In this paper, the scour downstream of a trapezoidal Piano key weir (PKW) is investigated under free and submerged flow conditions. For this purpose, an experimental model was used in a laboratory channel with a length of 10 m, width of 0.75 m, and height of 0.8 m at various discharges and tailwater depths. The results showed an increase and decrease in the maximum scour depth with an increase in the discharge and submergence ratio, respectively. In addition, increasing the particle Froude number enhanced the geometric features of the scour hole downstream of the PKW. The rate of bed changes slowed down over time. The average values of maximum scour depth, the distance of the maximum scour depth from the weir toe, maximum scour hole length and weir toe scour depth increased in the free flow compared to the submerged flow. The scour hole volume and scour hole area in the submerged flow were less compared to the free flow. The average maximum level of the sedimentary ridge in the free flow is less than the submerged flow. New equations for calculating the geometric characteristics of the scour hole downstream of the trapezoidal PKW are presented with acceptable accuracy.

中文翻译：

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自由流和潜流条件下A型梯形琴键堰下游冲刷试验研究

本文研究了自由流和淹没流条件下梯形钢琴键堰 (PKW) 下游的冲刷情况。为此，在不同排放量和尾水深度的长10 m、宽0.75 m、高0.8 m的实验室渠道中使用了实验模型。结果表明，最大冲刷深度分别随着流量和淹没比的增加而增加和减少。此外，增加颗粒弗劳德数增强了PKW下游冲刷孔的几何特征。随着时间的推移，换床的速度减慢。与潜流相比，自由流中的最大冲刷深度、最大冲刷深度距堰趾的距离、最大冲孔长度和堰趾冲刷深度的平均值均有所增加。潜流中的冲孔体积和冲孔面积均小于自由流。自由流中沉积脊的平均最高水位小于潜流中的沉积脊。提出了计算梯形 PKW 下游冲刷孔几何特征的新方程，精度可接受。