Keywords: 
• Gravity-driven sediment flows /
• fluid-mud layer /
• buoyancy-friction model /
• sediment transport /
• Jiangsu coast

Abstract: [Objective] An enhanced understanding of the underlying physical mechanisms governing the dispersal of terrestrial sediments into the deep ocean, together with their accompanying nutrients and contaminants, has long been one of the most fundamental components of sediment source-to-sink studies. Over the past three decades, wave- and current-supported gravity flows (WCSGFs) have been recognized as the predominant physical mechanism responsible for the cross-shelf transport of fine-grained sediment and the morphological evolution of numerous coastal and continental shelves worldwide. Despite their significant impact, it has continued to be an ongoing challenge to quantify the transport dynamics of WCSGFs due to their localized, episodic and ephemeral nature. Inadequate in-situ observations have hindered a comprehensive understanding of the transport processes of WCSGFs. [Methods] To address this gap in knowledge, a field campaign was conducted by deploying an instrumented tripod system from 07:00 h on November 25, 2018, to 08:30 h on November 29, 2018, off the central Jiangsu coast, China. A cross-shore bathymetric profile, obtained from a multibeam echo-sounder survey conducted in August 2018, served as a baseline. The instrumented tripod system was deployed at a depth of 7.00 m relative to the mean sea level indicated by the cross-shore bathymetric profile. [Results] Analysis of the collected time-series data revealed multiple instances of intermittent high suspended sediment concentration (SSC) values exceeding 5 kg?m-3, with durations ranging from 0.25 to 2.75 hours, indicative of fluid mud development. Notably, these fluid-mud events occurred during tidal slack water and exhibited a thickness of approximately 0.3 m. Vertical SSC gradients became prominent when SSCs reached around 5 kg?m-3, establishing a critical threshold for distinguishing between overlying flow and the near-bed fluid-mud layer. The presence of anomalously large near-bottom, offshore-directed current velocities coinciding with thin fluid-mud events unequivocally confirmed the occurrence of WCSGF events. In total, eight fluid-mud events were identified, of which five gave rise to WCSGF events. The observed WCSGF events were subjected to parameterization using a buoyancy-friction model, yielding a depth-averaged suspended sediment concentration within the fluid-mud layer equivalent to an average mass concentration over the bottom meter of the water column. [Conclusion] During storm events, unconsolidated sediments could be re-suspended by strong wave-induced shear stress, forming a fluid-mud layer that subsequently moved downslope under the influence of gravity, manifested as WCSGFs. In weak wave conditions, sediment settling from the overlying fluid during low slack water also had the potential to create a near-bed fluid-mud layer. When the settling sediment reached a critical excess density, WCSGF initiation ensued. Maintenance of WCSGFs depended on either current-induced bed stress or a combination of wave- and current-induced bed stress. Importantly, the observed WCSGF events were of short duration and were not observed during peak ebb and peak flood phases when stronger near-bottom currents prevailed. This suggests that the upward dispersion of bottom sediment within the near-bed fluid-mud layer contributed to the cessation of WCSGFs. The observed WCSGF events in the shallow water off the central Jiangsu coast provides yet another case study in support of the use of the classical theoretical existing buoyancy-friction model.