Revisited sediment budget with latest bathymetric data in the highly altered Yangtze (Changjiang) Estuary

Most river-deltas of the world have been extremely altered by anthropic activities, where a regime shift from net accretion into net erosion may have occurred. In this study, subtidal morphodynamic variations in the Yangtze Estuary (YE) are elaborately analyzed for their causalities using high resolution DEM data from bathymetric surveys conducted every two years from 2007 to 2019; sediment budgetary balance in the estuary has been updated accordingly with mean annual depositional/erosional sediment amounts. The results show that subtidal channel erosion has occurred almost over the whole YE with a mean annual erosional rate of −5.59 cm/yr during the study period. Peak erosion occurred both in the South Branch and the delta front with mean annual erosional rates of −12.64 cm/yr and −8.11 cm/yr, respectively. The erosion lessened in the middle estuary where sandy shoals prevail, and a net accretive rate of 3.13 cm/yr was solely measured at the Chongming shoal. Negative correlation between estuarine erosional volume and riverine sediment discharge demonstrates the great impact of upstream hydro-engineering projects on estuarine morphodynamic change, but large-scale estuarine engineering projects also exert remarkable influence on local morphodynamics, evidenced by the presence of deep erosional belts near the projects. Annually eroded sediment amounts were estimated as 166 Mt/yr and 206 Mt/yr in the distributary channels and the delta front during the period 2007–2019, and the former was almost offset by recent sand mining amount of 143 Mt/yr from the distributary channels for land reclamation. Moreover, contemporary eroded sediment mass and sand mining amount from the tidal river reach between Datong and Xuliujing were reckoned as 77 Mt/yr and 44 Mt/yr, respectively. Our analysis further show that recently 124 Mt/yr and 153 Mt/yr of sediment discharge into the tidal river reach at Datong station and the YE at Xuliujing station account for merely 27.5% and 37.7% of their corresponding values before the 1990s, respectively. Annual sediment amount bypassing the estuary into the open sea has decreased by 37.8% from pre-1990s 370 Mt/yr to recent 140 Mt/yr, leading to recently severe erosion in the delta front. It is therefore highlighted that the estuarine system in response to natural and anthropogenic pressures is very dynamic, and the impacts of various engineering projects in the source-to-sink system should be fully taken into consideration when predicting its morphodynamic response in the estuary.