Sediment transport mechanisms in sediment-starved subaqueous deltas: insights from storm-induced gravity flows

Subaqueous deltas worldwide are increasingly threatened by erosion, driven by the dual pressures of intensified storms and reduced fluvial sediment supply. An abandoned river delta, devoid of sediment input from its watershed, offers an ideal end-member case for investigating delta erosion processes. This study provides direct observational evidence of storm-driven sediment dynamics in such a sediment-starved delta, based on in situ measurements during both typical weather conditions and winter storms on the Abandoned Yellow River Delta, China. During storms, fluid mud layers, wave-induced seabed liquefaction, and gravity flows were directly observed. Fluid mud developed through two mechanisms: wave-induced liquefaction combined with strong bed shear stress; and suspended sediment settling during slack water under weak waves. To enable a more systematic assessment of gravity flow dynamics, we extended a previous analytical model by incorporating additional transport processes. Using this model, it was quantified that gravity flows contributed ∼ 49% of the total sediment transport exported from the subaqueous delta near the 10-m isobath, despite occurring during only ∼ 7% of the 18-day observation. These results highlight that storm-driven gravity flows can develop and play a pivotal role in controlling sediment balance even in sediment-starved subaqueous deltas. Our findings provide new insights into sediment dynamics of sediment-starved deltas under intensified storm forcing and offer a framework for understanding their long-term morphological evolution.