Delta evolution in the context of no sediment discharge has become a global concern, and an accretion-to-erosion conversion is occurring in the Yangtze estuary. This conversion could threaten Changjiang subaqueous delta development. Sediment erodibility is an important indicator of subaqueous delta vulnerability. However, the present and future erodibility of the Changjiang subaqueous delta remains unclear. In this study, 37 short cores were collected from the Changjiang subaqueous delta, and the critical shear stress of the sediment was measured using a cohesive strength meter (CSM) and compared with estimates based on an empirical Shields diagram. The sediment erodibility was analyzed by comparing the sediment critical shear stress with the bed shear stress simulated using a numerical model (i.e., FVCOM), and sediment activity was introduced to discuss the geomorphological change in the subaqueous delta. The CSM-derived critical shear stress is significantly higher than that derived from the empirical Shields formula, but it better shows the erodibility of the sediment. The annual surface sediment activity ranges from 5% to 30% based on the CSM, indicating low surface erodibility. Moreover, the critical shear stress in this region increases as water depth increases, but the bed shear stress shows the opposite trend. Therefore, the erodibility of the Changjiang subaqueous delta is lower than that of the shallow area, indicating no accretion-erosion conversion or continued vertical erosion under sediment starvation in the coming decades. These findings can provide suggestions for erosion assessment and management in large river deltas under decreasing sediment discharge.

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In AD1128, the Yellow River shifted its course from the Bohai Sea to the South Yellow Sea (SYS) due to anthropogenic dike excavation, starting the development of the Abandoned Yellow River Delta (AYRD) that lasted for more than 700 years (AD1128-1855). However, the sediment flux of the abandoned Yellow River into the sea is in a state of change due to human activities, and the growth process of the AYRD is not well understood. Here, we investigate the growth process of the AYRD and its sedimentary record characteristics over the last millennium based on three cores collected from the AYRD.The results show that the main sediment types in the AYRD are silt, mud and sandy silt. After AD1128, the grain size components in the sediments of the AYRD showed significant stage changes with the sand content first starting to decrease. The clay content increased and remained at a high percentage in the middle to late 14th century, followed by a sharp increase from the mid-16th to the mid-17th century, due to a further increase in sediment flux from the abandoned Yellow River into the sea. A slight increase in the proportion of sand content during the final stage of the transition from subaqueous delta to terrestrial delta is a distinctive feature of the sedimentary record, and this change persists for 10 ~ 90 years in different core records.This study further proposes a schematic model of the development of the AYRD: (a) before the 16th century, the sediments were deposited mainly in the estuary and nearshore, with rapid vertical accretion; (b) After the 16th century, the horizontal land formation was the main focus, and the rate of seaward extension increased rapidly. This model also reflects the following pattern: when the sediment flux from the river into the sea is certain, the rate of land formation is inversely proportional to the rate of vertical accretion. The dominant factors affecting the evolution of the AYRD are the sediment flux into the sea and initial submerged topography, with less influence from sea level changes. Hydrodynamic erosion by wave and tidal forces from the outer delta began to dominate after the interruption of sediment supply due to the Yellow River mouth northward to the Bohai Sea in AD1855. This study has important implications for understanding the growth and evolution of deltas under the influence of human activities.