Stepped Spillway Flow: Pre – aeration with Deflector

During floods, stepped spillways have the purpose of dissipating energy over flow passage through the chute and, therefore, minimizing stilling basins dimensions. Due to the advantages of energy dissipation and construction facility, these structures have been utilized on a broad scale in the entire world since the decade of 1970. However, stepped spillways have their utilization frequently limited unit discharges of approximately 15 m³/s/m, due to the risk of cavitation damage. Considering the growth of demand for higher flows on dam spillways and the advantages of stepped structures, there is an increasing interest in maximizing the unit discharge able to flow over a stepped chute without risks of cavitation damage. Several authors have been studying the effect of insertion of aerators elements on the chute in order to increase the maximum unit discharge. This paper is focused on the effect of a deflector insertion in the first step of a stepped spillway on the flow behavior and on mean pressures along the chute. Experimental tests were carried out on a physical model assembled at the Laboratório de Obras Hidráulicas (IPH/UFRGS). The model scale is 1:15, the stepped chute is 1V:0.75H sloping and 0.5 m wide, and the steps are 0.06 m high. The deflector has a thickness of 10 mm from the pseudo-bottom of the chute and exceed the step vertical face on 21,5 mm. Unit discharges from 0.054 m³/s/m to 0.436 m³/s/m were tested. Through visual analysis it was possible to observe that the flow is launched above the steps forming a jet which falls near the 5th step for the higher flows, and near the 8th step for the lower flows. Analyzing the mean pressures, it was possible to notice that, for the horizontal and vertical faces of the steps, upstream of the je impact location, the mean pressures were close to zero. These pressures were lower than the same measured at a stepped chute with natural aeration (without aerator elements) for the horizontal faces of the steps and higher for the vertical faces of the steps. Near the jet’s impact location, for both faces of the steps, a peak of positive pressures was observed, exceeding the same values on a chute with natural aeration. Downstream of this location, the pressures tend to the values acquired with natural aeration