Experimental Investigation of Scour Characteristics Downstream of Exemplary Rectangular Piano Key Weirs under Free and Submerged Flow Conditions

This laboratory study examines scour patterns downstream of a Type A rectangular piano key weir (PKW) under both free and submerged flow conditions, using a 10 m long, 0.75 m wide, and 0.80 m deep flume. Submerged flow conditions, which commonly occur in PKWs located in riverine settings when elevated tailwater levels during floods partially or fully submerge the structure, were explicitly reproduced in the experiments. Results show that scour dimensions beneath outlet keys are significantly larger than those beneath inlet keys, with higher densimetric Froude numbers correlating to larger scour holes. Under submerged flow, a 33% increase in the submergence ratio and a 67% decrease in the densimetric Froude number produced a 92% increase in maximum scour depth and a 90% increase in maximum scour length for sand beds, while gravel beds exhibited even greater increases (96% in depth and 99.5% in length). Scour was inversely proportional to the ratio of downstream to upstream total head—an increase of just 0.1% in this ratio reduced scour depth by 83% for sand and 90% for gravel. The influence of the densimetric Froude number on scour diminishes at higher values, whereas the sediment ridge height was highly sensitive to variations in densimetric Froude number, highlighting the complex interplay between hydraulic parameters and scour dynamics. Submerged flow, while reducing erosive capacity compared to free flow, introduced distinctive scour characteristics. Based on the results, new equations are proposed for scour estimation under both free and submerged flows. The study identifies the downstream-to-total head ratio (Hd/HT) as a key parameter strongly controlling scour behavior and reveals contrasting toe scour trends between sand and gravel beds.