Sediment Sources and Their Effect on Wave-Supported Gravity Flows in Abandoned River Deltas: Observational Insights

Wave-supported gravity flows are a key process driving near-bed sediment transport in deltaic systems. However, previous observations have focused primarily on active river deltas with abundant sediment supply. The sediment sources and dynamics of wave-supported gravity flows in abandoned deltas with low sediment availability remain poorly understood. In this study, in situ observations were conducted on the abandoned Yellow River Delta of northern Jiangsu Province, China, and four wave-supported gravity flow events were observed within 48 hr during a storm event. The sediment source mechanisms in the study area include local resuspension, wave-induced liquefaction, and tidal advection, and these different sources influence the dynamic characteristics of the flows. Wave-supported gravity flows developed solely by local resuspension from bed shear stress exhibited lower suspended-sediment concentrations and shorter durations. In contrast, when the sediment source mechanisms of wave-supported gravity flows involved wave-induced liquefaction or tidal advection, the wave-supported gravity flows achieved suspended sediment concentrations 7 to 14 times higher. Despite large variations in suspended sediment concentration, flow velocities among the events were not significantly different, ranging from 3.6 to 5.8 cm/s, a result attributed to increased bottom drag and viscosity in the high-concentration fluid mud layers. Furthermore, all observed events were short-lived (1–2 hr) and displayed a distinct tidal periodicity. These findings suggest that in fine-grained sediment limited abandoned deltas, tidal advection is the dominant mechanism triggering frequent, short-lived wave-supported gravity flows, effectively compensating for the lack of local sediment supply.