Effect of mutual interference of piers on their local scour phenomenon

Local scour is a phenomenon considering a three-dimensional flow system which sets up around bridge piers and abutments constructed in alluvial bed rivers. The process of local scour can bring about partial inability or complete breakdown of bridge supports and decks. The current investigation involves a series of analyses to determine the effect of mutual interference of bridge piers of various materials with varying distances between them and express the observations with the distinctive parameters of the analysis. With an isolated single pier as standard, tests with piers of five different materials were conducted under clear water conditions. The outcome revealed that the interference between two piers was most prominent at a spacing of two times the pier diameter which was then reduced at increased pier spacing. However, at a spacing of about eight times the pier diameter, the two piers behave as individual piers without affecting the scour depths around the other pier. Further, the surface roughness of the pier bears an inverse relation with scour depth and the interference effects get reduced with increasing surface roughness of the pier. In addition, the increase in the Froude number results in an increase in the scour depth around both upstream and downstream piers and the critical spacing ratio at which the interference is maximum increases with increasing Froude number. Following the experimental outcomes of the present analysis, a model for equilibrium scour depth at complex piers is formulated and validated.