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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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Upstream Propagation of Sea‐Level Signals in Fluvio‐Deltaic Environments: Time‐Lags and the Dynamics of the Fluvial Surface
Abstract Stratigraphic interpretation generally relies upon the assumption that the fluvio‐deltaic surface responds uniformly to sea‐level changes; however, recent theoretical work suggests that changes in its relief and concavity can influence the propagation of sea‐level information upstream and result in geologically long‐lived lags in the system response. We test this theoretical result using measurements from a experimental delta subject to high and low magnitude sea‐level oscillations. In both cases, changes in relief and curvature of the fluvio‐deltaic profile result in the proximal portion of the profile being out of phase with respect to sea‐level cycles, whereas the nearshore regions remain in phase. These results underscore the importance of delayed response to sea‐level variations in the upstream portion of river deltas, often resulting in net erosion during sea‐level rise and potentially complicating the reconstruction of paleo sea‐level from deltaic deposits.
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- PAR ID:
- 10445835
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 49
- Issue:
- 8
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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