SCOUR AROUND SPURS WITH AND WITHOUT RIP-RAP

Abstract

When an alluvial stream is partially obstructed by an obstruction like spurs, bridge piers and abutments the flow pattern in the channel around these structures is significantly changed. The obstruction such as a spur produces an adverse pressure gradient in its upstream. The boundary layer that develops upstream undergoes a three-dimensional separation resulting in the formation of horseshoe vortex. The shear stress around the spurs, bridge piers and abutments is significantly enhanced due to the formation of horseshoe vortex and it causes formation of scour hole around these structures, which, in turn, changes the flow pattern and bed shear stress. Several investigators have studied control of bridge pier scour by using various devices such as collar, rip-rap, piles etc. But not much information is available regarding the applicability of these devices in reduction of scour around spurs and abutments. In the present thesis an experimental investigation is made to study the effectiveness of rip-rap in reduction of scour around a spur. The experiments are restricted to the case of clear-water flows. The temporal variation of scour depth and the equilibrium scour depth around the spur are also studied. A thick rip-rap was found to reduce the scour depth by about 80 per cent, while a thin rip-rap was found to reduce the same by about 50 per cent. Use of a rip-rap having spatially varying thickness and a smaller areal extent produced a reduction of about 60 per cent in equilibrium scour depth.