Characteristics and time scale of local scour downstream stepped spillways

Stepped spillways are employed to reduce excess energy encountered with exiting ow from high hydraulic structures. Study of local scour evolution downstream of stepped spillways wilt, therefore, provide the required information to reap the benets made from these structures to minimize the scour hole dimensions. This paper provides the results of 67 experiments downstream of some stepped spillways subjected to dierent Froude numbers, basin lengths, tail-water depths, sediment sizes, and two dierent sloped spillways. The experiments were continued for 6, 8, 12, and 24 hours from which 824 proles and 85000 data points were recorded and analyzed. The results show that, in certain circumstances, the dimensions of scour hole increase in accordance with particle Froude number. It was also observed that an increase in the slope of spillway would result in reduction in the geometries of scour hole. Under certain conditions, as the tail-water increases, the depth of scour hole increases and elongates the hole. The relations of duration of scour evolution downstream of stepped spillway are presented in this paper. Finally, it was observed that the stepped spillway would considerably decrease the dimensions of scour hole compared with ogee spillways, re ecting the excess energy downstream loss of stepped spillway.