Design of Stable Riprap around Aligned and Skewed Rectangular Bridge Piers

Using riprap stones is a common method to prevent scouring around bridge piers. Many parameters affect stable riprap size. In the present work, stability of riprap stones around rectangular bridge piers are studied experimentally under clear-water conditions. Round-nose rectangular piers with three different aspect ratios of 3, 5, and 7 were tested. Furthermore, each pier was tested aligned with the flow and skewed at 5, 10, and 20° to the flow. Failure of the riprap layer occurred either because of high shear forces at the upstream face of the pier or under a combination of forces attributable to wake vortices and shear stresses downstream of the pier. Analysis of experimental data showed that relative flow depth, relative stone size, and effective pier width are the important factors affecting the stability of the riprap layer. At different stone sizes and pier skew angles, the region at which the riprap layer fails varies. Based on more than 140 sets of experiments, a design equation is presented for stable riprap size.