PSI - Issue 84

Pietro Giaretta et al. / Procedia Structural Integrity 84 (2026) 449–456

450

Nomenclature pier diameter 50 median grain size scour depth 4 she Sheppard coefficient used in the CSU formula particle density gradation coefficient 1. Introduction

Bridges are critical points where hydrographic networks intersect with road or rail infrastructures. The presence of piers or abutments within the riverbed which can induce scour potentially compromising bridge safety during both flood events and ordinary flow conditions. The importance of river crossings is twofold. They are crucial during hazardous events, facilitating rescue operations and evacuation. Moreover, they significantly impact surrounding social and economic activities (Salandin, 2021). Despite their importance, bridge failures are common worldwide, with over 50% related to hydraulic causes, primarily local scour at piers (Landers, 1992; Wardhana and Hadipriono, 2003; Xiong et al., 2023). Local scour, the erosion of sediment around bridge piers, can undermine foundations and lead to collapse if not adequately addressed in design. This poses a particular threat to existing bridges with shallow foundations, which may not account for cumulative scour from successive flood events. For new bridge, this vulnerability can be effectively mitigated by deepening foundations below the estimated maximum scour depth (Lagasse et al., 2007). The presence of a bridge pier within the riverbed invariably leads to local scour formation. As the flow encounters the pier-foundation system, the obstacle alters the flow field, generating coherent turbulent structures capable of sediment removal (Ettema et al., 2011). As the flow approaches the pier, a downflow generates, increasing in strength as it moves downward, forming a groove in front of the pier (Ettema, 1980). Once initial scour occurs, a horseshoe vortex develops due to the boundary layer separation at the upstream rim of the scour hole. The vortex effectively transports sediment particles downstream of the pier. As the scour hole enlarges, the strength of the horseshoe vortex diminishes, as along with the amount of material transported downstream. Flow separation at the pier sides creates unstable shear layers that roll up into eddy structures, forming wake vortices (Chiew, 1984). These turbulent structures transport the material destabilized by the horseshoe vortex away from the pier. The intensity of wake vortex decreases with the distance from the pier, leading to the deposition of eroded sediments downstream of the obstruction (Ettema et al., 2006). Over the years, numerous authors have proposed formulations to estimate local scour depth based on pier geometry, flow characteristics, and riverbed material, primarily through laboratory experiments. However, local scour is governed by a wide range of hydraulic, morphological, and geometric parameters (Melville and Coleman, 2000), resulting in significant uncertainty in predicting maximum scour depth. Most existing studies focus on circular piers, with considerably less attention given to elongated piers. When elongated piers are skewed relative to the flow direction, the resulting flow field becomes more complex, producing scour behavior that differs markedly from that observed for circular piers. Limited research has addressed the influence of foundation at skewed piers on scour development, considering the many geometric variables involved, such as the foundation’s planform offset relative to the pier, and the elevation of the foundation top relative to the riverbed. In analyzing the local scouring through laboratory experiments for elongated piers, the skew angle relative to the flow direction significantly influences the scour geometry. As the skew angle increases, the maximum scour depth generally increases as well (Laursen and Toch, 1956). Moreover, skewing the pier shifts the location of maximum scour depth from upstream to downstream (Breusers and Raudkivi, 1991). These authors suggest avoiding angle of attack greater than 5°-10°, preferring row of cylindrical columns to elongated piers. The presence of foundation elements, such as footings, caissons, or pile caps, adds significant complexity to the scour process. Chabert and Engeldinger (1956), experimenting with a circular pier on a circular foundation, reported that positioning the foundation top below the riverbed surface can effectively reduce scour depth. However, in practice,