Tropical Cyclones Significantly Alleviate Mega-Deltaic Erosion Induced by High Riverine Flow

The drastic decline in sediment discharge experienced by large rivers in recent years might trigger erosion thus increasing the vulnerability of their extensive deltas. However, scarce information is available on the erosion patterns in mega-deltas and associated physical drivers. Here a series of bathymetries in the South Passage, Changjiang Delta, were analyzed to identify morphodynamic variations during high riverine flow and tropical cyclones (TCs). Results indicate that high river flow during flood season triggers large-scale net erosion along the inner estuary, generating elongated erosion-deposition patches. Erosion magnitude gradually weakens moving seaward with few localized bottom variations in the offshore area. TCs transport sediment landward and are accompanied by an overall weak erosion, with a less organized spatial pattern of erosion-deposition. TCs can therefore significantly alleviate erosion, reducing the sediment loss induced by riverine flows by over 50%. These results highlight the role of TCs on the sediment dynamics of mega-deltas.