Shallow foundations for the support of vertical-wall bridge abutments: Interaction between riprap and contraction scour

The database from flume experiments focused on the performance of riprap layouts based on field installations and FHWA HEC-23 design guidelines against clear-water abutment scour combined with Computational Fluid Dynamics (CFD) is used to investigate how flow fields at single span bridge openings, dominated by flow contraction, adjust in response to variations of bed roughness and cross-section geometry due to riprap installations. These adjustments increase bed shear stress magnitudes on the unprotected erodible bed leading to underestimated contraction scour depths therefore creating instability, and ultimately causing edge failure of the riprap. Based on the combined physical/numerical modeling approach an edge failure-resistant riprap layout is proposed. Furthermore, the CFD approach provides an insight into shear stress magnitudes within a nonuniform bed roughness in the bridge opening, and a comprehensive flow depth-riprap interaction model to define limits for “hydraulically narrow” bridge openings that might be prone to edge failure of the scour protecting riprap.