Influence of Piano Key Weir Crest Shapes on Flow Characteristics, Scale Effects, and Energy Dissipation for In-Channel Application

AbstractDue to their small footprint and efficient hydraulic performance, piano key weirs (PKWs) have been gaining growing attention in the past two decades. Besides the primary geometric parameters like magnification ratio and weir height, the crest shape is another crucial factor influencing the overall performance of this type of hydraulic structure. Beyond discharge aspects, the present study investigates how the flow characteristics, energy dissipation, and scale effects of a piano key weir are influenced by different crest configurations. Overall, results indicate that crest shape majorly affects the nappe trajectory and has significant influence on the hydraulic characteristics of a PKW. The downstream crest shape was observed to be the key element governing the outflow characteristic at low heads, whereas the crest shape in the upstream side was found to be more critical for upstream relative heads within the range of 0.4–0.6. With the resulting nappe behavior, half-round crest was found to provide the highest discharge capacity, while flat crest was the most efficient configuration in terms of energy dissipation. Furthermore, relative to rectangular crests, models with rounded crests were observed to have more significant scale effects at low heads and deviated downstream flow regimes. To facilitate the practical design of such structure, empirical equations were developed to predict the discharge coefficients and downstream residual energies.