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ABSTRACT Deltas are crucial for land building and ecological services due to their ability to store mineral sediment, carbon and potential pollutants. A decline in suspended sediment discharge in large rivers caused by the construction of mega‐dams might imperil deltaic flats and wetlands. However, there has not been clear evidence of a sedimentary shift in the downstream tidal flats that feed coastal wetlands and the intertidal zone with sediments. Here, integrated intertidal/subaqueous sediment samples, multi‐year bathymetries, fluvial and deltaic hydrological and sediment transport data in the Nanhui tidal flats and Nanhui Shoal in the Changjiang (Yangtze) Delta, one of the largest mega‐deltas in the world, were analysed to discern how sedimentary environments changed in response to the operations of the Three Gorges Dam. Results reveal that the coarser sediment fractions of surficial sediments in the subaqueous Nanhui Shoal increased between 2004 to 2021, and the overall grain size coarsened from 18.5 to 27.3 μm. Moreover, intertidal sediments in cores coarsened by 25% after the 1990s. During that period, the northern part of the Nanhui Shoal suffered large‐scale erosion, while the southern part accreted in recent decades. Reduced suspended sediment discharge of the Changjiang River combined with local resuspension of fine‐grained sediments are responsible for tidal flat erosion. This study found that the spatial pattern of grain‐size parameters has shifted from crossing the bathymetric isobaths to being parallel to them. Higher tide level and tidal range induced by sea‐level rise, an upstream increase in bed shear stress and larger waves likely further exacerbated erosion and sediment coarsening in deltaic flats. As a result, this sediment‐starved estuary coupled with sea‐level rise and artificial reclamations have enhanced the vulnerability of tidal flats in Changjiang Delta, this research is informative to the sedimentary shift of worldwide mega‐deltas.
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Tropical Cyclones Significantly Alleviate Mega‐Deltaic Erosion Induced by High Riverine Flow
Abstract 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.
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- PAR ID:
- 10455064
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 19
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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