HYDRAULICS OF SKIMMING FLOWS OVER STEPPED CHANNELS AND SPILLWAYS

Stepped chutes have become recently a popular method for discharging flood waters. The steps increase significantly the rate of energy dissipation taking place on the spillway face and reduce the size of the required downstream energy dissipation basin. This paper reviews the hydraulic characteristics of skimming flows. The onset of skimming flows is discussed. New results are presented to estimate the flow resistance along stepped chutes. The study indicates some major difference between the flow patterns on steep and flat stepped chutes. An analogy with flow over large roughness is developed. Then the calculations of the start of air entrainment are detailed. The results indicate that free surface aeration occurs much more upstream than on smooth spillways. The effects of flow aeration are later discussed.