Sedimentary dynamics in the subaqueous delta of the abandoned estuary: Effects of strong wave activity and riverine re-supply

Global climate change and the decline in riverine sediment supply are profoundly reshaping the fate and trajectory of numerous river deltas around the world. Notably, subaqueous deltas of abandoned estuaries (SDAEs) resulting from river diversion represent an extreme example of zero riverine sediment supply, yet studies of these environments remain scarce. Based on in-situ bottom boundary layer observations in both spring and summer in the SDAE of the Yellow River, we examined the variation patterns and controlling mechanisms of sediment dynamics —under the effect of strong wave activity and the new secondary distribution. Strong wave events exhibited a pronounced dual influence on SSC; alongshore wave substantially increase SSC (>21.28 g/L) by enhancing sediment resuspension and liquefaction, whereas onshore wave significantly reduce nearshore SSC through water mass replacement (with values rapidly dropping from 13.11 g/L to 1.14 g/L). Riverine re-supply due to the new secondary course of the Yellow River has altered sediment transport pathways, enabling SSC in the SDAE to reach as high as 22.34 g/L under calm weather conditions, and resulting in a net alongshore sediment flux of 117 kg/m². We identified and established three conceptual models of fluid mud formation mechanisms: wave-supported, wave-current coupled fluid mud (WCFM), and river advection. We identified and established three conceptual models of fluid mud formation: wave-supported, wave-current coupled fluid mud (WCFM), and river advection. Identification of the WCFM highlights the sustained impact of strong wave activity on sediment transport within the SDAE. These findings advance our understanding of sediment dynamics in the SDAEs and provide valuable theoretical references and comparative examples for the studying the global evolution of abandoned or sediment-starved delta.