The Effect of Side Wall Angle on Downstream Scour of Trapezoidal Key Piano Weir

The main problem downstream of hydraulic structures, such as weirs, is scouring and movement of bed materials.  The piano key weirs (PKW) is one the types of weirs. Due to the fact that the geometric shape of the weir affects the condition of the downstream scour, therefore in this study, the downstream scouring of the piano key weirs with a trapezoidal geometric shape is studied. In this paper, the scour profile and the rate of change were compared with the change in the lateral wall angle of the weir.

For this purpose, four experimental models of this type of weirs were made and these models were tested in a laboratory flume at different flow rates. Also, to investigate the effect of tail water depth on scour profiles, one of the models was investigated at three different tail water depths.

The results of scouring characteristics showed that in trapezoidal piano key weirs, the maximum depth of the hole is created under the output key and by increasing the discharge and decreasing the tail water depth, the geometric dimensions of the scour hole increase in trapezoidal models. Also, in trapezoidal piano key weirs, by increasing the side wall angle of the weir, the geometric dimensions of the scour hole are reduced. In all flows, by increasing every 3 degrees of the side wall angle of the piano key weir, maximum scour hole depth on average, 5%, Spatial position distance, maximum scour hole depth and scour hole span length, respectively, and it decreases by 8% and 3% on average. Also, when Frd>65% is, this difference becomes insignificant and there is not a significant difference in the shape of the scour hole for PKW models.