Opposite to mechanical dredging by machine, hydraulic flushing or drawdown flushing is a technique utilizes the nature power of flow, by lowing the water level downstream, to excavate sediment deposit from either a reservoir or a settling basin. Flume experiments are performed and two phenomena are found significant in the processes of flushing. One is moving drop flow and the other. moving hydraulic jumps. Drop flow takes place at a point when water-level lowing occurs at the downstream of that point due to erosion of downstream sediment. Hydraulic jump following the drop flow is a transition from supercritical to subcritical flow condition. Together, the process of hydraulic flushing consists of a series of trans-critical drop flows (or cascade) along with moving scouring. Volume erosion ratio (VER) is defined as the percentage of accumulated volume eroded from original deposit. VER is plotted against time of operation for quantifying the efficiency of hydraulic flushing. Typical pattern of VER is a curve rapidly increase at the beginning and gradually become a constant as an equilibrium slope reached. Opening of sluice gate, slope of flume, and inflow discharge are three factors affecting the VER curve in terms of the final stage, but not the time required. According to the regression formula based on experimental data,weighting among three factors reveals that the most important factor is the opening of sluice gate, second the slope of flume, and inflow discharge is the least important parameter.With optimal control, a best flushing rate can be obtained, which can be interpreted as saving of water volume for same sediment removal within an acceptable duration.
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