The Effect of Different Block Shapes on the Downstream Local Scour in Piano Key Weir

Abstract

Piano key weirs (PKWs) have a higher discharge capacity than their traditional form, the labyrinth weirs. Due to the high efficiency of PKW, the investigation of their downstream scour is important and requires mitigation strategies. For the first time, this study investigated scour reduction using different block shapes in the outlet keys of a C type trapezoid PKW. The shapes of the blocks are rectangular cubes, trapezoidal cubes (prisms), and cylinders. The range of the densimetric Froude number was between 1.038 and 2.903, while the range of the ratio of the total flow head upstream of the weir to its downstream was between 0.252 and 0.461. The findings revealed that scouring was reduced by 12.7 and 27.3% with an increase of 200 and 300% in the tailwater depth and by 14.5 and 29.3% with a 12.5 and 25% decline in the flow rate. Blocks in the outlet keys significantly reduced the maximum scour depth, with rectangular blocks being the most effective (55.7% reduction), followed by trapezoidal (32.2%) and circular (11.3%). Blocks also elongated the scour hole and shifted the maximum scour depth away from the weir toe. The scour index was lower in weirs with blocks, indicating reduced overturning risk.