Experimental study of scour evolution downstream of Linear and Curvilinear Piano Key Weir configurations

Scour downstream of Piano Key Weirs can affect the stability and performance of hydraulic structures. This study evaluates the influence of outlet-key geometry on scour development by comparing a conventional Linear Piano Key Weir with a Curvilinear Piano Key Weir incorporating an ogee-shaped outlet profile. Both configurations were designed with identical dimensions and inlet-key geometry to isolate the effect of outlet modification. Laboratory experiments were carried out under clear-water conditions using a uniform movable sediment bed and discharges of 4, 6, 8, and 10 L/s. Longitudinal scour profiles were measured after equilibrium conditions were reached. The results showed that scour depth increased with discharge because of greater flow momentum and turbulence. In both configurations, the outlet-key region experienced more severe scour than the inlet-key region. However, the Curvilinear Piano Key Weir consistently produced lower equilibrium scour depths and smoother scour-hole profiles. The modified outlet profile promoted improved flow spreading and reduced jet concentration near the bed. Dimensionless analysis showed that normalized scour depth increased with hydraulic intensity for both configurations, although the increase remained smaller for the Curvilinear Piano Key Weir. Comparison with published studies confirmed consistent trends and highlighted curvilinear outlet geometry as a passive scour-reduction measure.