Influence of outlet keys slope on downstream bed topography in trapezoidal piano key weirs: An experimental investigation

Abstract

Assessing scour depth downstream of piano key weirs (PKWs) is crucial for maintaining their structural integrity. The slope of the outlet keys significantly influences flow velocity and the associated risk of downstream scouring. This study experimentally investigates the impact of various factors on scour downstream of trapezoidal PKWs, specifically focusing on weir outlet keys slope (So), particle Froude number, sediment size, bed levels, and relative drop height. The research identifies three distinct stages of scouring. Notably, changes in So significantly affect the hydraulic flow over both the inlet and outlet keys. Reducing So by 57 % led to increases of approximately 32 %, 34 %, 49 %, and 54 % in relative maximum scour depth, distance from the weir, length, and volume of the scour hole, respectively, while relative weir toe scour decreased by about 31 %. Additionally, a decrease in So shifted the locations of the maximum scour depth and sediment ridge downstream, enhancing the symmetry of the scour hole. Increasing the particle Froude number and relative drop height, along with decreasing tailwater depth and sediment size, further intensified the scour hole and sediment ridge characteristics. This study provides critical insights for mitigating downstream scouring in PKWs through a comprehensive analysis of varying So. The proposed optimal value for So, along with newly developed design equations, offers practical guidance for engineers and water resource managers in enhancing the stability of hydraulic structures.

Keywords

• Piano key weir
• Scouring
• Bed topography
• Weir geometry
• Outlet keys slope
• Scour index