Experimental Investigations of Air Entrainment in Transition and Skimming Flows down a Stepped Chute: Application to Embankment Overflow Stepped Spillways

Stepped spillways have been used for about 3,500 years. The last few decades have seen the development of new construction materials, design techniques and applications : e.g., embankment overtopping protection systems. Although it is commonly acknowledged that free-surface aeration is significant in stepped chutes, experimental data are scarce, often limited to very steep slopes. The paper presents an experimental study conducted in a large-size stepped chute. Observations demonstrate the existence of a transition flow pattern for intermediate flow rates between nappe and skimming flows. Detailed air-water flow measurements were conducted in both transition and skimming flows, immediately downstream of the inception point of free-surface aeration where uniform equilibrium flow conditions were not achieved. In skimming flows, a complete characterisation is developed for the distributions of void fraction, bubble count rate and velocity, and flow resistance is estimated including drag reduction effects. Transition flows exhibit significantly different air-water flow properties. They are highly aerated, requiring the design of comparatively high chute sidewalls. The design of embankment overflow stepped spillway is discussed in the light of the new results and design recommendations are provided. Major design issues include the step stability at the plunge point for high tailwater levels, the interactions between free-surface and seepage flows which could lead to further drag reduction, and secondary currents at the connection between steps and abutments.