Characteristics of fluid mud in the Yangtze Estuary: Storm, tide, and slope-triggered sediment dynamics and effects

Fluid mud is widely found in estuarine and coastal areas and significantly affects sediment transport, naviga-tional safety, and ecological functions. The formation mechanisms of fluid mud are not yet fully understood because of their complex dynamics, including the effects of riverine discharge, tides, storms, and gravity-driven flows. In this study, the in situ density of 18 fluid mud events was measured along the deep navigation channel (DNC) of the Yangtze Estuary from 2010 to 2016, along with hydrodynamic factors such as riverine discharge, wind, waves, tidal stratification of salinity and suspended sediments, and channel slope. Our results indicate that the scale of storm-triggered fluid mud events in the DNC was 10-60 km long and 0.5-4 m thick, and tidal residual flow circulation and salinity stratification co-formed fluid mud and determined its structure along the DNC. In addition, the coupled stratification induced by the suspended sediment concentration and salinity affected the thickness of fluid mud, and the strengthened stratification induced by the decreasing tide current affected the thickness of high-density fluid mud. Furthermore, gravity-driven flow induced by the steep slope on both channel banks enhanced the accumulation of fluid mud in the DNC. Finally, fluid mud accounted for 20% of the annual silted sediment in the DNC, and the upper low-yield-stress layer of fluid mud (density <1250 kg/m3) could be utilized as the potential nautical depth during fluid mud events, accounting for 20-75% of the entire thickness of the fluid mud. The results of this study can improve the understanding of fluid mud processes and mechanisms and promote cost-effective dredging activities, particularly in muddy tide-dominated estuaries.