Prediction of bank erosion for alluvial channels below reservoirs

Abstract

The impact, of a reservoir on downstream channel erosion has received increasing attention since the 1920's. The sediment-free water released from the reservoir could induce severe problems on downstream channel erosion because the reservoir trapping sediment load disturbs the natural balance between sediment supply and sediment transport. To satisfy its transport capacity, the flow scours the channel and picks up sediment load. The reservoir-induced erosion superimposes on the natural erosion process and will continue until a new equilibrium is reached. The erosion rate induced by an upstream reservoir is estimated by an erosion-rate model proposed in this study. The parameters in this model are defined by algebraic relations in terms of channel characteristics. These relations are derived from regression analyses based on the data published by Williams and Wolman (1984). These empirical relations can be used to determine the unknown parameters for a given stream by input of the channel slope, flow discharge and velocity. The validity of the proposed model is tested by the published field data, and the stream bank erosion of a meander bend in a natural state is estimated by using Odgaard's (1987) Equation, which expresses bank erosion in terms of channel characteristics. Bank erosion below a reservoir is the result of natural erosion and reservoir-induced erosion. The computation of estimation is facilitated by a computer program. EROS.FOR. which is written in FORTRAN. This method is illustrated by examples of the Colorado River and Brazos River, both in Texas. Com paring the estimated data with the observed data on these rivers shows that the method provides reasonable and acceptable estimates for bank erosion of alluvial channels below reservoirs. The method is easy to use and is valid for gross planning purposes.