Survey the effect of downstream crest chute slope on the natural air entrainment point in skimming flow in stepped spillway

Stepped spillways are of most important hydraulic constructions to dissipation energy of water flow. These constructions are used in opening channel to transfer water from upper level to lower level. Because of considerable effect of steps on flow dissipation energy and also the resulted developments in construction of roller competed concrete RCC, the mentioned constructions have drawn hydraulic engineers’ attention. In this research, physical models with three water chute slope in downstream face were used to survey natural air entrainment point in skimming flow. So three different slopes 1:1, 1:0/9, 1:0/7 (vertical & horizontal) were used in chute section after the standard crest parabola and before the steps start. Then the models were installed in hydraulic lab flume of Shahid Chamran University. The experiments were taken for 5 different flows per unit width (with amplitude 0.036 to 0.072 m2/s). The observations show that entrainment happens in water surface at certain distance from crest during the flow passes over the crest (Li). Results showed that crest downstream face geometry can cause change in flow hydraulics leading to change in boundary layer development because of different created (Li). Also it was seen that the 1:1 crest in comparison with other slopes, performs better hydraulically and creates a shorter entrainment point length (Li). In this paper the important results have been shown in the form of different tables and curves.