Experimental Investigation of Local Scour Around Complex Bridge Foundations Under Highly Asymmetric Tidal Flow: A Case Study of the Qiantang River Estuary

Local scour at composite bridge foundations in macrotidal estuaries is governed by both foundation geometry and reversing-flow history, whereas most design equations assume steady unidirectional flow. A 1:80 undistorted movable-bed model was used to investigate three foundation configurations of the Qianjiang Third Bridge with two prototype-derived fast-flood, slow-ebb hydrographs per configuration. Pointwise scour evolution was measured using an optoelectronic interface probe, and post-test bed surveys provided global maximum scour depths. Most erosion occurred during the short flood phase; the weaker ebb phase produced limited backfilling, and pointwise scour approached dynamic equilibrium after approximately three complete tidal cycles. Prototype-scale global maximum scour depths were 11.8–12.2 m for Model 1, 9.5–9.9 m for Model 2, and 8.0–8.4 m for Model 3. Inverse calibration of steady-current tests yielded geometry coefficients of 0.72, 1.08, and 0.88 relative to HEC-18 equivalent obstruction widths. An exploratory tidal-asymmetry relation, Kη=Kbηβ, was calibrated using nine cases with a common Oh-type baseline. The fitted β was 0.329, and the in-sample mean absolute percentage error decreased from 29.0% for Kb alone to 13.6%. Because the dataset is small and the relation did not transfer consistently to HEC-18-based cases, local recalibration remains necessary. The results provide a traceable experimental basis for site-specific scour assessment of complex foundations with strongly asymmetric tides.