Stability of Riprap Used to Protect Bridge Piers

摘要

Bridge piers subjected to floodwaters induce strong secondary currents that causelocal scour holes in alluvial streambeds. Local scour holes may form to the extent that the pier foundation is undermined. A common method of protecting the streambed is the placement of a riprap apron. The effect of the secondary currents on the bed material must be known before sizing the rock protection. Small scale model experiments were performed in laboratory flumes to study the impact of piers on local velocities and the stability of riprap protection. Rectangular and cylindrical flow obstructions were used to model bridge piers. The influence of obstruction shape, relative depth of gravel placement, relative gravel size, relative flow depth, and relative approach flow roughness on near-bed velocities and critical displacement conditions was investigated. Obstruction shape, relative depth of gravel placement, and relative flow depth were found to significantly influence the near-bed velocity. Significant factors in the analysis of the critical flow conditions were obstruction shape, relative depth of gravel placement, and relative gravel size.