Cumulative impact of human activities on hydro-sediment dynamics and morphodynamics in the highly altered Yangtze Estuary

Estuarine dynamics is influenced not only by upstream human activities but also significantly by large-scale estuarine engineering projects (LSEPs). In recent decades, multiple LSEPs have been constructed in the Yangtze Estuary, and the impacts of single project have been well studied, but their cumulative impacts remain elusive. This study investigates Yangtze Estuary morphological changes from 1958 to 2020 by chart bathymetric data. Results reveal a shift from rapid accretion (1958–1983) to moderate accretion (1983–2011), and then to moderate erosion (2011–2020), almost coeval with declining fluvial sediment discharge in response to the upstream human disturbances. Moreover, increasing LSEPs have complexified estuarine erosion and deposition patterns over local to regional scales in the past two decades. The impacts of LSEPs on estuarine dynamics are further examined by a coupled model based on Delft3D in the Yangtze Estuary under six scenarios. Results indicate that the residual water flux and residual sediment flux both form a convergence center at the South Passage mouth, aligning with the persistent muddy depo-center. The hydro-sediment dynamics intensify at the upper reaches of the South Passage, North Passage, and North Channel while weaken at the lower reaches under the cumulative effects of three constructed LSEPs. However, the planning Hengsha Shoal Offshore Groins project will cause contrasting effects in the North Channel. As the LSEPs increase, estuarine sediment accretion intensity decreases, declining the net suspended sediment budget by ∼ 8.7 % after four LSEPs. These findings highlight numerical modeling is a powerful tool for worldwide estuarine planning and management.