Field study of local scour around bridge foundations on silty seabed under irregular tidal flow

Local scour induced by tidal flow is a major hazard for sea-crossing bridge foundations in coastal areas. The irregular tidal flow and silt-muddy seabed environment generates a very complicated flow turbulence and sediment transport. This paper, for the first time, presents the field measured scour holes around the Hangzhou Bay Sea-Crossing Bridge (HBSB) under irregular tidal flow dominated water regions. The high-resolution seabed topography around 19 bridge foundations in the south waterway of HBSB is measured from 2015 to 2020 by using a multi-beam sonar system. The field measured data are analyzed to investigate the local scour characteristics under irregular tidal flows. Results indicate that under irregular tidal flows with stronger flood tide, the local scour hole around every bridge foundation is slightly asymmetric and the scour depth downstream is larger than that upstream. The slope angles at upstream, downstream and lateral sides of local scour hole are about 13°～18°, which are much smaller than the Angle of Repose (AoR = 35°) due to bidirectional scouring generated by tidal flows. During 2015～2020, the maximum scour depths around all bridge foundations range between 1.2 De～2.5De (where De being the effective diameter of bridge foundation). The maximum scour depths vary frequently with a fluctuation within 0.6De. Comparison between the field measured and predicted scour depth using three available scour depth predication equations show that all prediction equations overestimate the scour depth. As such, considering the discount effect of the varied tidal flow compared with steady current, an improved maximum scour depth prediction equation is proposed to estimate the scour depth under tidal flow on a silty seabed. Results show that the predicted scour depth using the proposed equation has a relative error of ±30%.