193 / 2024-09-11 10:00:44

Geomorphic evolution of the Yangtze submarine shoal: Artificial and natural impacts

Reduced sediment flux,Geomorphic evolution,Yangtze submarine shoal,Artificial and natural impacts,Quantitative impacts

Session 17 - Advances in Coastal Hydrodynamics and Sediment Dynamics for a Sustainable Ocean

Global deltas are facing severe challenges under the impacts of human activities and climatic change, but it has been challenging to quantify their contributions due to nonlinear natural processes and lack of long-term geomorphological data. Time-series bathymetric data were collected at submarine Hengsha Shoal in the Yangtze Delta during 11 repeat surveys over 60 years. Our results show that the Hengsha Shoal exhibited overall expansion during the past ~60 years. Vertically, this shoal is currently experiencing a state of slight erosion as −1.5 cm/yr. Spatially, the Hengsha Shoal is currently changing from horizontal expansion to vertical accretion. Considering the spatial patterns of erosion/deposition and residual currents and sediment, modern geomorphic changes in the Hengsha Shoal appear to be dominated by internal sediment transport from deep to shallow waters. The Hengsha Shoal, currently half-surrounded by the Hengsha Reclamation Project and the Deep Waterway Project, has been growing in certain periods. Specifically, the accumulation/erosion during the projects period was much stronger than those during the pre- and post-projects periods. Morphological change due to deltaic engineering could be as high as 19 times of that induced by sediment decline. Without human impacts, it would have been experiencing net erosion since the operation of Three Gorges Dam in 2003. The minimum riverine sediment supply to maintain the shoal’s morphology was 229 Mt/yr, approximately 40 Mt/yr lower than that required to maintain the geomorphological stability of the subaqueous delta. Sediment erosion in the subaqueous delta may transported landward, making the shoals more resilient than the subaqueous delta. Slopes are getting steeper. For future research on global deltaic evolution, it is critical to quantify the impact of deltaic human activities during the Anthropocene Epoch.