Numerical and experimental analysis of scour downstream of piano key weirs

In the current study, the scour behavior and scour rate downstream of PKWs with rectangular and trapezoidal geometries are investigated and compared for the first time. For this purpose; two experimental models of rectangular and trapezoidal PKWs were constructed and tested at different discharges and three different tail water depths. Three height differences between the sedimentary bed and the weir's base were considered to investigate the effects of drop height on scouring. The scour measurements showed that the scour hole characteristics increased with the increase in the discharge and the decrease in the tailwater depth in both models. The rectangular PKW model created higher scour depths than the trapezoidal model; however, this difference decreased with the increase in discharge and upstream head. Moreover, as the height difference between the sedimentary bed and the weir's base increased, the difference between the maximum scour depth of the two models decreased, and their scour profiles became increasingly similar. Numerical modeling of flow pattern under the same hydraulic conditions was used to interpret the scour profiles of rectangular and trapezoidal models of PKWs. Several equations were formulated for predicting the maximum scour hole depth, its location, and the scour hole length downstream of the models based on the regression method, which can help designers with designing and assessing the safety of weirs. In these equations, the most influential parameter is the head difference between upstream and downstream of the weir. The accuracy of the new formulas for score hole dimensions was compared with previous investigations, especially with equations for falling jet scour. More accuracy was achieved for both types of PKWS scour hole characteristics.

Automated summary

This study investigates the scour behavior and scour rate downstream of PKWs with rectangular and trapezoidal geometries and compares them for the first time. Experimental models of both types of PKWs were constructed and tested under different discharges and tailwater depths. The results show that the scour hole characteristics increase with discharge and decrease in tailwater depth in both models. The rectangular PKW model produces higher scour depths than the trapezoidal model, but this difference decreases with increasing discharge and upstream head.