Experimental study on the influence of abrupt slope changes on flow characteristics over stepped spillways

Application of stepped spillways increases the energy dissipation rate along the spillway and may reduce the dimensions of the terminal energy dissipation structure. This pronounced energy dissipation makes stepped chutes attractive under various conditions, namely as service spillways on RCC gravity dams and on valley flanks near earth dams. For both, in some cases, an abrupt slope change may be required to be implemented on stepped chutes in order to follow the site topography and to minimize the needed excavations and hence respective costs. An abrupt slope change along stepped spillways can influence the flow properties such as the air entrainment, velocity and pressure distribution, and the energy dissipation. A quite limited number of stepped spillways have been built with an abrupt slope change, whereas no systematic scientific investigation for designing such type of configuration has been conducted to date. Accordingly, comprehensive information on the effect of an abrupt slope change on the flow features is missing. Therefore, the present experimental research work aimed to examine the effect of an abrupt slope change (from steep to mild) on the skimming flow features, by analysing the air entrainment, flow bulking, velocity and dynamic pressure development and energy dissipation along the stepped chute. Physical modelling was conducted in a relatively large scale facility with slope changes from 50º to 18.6º and 50º to 30º. Detailed air-water flow measurements were conducted at several cross-sections (step edges) along the chute, upstream and downstream of the slope change. In addition, dynamic pressure measurements were obtained on both vertical and horizontal faces of several steps in the vicinity and far downstream of slope change cross-section. The results indicated a substantial influence of abrupt slope changes on the flow properties for the tested range of flow rates, particularly in comparison with typical results for constant sloping stepped spillway flows. Four main local sub-regions have been found to describe the typical air-water flow patterns in the vicinity and further downstream of the slope change, namely with regard to the mean (depth-averaged) air concentration, air concentration distribution, pseudo-bottom air concentration, air-phase frequency and characteristic flow depths. The relative head loss corresponding to the reach under the influence of the slope change was found to vary between 38% to 51%, for the tested range of flow rates and slope change configurations. Mean pressures up to approximately 21 times the equivalent clear water depth (approximately 13 times the step height) were observed on the horizontal step faces in the vicinity of slope change cross-section for the tested range of flow rates. In conclusion, for the first time, the influence of an abrupt slope change on skimming flow properties on stepped spillways was investigated with systematic experiments on two slope change configurations and for a wide range of relative critical flow depths. This thesis report describes and discusses the achieved results mainly on the air entrainment and flow bulking, velocity and dynamic pressure distributions, as well as the energy dissipation.

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