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A contribution to the study of local river-bed scour around bridge piers Van der Gugten, Cornelis Adrianus

Abstract

This thesis presents a review of the reported research on local sand-bed scour around bridge piers, describes the mechanics of local scour with particular reference to the horseshoe vortex, describes experiments on the effect of the vertical velocity distribution of the approach flow on local scour, and reports the results of these experiments. The review of the previous research on local scour shows that the pier size is the most important parameter affecting the equilibrium depth of local scour, while the pier shape is of secondary importance. Two flow parameters are found to be important; the flow velocity and the flow depth, although their relative importance depends on the regime of the flow being considered. The primary scouring agent is seen to be the horseshoe vortex system, which is a system of linked vortices that arises out of the vorticity always present in shear flows, due to the interaction of the pier and the flow. The experiments that were done consisted of observations and measurements of vertical velocity distributions, scour depths, and vortex patterns, for a circular cylinder in a laboratory flume. These experiments showed that the vertical velocity distribution of the boundary layer flow approaching the pier affects the structure of the horseshoe vortex system and the equilibrium depth of scour at the pier nose. The Thesis concludes with a Summary and Conclusions, including recommendations for further research.

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