Reservoir sedimentation modelling and prediction of project lifetime

Abstract

Sedimentation causes an estimated annual reduction of 1 percent in the total capacity of all reservoirs worldwide, the figure rising to as much as 30 percent regionally. Sediments also block intakes in reservoirs and damage tunnels and turbines. The use of models to simulate reservoir sedimentation enables the key parameters of reservoir life to be predicted over the long term. In particular, they can provide information on the distribution of sediments within a reservoir; a key issue for the project’s lifetime and the planning of management measures. The rate of reservoir sedimentation depends mainly on the size of a reservoir relative to the amount of sediment flowing into it: a small reservoir on an extremely muddy river will rapidly lose capacity; a large reservoir on a very clear river may take centuries to lose an appreciable amount of storage. To predict sedimentation rate with reasonable accuracy involves the estimation of trap efficiency of a reservoir. Different methods of estimation of the reservoir trap efficiency are discussed. The Generalized Sediment Transport Model for Alluvial River Simulation, version 3 (GSTARS3), which simulates and predicts sedimentation processes in lakes and reservoirs is presented. GSTARS is a series of computer models developed by the U.S. Bureau of Reclamation. The stream tube concept is used to solve one-dimensional equations for each stream tube independently and obtain semi-two-dimensional variation of the hydraulic conditions along and across stream tubes for rivers and reservoirs. Sediment transport, scour, and deposition processes are simulated along each stream tube independently to give a semi-three-dimensional variation of the bed geometry. GSTARS models apply the theory of minimum stream power to the determination of optimum channel width and channel geometry. The concepts of channel side stability, and active, inactive, and armoring layers are used in all GSTARS models for realistic long-term simulation and prediction of the scour and deposition processes in rivers and reservoirs. Similarly, an EXCEL programme (SED-RES) to compute the sedimentation in a reservoir for given sediment transport and characteristics at the upstream reservoir boundary is discussed, with an example application to illustrate. In addition, a mass balance differential equation for reservoir sedimentation is formulated for numerical solution. Based on a simplified consideration for the rate of sedimentation in a reservoir, a closed form solution of the differential equation is derived for application in the prediction of project lifetime. Methods and approaches for sediment removal from reservoirs such as sluicing, venting and flushing, as well as for reducing reservoir sedimentation in general, are briefly reviewed.