Influence of flow unsteadiness on local scour at bridge piers: An energy-based perspective

Local scour at piers is a major cause of bridge failures worldwide. While most existing studies assume steady flow conditions, actual flood events are inherently unsteady, with distinct rising and falling limbs that significantly modify scour dynamics. This study investigates the influence of flow unsteadiness during the rising flood stage on local scour processes through combined theoretical analysis and flume experiments. A novel energy-based theoretical framework is proposed, wherein the effective scour work of the flow equals the energy gained by the scoured sediment. Laboratory tests with various hydrograph shapes were carried out to quantify the effective scour work under unsteady flow conditions. The results indicate that the flow unsteadiness contributes to an increase in effective scour work, thereby exacerbating local scour around bridge piers. This enhancement is positively correlated with the rising rate of flow and can be attributed to the added mass force induced on the pier and the reduction in the critical velocity for incipient scour, with the latter playing a more dominant role. When the rising rate of flow is smaller than 10 −5 m/s 2 , the effect of unsteadiness is negligible. However, flow unsteadiness contributes more than 20% to the effective scour work when the rising rate of flow is approximately 10 −4 m/s 2 and may exceed 50% in extreme events. Finally, a theoretical model based on the balance between the effective scour work of flow and the energy gain of sediment is proposed to predict the temporal evolution of scour depth under unsteady flows. These findings elucidate the importance of considering the influence of flow unsteadiness on local scour at piers and provide a physics-based method to improve scour prediction under unsteady flow conditions.