The role of a remote tropical cyclone in sediment resuspension over the subaqueous delta front in the Changjiang Estuary, China

Abstract

This study investigates the sediment resuspension and the resultant morphological changes over the subaqueous delta front in the Changjiang Estuary (CJE). An in-situ observation platform equipped with multi-sensor instruments was deployed in the CJE to measure the currents, waves, and suspended sediment concentrations (SSC) when a typical remote tropical cyclone, Sanba, passed through the waters over 540 km away from the CJE from September 15 to 17 in 2012. In addition, a current-wave-coupled numerical model was employed to simulate the spatial and temporal variations of the waves, currents, and sediment transport. The numerical simulation accurately captures the sharp increases in wave-induced bed shear stress and associated SSC, as indicated in the field measurements during the tropical cyclone period. Remarkable erosional patterns are observed over the subaqueous delta, which highlight the role of the remote tropical cyclone in sediment suspension and short-term morphological changes in the CJE. Moreover, the simulated erosional pattern agrees well with the sediment coarsening area observed between the 2 and 10 m isobaths. Analyses of the wave spectrum suggest that the bed shear stresses induced by the swells prevail over those induced by the wind waves during the tropical cyclone period. In addition, two simulation runs with ideal wind fields were performed to clarify the contribution of the swells propagated from the outer sea to the sediment transport. Overall, this study suggests that the erosion trends of the subaqueous deltas observed in the CJE are largely determined by the waves, and the swells produced by tropical cyclone events can significantly facilitate such a trend.