Accretion-erosion conversion in the subaqueous Yangtze Delta in response to fluvial sediment decline

Identifying the pattern of delta morphological change under decreasing sediment flux due to damconstruction is essential for sustainablemanagement in such densely populated coastal areas. In this study, we investigated the morphological processes of the Yangtze mouth bar and prodelta based on bathymetric data on a decadalinterannual scale (1958, 1978, 1997, 2002, 2007, 2010, 2013 and 2015). We found that strong accretion (205.1 Mm(3) yr(-1)) occurred during 1958-1978, when a high sediment load (465 Mt yr(-1)) was supplied by the Yangtze. Afterwards, the net accumulation rate decreased to 31.9 Mm(3) yr(-1) in 1978-1997 and 114.6 Mm(3) yr(-1) in 1997-2002 as a result of riverine sediment loads decreasing to 390Mt yr(-1) and 314 Mt yr(-1), respectively. Surprisingly, the net accumulation rate increased to 130.8 Mm(3) yr(-1) in 2002-2007, though the sediment load sharply decreased to 177 Mt yr(-1). This anomaly was attributed to the construction of training walls within the mouth bar area, which induced significant accretion in groyne-sheltered areas and nearby regions. Along with a further decrease in sediment load, the entire study area converted to net erosion of-200.4Mm(3) yr(-1) in 2007-2010 and -152.2Mm(3) yr(-1) in 2010-2013. Stronger erosion in the former period was partly caused by intensive dredging activities in themouth bar area. The critical sediment discharge for the Yangtzemouth bar and prodelta to retain net accretion was estimated to be ca. 218 Mt yr(-1). If deducting the impacts of estuarine engineering projects on accretion/erosion during 1997-2010, the critical sediment discharge is adjusted to ca. 234 Mt yr(-1). In combination with previously reported accretion-erosion conversion elsewhere in the Yangtze Delta, we inferred that most portion of the subaqueous delta has most likely converted from net accretion to net erosion in response to fluvial sediment decline, and the mouth bar area showed the latest conversion among portions of the delta. Integrated assessment and adaptive strategies are urgently required for the Yangtze Delta to survive the coming erosional stage. (c) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study observed a transition from net accretion to net erosion in the Yangtze mouth bar and prodelta over the past decades. The critical sediment discharge for net accretion was estimated to be around 218 Mt yr(-1), but the entire study area has now converted to net erosion due to fluctuations in fluvial sediment supply.