experimental study of effect of collar on local scour reduction around vertical and 45˚ wing wall abutments

local scour around the abutment is of utmost importance in bridge failures. numerous methods such as riprap have been proposed to reduce the destructive effects of scouring; one of the ways to harness and reduce the local scour depth is installed protective collars around the abutment. in this paper effect of collar elevation (upper level, bed level and lower level), collar geometry and its size as a multiplier of the length of the abutment ( ) have been experimentally investigated. all experiments were performed at the threshold of bed material motion under clear-water conditions and uniform-sized bed material. in this study the full-collar and partial collar performance as a bed protective on the two types of abutments; vertical wall and 45˚ wing wall abutment were investigated. the abutment considered in all experiments was short (ratio of abutment length to approach flow depth less than unity). results from the experiments indicate that by increasing the collar width the local scour reduced and the collar width is utmost importance parameter in scour depth reduction. the experimental results presented show in the best case, percentage of scour depth reduction that around the vertical wall (collar at lower bed elevation) and wing wall abutment (collar at streambed elevation) which is 2.25 times wider than the abutment width, was obtained 87.8 and 67.8.