Wind-induced residual current as a driver of sediment flux intensification in a shallow, micro-tidal bay

Wind-induced currents are the major forces responsible for sediment resuspension and transport in micro-tidal bays. The hydrodynamics and sediment transport mechanisms were investigated in Onsan Bay, a heavily contaminated, micro-tidal area on the southeastern coast of Korea, designated as a “Special Management Coastal Zone” due to severe pollution. At two mooring stations (M1: central part of the bay; M2: entrance of the bay), in-situ measurements using acoustic Doppler current profilers (ADCPs) were conducted to examine the impact of wind-induced residual currents on the sediment flux over four weeks. During the mooring period, residual currents (ū) in both stations showed classical estuarine circulation characterized by seaward (landward) flows at the surface (bottom) layers. The suspended sediments at both stations were transported seaward (landward) at the surface (bottom) layer mainly through the residual currents (mean-flow flux Fmean: > 70% of the total flux). Under northerly winds, the bottom ū at M1 and M2 strengthened, with a higher increment at M1. This result implies that the intrusion of alongshore currents through the bottom layer strengthened under northerly winds. The landward Fmean at M1 (M2) was 1.4 (1.2) times higher under northerly winds than southerly winds, resulting in the quadruple “intensification” of net sediment flux. This observation was attributed to the enhanced landward water transport and the weak sediment resuspension by wind-induced residual currents. This suggests that the northerly winds might be a primary factor intensifying the landward sediment fluxes, potentially resulting in the increased sediment deposition into the bay. The findings provide insights into managing sedimentation in contaminated coastal bays and highlight the importance of wind effects on sediment transport in micro-tidal bays.