Local scour around a complex bridge pier under bidirectional and unidirectional flow

The local scour around bridge piers poses a significant threat to the stability of coastal and fluvial structures. Although extensive research has been conducted on scour under unidirectional currents or waves, studies on bidirectional tidal flow remain limited. In this study physical modelling is employed to investigate scour around a complex pier (column, pile cap and pile group) under bidirectional tidal and unidirectional flow in a flume. A terrain scanner is used to obtain bed elevations around the pier. Local scour holes and sand ripples induced by bidirectional and unidirectional flows are discussed. The results reveal that bidirectional flow produces symmetrical scour holes with reduced maximum depths owing to periodic sediment backfilling. The scour depth and morphology are highly sensitive to the flow attack angle, with 45° causing the most severe erosion. These findings underscore the importance of incorporating tidal directionality and pier geometry in scour prediction models for safer design in tidal environments.