Measurement of boundary shear stress in non-uniform open channel flow

Abstract

This paper describes experiments which were carried out to determine the increase in boundary shear stress associated with the zone of suction over a submerged type of river water intake. Conventional techniques for shear stress measurement could not be used because of the non-uniform nature of the flow. Two different experiments were devised, and the results were compared with predictions obtained from a mathematical model, based on a mean flow momentum equation written for successive bed elements in the suction zone. In one set of experiments, a grain population of known threshold shear stress, measured in uniform flow, was placed in the suction zone and the effect of suction velocity on the flow required to induce threshold conditions for these grains was determined. At threshold, shear stress was assumed to be equal in the two cases (of uniform flow, and flow with suction), the direct effect of suction on grain stability having been eliminated by suppressing suction in the immediate vicinity of the "indicator" grains. Shear stress was also deduced from the rate of bed erosion determined by measuring the depth of scour caused by different suction velocities. The latter method was found to be more reliable, and the results were in reasonable agreement with the predictions of the mathematical model.