Air entrainment and pressure development in skimming flow on an abrupt slope change on stepped spillways

Numerous stepped spillways were built to date, namely on RCC gravity dams or on valley flanks besides embankment or rockfill dams, while some others are currently under design or construction. In some cases, slope changes may be required on stepped chutes, due to the topography, leading to reduced excavation and related economic benefits. A quite limited number of stepped spillways have been built with slope changes, whereas no systematic scientific investigation for designing such type of configuration has been conducted to date. Thus, there is a lack of information with regards to the main flow features along abrupt slope changes on stepped spillways. The present study is focussed on an experimental investigation of the air entrainment and pressure development on skimming flow on stepped spillways, in the vicinity of an abrupt slope change. Physical modelling was conducted in a relatively large scale facility, where detailed air-water flow measurements were gathered upstream and downstream of a 50˚-30˚slope change. In addition, dynamic pressure measurements were obtained on both vertical and horizontal faces of several steps in the vicinity of the slope change. The results on the air entrainment and pressure development are presented and discussed. A substantial influence was observed on the air entrainment and pressure development pattern, in comparison with typical results for constant sloping stepped spillway flows.