洩降沖刷過程之物理現象與影響因子研究

洩降沖刷是以水流自然力量沖刷泥沙，在水庫與沈砂池等蓄水體中，爲排除淤砂在操作上有藉放水洩降水位以沖刷前期淤積者。洩降沖刷過程中有兩個重要的水理現象，分別是跌水及水躍，當下游水位降低至不影響上游水位時發生跌水，當水流由坡度陡的超臨界流進入坡度緩的亞臨界流時發生水躍，在洩降過程時常見到跌水與水躍同時存在。無因次體積沖刷累積量歷線具有初期快速增加然後緩慢趨於穩定的特性，閘門開度、入流量及渠槽坡度的改變，將使無因次體積沖刷累積量的時變率改變，但對無因次體積沖刷累積量快速增加的終止時間影響不大。三個影響因子皆與穩定的無因次體積沖刷累積量有單調且正比的關係，各因子的影響程度依次為閘門開度＞渠槽坡度＞入流量。

關鍵字

沈砂池；洩降沖刷；水力沖刷；溯源侵蝕；跌水

並列摘要

Opposite to mechanical dredging by machine, drawdown flushing or hydraulic 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 settling basin. Flume experiments are performed and two phenomena are found significant in the processes of flushing. One is moving waterfall and the other, moving hydraulic jumps. Waterfall takes place at a point whenever erosion induced water-level lowing occurs downstream at that point. Hydraulic jump following the waterfall is a transition from supercritical to subcritical flow condition. Together, the process of hydraulic flushing consists of a series of trans-critical drop flows along with moving scouring. Dimensionless Volume erosion (DVE) is defined as the percentage of accumulated volume eroded from original deposit against time, which is an index to quantify the efficiency of hydraulic flushing. Typical pattern of DVE 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 DVE curve in terms of the final stage, but not the time required. According to the regression formula based on experimental data, the weighting among three factors are first 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.

並列關鍵字

Settling basin ； Drawdown ； Hydraulic flushing ； Retrogressive erosion ； Drop flow

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