Reservoir sedimentation

In several tropical and sub-tropical regions reservoir sedimentation severely limits the useful life of reservoirs. Therefore, detailed studies of reservoir sedimentation are becoming more commonplace in order to predict the loss of available storage as a function of time.

Empirical methods have been introduced for predicting sediment deposition patterns in reservoirs, for calculating reservoir trap efficiency, and for estimating reservoir half life (i.e. the time taken to fill 50% of the reservoir storage with sediment). Physical modelling can be used to examine local patterns of deposition and erosion in the vicinity of the dam site and intake structures. Mathematical models can be used to forecast reservoir sedimentation, and to investigate the influence of changes in different parameters, for instance changes in water discharge, sediment yield, reservoir operation, etc.

The most reliable way of determining reservoir sedimentation is by direct measurements, for example by repeated surveying along fixed sections, and by analysing and dating the sedimentary sequence in sampled sediment cores. Some examples are given from the reservoirs, named in the frame to the left, where X-ray radiographic and X-ray densitometric methods have been used for the determination of reservoir sedimentation.

Water hyacinths (Eichhornia crassipes) in the upstream part of the Cachí reservoir in Costa Rica.

Dense, floating mats of water hyacinths are a severe environmental problem in many tropical and sub-tropical water reservoirs. The water hyacinths interfere with navigation and the siltation is increased. Low oxygen conditions develop beneath the water hyacinth mats and gas-rich layers with debris of water hyacinths are formed in the bottom deposits.

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