Air-water flow in a plain stilling basin below smooth and stepped chutes

In recent decades, stepped chutes followed by a stilling basin became a standard for spillways of dams. The comprehensive knowledge of stepped chute approach flows on internal air-water flow properties of the hydraulic jump is still too limited for an appropriate design of stilling basins. Therefore, an experimental campaign was performed on a large-scale physical model of a plain stilling basin preceded by a 30 degrees sloping smooth or stepped chute. Stepped chute approach flows induce lower deaeration rates along the jump roller as compared to smooth chute approach flows, indicating longer dimensionless jump lengths, normalized by the tailwater depth. Pronounced bottom air concentrations were observed within the first 40% of the jump length in the stilling basin downstream of the stepped chute. Thus, despite the significantly higher pressure fluctuations, this zone seems better protected against cavitation damage than in stilling basins following a smooth chute.