Pressure Flushing of Cohesive Sediment in Large Dam Reservoirs

Abstract

Pressure flushing of deposited sediments behind large dam walls causes development of a funnel-shaped crater due to the induced vortex and considerable shear flow around the outlets. Laboratory experiments were conducted to investigate interaction of flow and sediment parameters on the development and final size of the scour cone. The volume of the flushed sediment increases with increase of discharge, and decreases with increase of sediment bulk density and the water level above the sediment. While the water depth over the sediments is the most significant parameter in the collapse of sediments above the intake and the initial development of the cone, sensitivity analysis indicated bulk density of cohesive sediment to be the most effective parameter to determine the final size of the scour cone. Using experimental results, two equations were developed to estimate scour cone volume and length with readily defined and measurable parameters of flow and sediment. The field measurements confirm the capability of the equations for large-scale application and flushing practice.