Protecting bridge piers against local scour using a flow-diversion structure

Abstract

Previous studies indicate that local scour is a leading cause of most waterway bridge failures during flood events. Using armouring countermeasures such as riprap and gabions is a conventional way of reducing scour around bridge piers, but is very costly and time consuming. As an alternative, a flow-diversion structure is proposed that has a triangular prismatic shape with dimensions much smaller than the actual pier and should be installed upstream of the pier. To assess its performance, experiments were conducted under clear-water scour conditions. After achieving equilibrium bed conditions, the bed profile was measured and the maximum scour depth and volume of the scour hole were determined for each experimental test. The results indicated that the clear distance between the pier and the countermeasure to achieve the maximum reduction in local scour was 1·5 times the pier diameter. For this condition, the proposed countermeasure reduced the maximum scour depth by 38% and the volume of the scour hole was decreased by around 61%. To determine the influence of the countermeasure on flow field around the pier, three-dimensional velocity components were measured at grid points using a micro-acoustic Doppler velocity meter. Analysis of the results indicated that the proposed structure could change both the magnitude and direction of the velocity components upstream of the pier and consequently induce a significant reduction in local scour depth and volume around the pier.