Experimental Study of Scour Induced by Temporally Oscillating Hydraulic Jump in a Stilling Basin

Abstract

A hydraulic jump can be washed out or drowned if the downstream tailwater depth is lower or higher than the downstream sequent depth, respectively. Stilling basins built downstream of water control structures are designed to freeze the jump within the confines of the basin to dissipate energy and keep the jump from washing out. The structure of the flow associated with both washed-out and drowned jumps may result in significant scour downstream of stilling basins, which can in turn pose a threat to the stability of hydraulic structures that the stilling basin is intended to protect. This paper presents novel experimental results of observations of hydraulic jumps at the stilling basin downstream of a low-head spillway model that capture the occurrence of a temporally oscillating hydraulic jump (a jump that alternates between drowned and washed-out types in a quasi-periodic manner). Such a type of jump is found to result in the maximum scour conditions downstream of the stilling basin, which are larger than those induced by well-formed hydraulic jumps. The observations presented herein underscore the point that the conditions that result in temporally oscillating hydraulic jumps should be avoided as part of the design of stilling basins downstream of water control structures to reduce the risk of scour-induced damage.