Scour Downstream of a Piano Key Weir with and without a Solid Apron

Piano Key Weirs (PKWs) are often preferred over other spillways for the dam rehabilitation due to high discharging capacity. The present study investigates scour downstream of three cycles of PKWs with and without solid apron under both free and submerged flow conditions. The scour formations downstream of the inlet and outlet keys were investigated by varying discharges and tail water depths. Three different types of aprons were used under varied discharges and tail water depths. Falling and impinging jets emanating from inlet and outlet keys were noted as being responsible for the formation of ridge and dip, respectively. The maximum depth of scour was 40%–80% of the total weir height under different discharges. For high submergence, negligible scour development was reported. The average reduction in the scour depth with a solid apron lies in the 60%–80% range. Empirical equations were developed using a least square technique for the computation of maximum scour depth for both the conditions, that is, with and without a solid apron. Statistical and graphical analysis indicates better conformity of the proposed equation with the previous investigation. Sensitivity analysis indicates that the ratio of tailwater depth to the median size of a sediment particle ($y_t/d_{\mathit{50}}$) and the ratio of the crest length to the width of the weir ($\mathit{L}/\mathit{W}$) are the most sensible parameters affecting maximum scour with and without a solid apron, respectively.