Effects of Roughness on Riprap Stability around Cylindrical Bridge Pier in 180 Degree River Bend

Bridge is the most important structure in road building where the route crosses a river. Local scour around bridge piers built over a curve in a river is an important reason for its failure. Correct placement of ripraps around the base of piers protects the streambed from scouring by the turbulent flow, which could undermine the pier, resulting in the settlement that terminates in the bridge collapse. Roughening the smooth surface of the pier base and using rough-hewn stones for the riprap is a logical method for their protection. Controlled laboratories experiments were conducted in a 180 degree bent Plexiglas flume where two cylindrical piers had been installed in the bend, one with smooth and the other with the roughened surface. Treatments consisted of two bulk densities and four diameters for the riprap with only one flow rate of freshwater. Results indicated that stability of the riprap depended on the Froude number at the failure threshold, and the relative diameter of the riprap. Furthermore, the presence of roughness on the pier base surface enhanced its stability.