Effect of Contraction Ratio on Local Scour Around Bridge Abutments

Abstract

This paper presents the results of a laboratory study of short duration (5 min to 2 hrs) clear water scour tests around bridge abutments in erosive bed (medium sand). The type of the used bridge abutments was vertical with splayed entrance and exit. The time evolution of the erosion process was analyzed with respect to local and global characteristics of the scour hole. Special concern was given to the effect of the contraction ratio, e = B/b, between the contracted width at bridge site and the main channel width on the scour process. In addition, experiments were conducted to combat the scour hole by using pipes or culverts, through the abutment body to reduce the effect of flow constriction. Experimental results showed that the maximum depth of local scour was located at the upstream corner of the abutment. It was concluded that the optimum constriction for a minimum scour hole parameters was approximately 0.65. Also, it was found that using pipes or culverts, through the abutments would reduce the scour hole dimensions by approximately thirty percent. In addition, the scour parameters were found to increase with time and the Froude number while the rate of scour was found to decrease with time. Equations relating the maximum depth and length of scour hole with dimensionless terms were introduced for practical use.