Keynote lecture: Flushing sediment through reservoirs

Reservoir sediment deposition, which is a major factor in limiting the life of darns and reservoirs, has led to an increased interest in sediment flushing to sustain useful project life. Experience has shown that full reservoir drawdown with low-level outlets leads to retrogressive erosion, which is most effective at flushing sediment through reservoirs. A narrow flushing channel is incised into the sediment bed during reservoir drawdown. Recent laboratory and field studies have increased our understanding of flushing channels, within which most of the sediment erosion and transport takes place. The fluvial relationship between channel width and the square root of the flow applies to the laboratory flushing channel when the flow is first confined within the channel. The laboratory data agrees with a curve based on field data from Sanmenxia Reservoir in China. The flushing channel will widen during the rising limb of a flood, but will remain constant during the falling limb. A preliminary relation based on laboratory data is proposed for the rate of channel widening. When considering sediment flushing, one should investigate the project functions, the flow and sediment delivery characteristics, the downstream effects of increased turbidity and water pollutants, and the economic risk.