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

Resilience of global large tide-dominated deltas: Subaerial land change and subaqueous delta transformation

Tide-dominated deltas,Reduced sediment flux,land gain/loss,the Yangtze Delta,Subaerial and subaqueous delta,Deltaic human activities

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

Global river deltas are undergoing a variety of geomorphic evolution due to reduced sediment fluxes, deltaic human activities and climate change. Among them, tide-dominated deltas, possessing a certain level of resilience, are relatively more complex in response processes. This study demonstrates the temporal changes in horizontal land area in ten large tide-dominated deltas since the mid-1980s utilizing >12000 Landsat archive images, and illustrates the transformation process of the Yangtze subaqueous delta by the data across four generations during the past ~70 years. These ten large tide-dominated deltas have shown an overall trend of net land gain over the past 35 years at a mean rate of 75.1 km²/yr, with super deltas leading the way, especially the Amazon, Yangtze, and Ganges-Brahmaputra deltas. In contrast, the other deltas, facing insufficient sediment supply, experienced varying degrees of land loss, either in a continuous or intermittent pattern. Based on the case study on the Yangtze River Delta, it is found that its accelerated land gain in recent decades contradicts the continuous decline in sediment input, which is primarily attributed to the artificial reclamation of tidal flats. The reason we do not see the erosion is because the visible subaerial delta is protected by coastal engineering projects and the invisible subaqueous deltas, probably undergoing erosion, is hard to detect. Deltaic human activities, such as restoration and deforestation in the Colorado and Irrawaddy deltas, have also been observed to interfere with deltas, which may intermittently alter the land loss status of the visible subaerial delta. Regarding their subaqueous delta in transition, both accumulation in shallow waters and erosion in deep waters were simultaneously observed. The erosion in the inner delta (including high-lying shoals, especially at the shoreward edge and along the channels, as well as tidal channels, primarily from channel widening and deepening), the declined fluvial sediment input constrained at the river mouth by strengthening oceanic forces, and the eroded sediment in the deep waters being transported landward, may together contribute to the expansion of the shallow-water shoals. This mechanism was jointly verified by the field measured data from the Yangtze River Delta and the simulations from idealized models. Future research on the evolution of tide-dominated deltas should cover both the subaerial and subaqueous deltas, not only focusing on horizontal land changes but also on the vertical changes in sediment volume.