efficiency

The reservoir, located in the Rio Reventazón, is an important supplier of electrical power to Costa Rica. It is operated by ICE (Instituto Costarricense de Electricidad).

The problems experienced were related to a high sediment transport into the reservoir, that built up in front of the intake. After a few years the reservoir had to be flushed, to get rid of the sediments that would otherwise have entered the intake to the turbines. When flushing, the reservoir has to be emptied of water completely, which of course leads to loss of power generation, and loss of money. Also, preliminary measurements gave questinable values for the efficiency of flushing (about 300%!).

The study involved measuring the inflow of sediments, the outflow of sediments especially during flushing, the pattern of sediment accumulation within the reservoir, and the pattern of erosion within the reservoir during flushing. Turbidimeters were used for monitoring sediment transport. Side scan sonar, sub-bottom profiler, repeated echo sounding, and sediment coring and X-ray analysis, were used to investigate the accumulation and erosion within the reservoir.

The results were that flushing was rather effective, largely due to the shape of the reservoir with a rather narrow gorge (cf. the map below). On the terrasses in the reservoir flushing was less effective, because water hyacinths got stranded and trapped the sediments causing a mean annual accumulation rate of about 5 cm. The accumulation rate in the old river network amounts to about 2 m per year before compaction, but as noted, this is effectively removed by flushing.

The investigation was carried out by Margareta Jansson, Åke Sundborg, Valter Axelsson, and Ulf Erlingsson, in cooperation with ICE.

The English Report:
ISBN 91-506-0913-0, ISSN 0375-8109
En Español:
ISBN 91-506-0948-3

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