Comparative Analysis of Tailwater Influence on Downstream Flow Conditions of an A-Type Trapezoidal Piano Key Weir and a Linear Weir

Abstract A piano key weir (PKW) is a hydraulic control structure that offers significantly higher discharge capacity than its linear counterpart, primarily due to the extended effective spillway crest. When operated as in-channel applications, rising downstream water levels can interfere with free-flow conditions, causing submergence of the weir and reducing its hydraulic performance. Moreover, variations in tailwater levels as well as the weir geometry can significantly influence the hydraulics and flow patterns in the downstream reach of the weir. This study investigated the impact of tailwater on the downstream flow regime of an A-type trapezoidal PKW within an experimental model. By comparing flow velocities and flow depths with those observed downstream of a linear weir under similar boundary conditions, the influence of the weir geometry on downstream flow behavior was evaluated. Results indicate that the PKW generates a complex, three-dimensional flow field directly downstream of the weir, characterized by free-falling nappes and jet formations, whereas the linear weir induces a more two-dimensional flow structure. As tailwater levels increase, the visual complexity of the PKW flow diminishes, and both weir types exhibit reduced air entrainment due to decreased drop height. Energy dissipation at the PKW occurs more locally, driven by plunging jets and submerging nappes, whereas the linear weir promotes supercritical flow at low tailwater levels, followed by a hydraulic jump farther downstream. This shift in the dissipation zone may enhance the potential for sediment transport processes in the vicinity of the weir structure. Farther downstream, beyond the aerated region, velocity profiles evolve from logarithmic to irregular shapes, with decreasing boundary layer thickness at higher flow depths. This trend is less pronounced at the linear weir, for which profiles remain in a narrower range of profile development. However, the PKW has more distinct secondary flow effects, such as the velocity dip phenomenon, which intensifies with rising tailwater levels and across the channel width. Additionally, elevated bed shear stress and Reynolds numbers suggest increased turbulence relative to that of the linear weir, implying a greater potential for sediment transport. These findings highlight the importance of weir design and flow depth in influencing downstream flow behavior and hydraulic characteristics.