Characteristics and Time Scale of Local Scour Downstream Stepped Spillways

Document Type : Article

Authors

1 Hydraulic Structures, Irrigation and Reclamation Engineering Dept., University of Tehran, P.O. Box 31587 - 4111, Karaj, Iran

2 University of Tehran Karaj, Irrigation and Reclamation Department, Professor in Hydraulic Structures, P.O. Box 31587-4111, Alborz, Iran

3 , Irrigation and Reclamation Engineering Dept., University of Tehran, P.O. Box 31587 - 4111, Karaj, Iran

4 Head of Hydraulic Structures Division, Water Research Institute, P.O. Box 16765 - 313, Tehran, Iran

Abstract

Stepped spillways are employed to reduce excess energy encountered with exiting flow from high hydraulic structures. Study of local scour evolution downstream of stepped spillways wilt therefore, provide information to required get benefits from these structures to minimizes the scour hole dimensions. This paper provides the results of 67 experiments downstream of some stepped spillways subjected to different Froude numbers, basin lengths, tail-water depths, sediment sizes and two different sloped spillways. The experiments were continued for 6, 8, 12 and 24 hours from which 824 profiles 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 increase in the slope of spillway would result in reduction in the geometries of scour hole. At certain conditions, as the tail-water increases, the depth of scour hole increases and elongate the hole. The relationships for 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 which is reflects the excess energy loss downstream of stepped spillway.

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