Mobile Riverbed Scour Downstream of a Piano Key Weir

In parallel to their most frequent application as an auxiliary dam spillway, piano key weirs (PKWs) are also installed on low-head barrages in combination with run-off-the-river power plants. Their efficient rating curve, advantageous behavior under driftwood blockage, and the absence of mechanical elements are optimal for the hydraulic conditions associated with these run-off-the-river projects. However, in the absence of technical protection measures, scouring of the riverbed can occur at the downstream foundation. The unhindered scour formation and corresponding ridge generation caused by a PKW are investigated in this paper. Physical model tests were conducted under various conditions, providing a general estimation of the equilibrium scour and ridge dimensions. The maximum scour-hole depth dominates the shape of the hole and of the ridge. It was further shown that the maximum scour slope corresponds to the angle of repose of the sediments on the riverbed. Finally, a comparison of the present PKW scour data with predictions provided in the literature revealed that PKW-generated scour might be considered as jet-induced scour.