Effect of 30-Degree Sloping Smooth and Stepped Chute Approach Flow on the Performance of a Classical Stilling Basin

Advances in dam construction techniques have significantly increased the number of stepped spillways implemented worldwide. Although stepped chutes provide enhanced energy dissipation along the chute, as compared to smooth chutes, an adequate energy dissipater is usually needed at their toe to govern the remaining energy. Stilling basins downstream of stepped spillways are currently designed using the approaches developed for smooth chutes. As a stepped surface alters the structure of the approaching flow, such practice is questionable. This paper reports a study on the effect of stepped chute approach flows on the performance of a classical stilling basin. Physical modeling was conducted using a large-scale facility of a smooth and stepped spillway with a 30° sloping chute. Experiments were performed under different discharges, two step heights, and variable approach flow aeration. The characteristics of the hydraulic jump were described, focusing mainly on flow depth, bottom pressure, and length. The results indicated a significant effect of stepped chute approach flows on bottom pressure and length of the hydraulic jump. Near the jump toe, pronounced fluctuating and extreme pressures were observed after stepped chutes and attributed to the higher turbulence level of the incoming flow. The normalized hydraulic jump lengths were found to be about 17% longer downstream of stepped chutes as compared to smooth chutes.