Experimental study on local scour at complex bridge piers under steady currents with bed-form migration

Local scour at pile-supported bridge foundations in coastal and offshore areas are an essential factor to engineering design and evaluation. This study experimentally investigates live-bed local scour at complex bridge piers under the influence of bed-form migration. The results show that the variation of the mean scour depth as a function of flow intensity ratio U/Uc (Uc= threshold velocity for incipient motion) depends on the pile-cap elevation and is significantly different to single piers. Instantaneous bed level, cross-correlation analysis, and power spectra show that bed-forms approaching an exposed pile group may be damped, leading to mean bed level fluctuation that are slightly smaller than the bed-form heights. In contrast, when the pile-cap is completely buried by the original bed but can be exposed during scour, approaching bed-forms tend to keep their individual features and cause larger scour fluctuation at pile-cap's leading edge. The enlargement is due to the concurrence between the enhanced (or weakened) scouring capacity and the decreased (or increased) upstream sediment supply. The scour predictor proposed by previous studies is extended to live-bed range and performs better than existing predictors. •Features of current-induced scour at complex piers with bed-form migration and different pile-cap elevations are discussed.•Pile-cap’s position can affect both the time-averaged scour depth and the real-time scour fluctuation.•Cross-correlation and spectral analysis show the process of bed-forms interacting with the pier and being damped.•A fully buried pile-cap leads to the enlargement of scour fluctuation.•A new scour prediction method is proposed for complex bridge pier with improved accuracy.