Different response of hydrodynamics and near-bottom sediment transport to a cold front in the Changjiang Estuary and its submerged delta

Abstract

Cold fronts are expected to exert a significant impact on hydrodynamics and sediment dynamics in estuaries, but the exact influence remains insufficiently understood. In this study, we provided an in-depth examination of cold front impact based on a 13-day in-situ observation in the South Passage of the Changjiang Estuary. We deployed two bottom-mounted tripod systems and several buoy stations to collect current velocities, wave heights, wind speeds, near-bottom suspended sediment concentrations (SSC), and bed level changes. Data analyses indicated that the maximum wind speed reached over 20 m/s, with significant wave heights of 1.38 m and 2.66 m inside and outside the South Passage, respectively. Wave-induced bed shear stress predominantly contributed to the total shear stress, and reached up to 0.81 N/m² outside the South Passage. Near-bottom SSC increased by 31 % and 20 % inside and outside the South Passage, respectively, during the post-frontal period, compared to the frontal passage period. The cold front event intensified both the longitudinal circulation in the South Passage and the transverse transport intensity outside the South Passage, leading to a shift in sediment transport direction and landward sediment transport towards the South Passage during the post-frontal period. The cold-front-induced bed shear stress resulted in sediment resuspension in the submerged delta, thereby facilitating a more homogeneous vertical distribution of suspended sediment in the bottom boundary layer. These findings provide insights into the impact of episodic events on water-sediment exchanges at the estuary-ocean interface.