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The radial sand ridges found off the coast of Jiangsu (China) are major morphological features in the southern Yellow Sea and undergo rapid changes in morphology. Despite extensive observational and modeling efforts, the physical processes controlling sediment transport and associated morphological evolution over the ridges remain not well understood. Here we analyzed data collected from systematic hydrodynamic and sedimentary surveys including records from moorings and bottom-mounted tripods deployed at channels and adjacent ridges. We explored the role of advection and local resuspension processes in controlling morphological changes and how they are modulated by tidal asymmetry. Bottom shear stresses due to mean tidal currents, waves, and wave-current interactions were estimated to examine the effects of tidal asymmetry, advection and resuspension processes on suspended sediment concentration (SSC) variability. The simplified depth-averaged model of Bass et al. (2002) was used to explain the phase relationships between fine sediment suspensions and tidal currents. We found that resuspension is the major controlling factor of bottom SSC during mean and spring tides, while advection is the dominant process during neap tides. The asymmetries in water level and tidal current lead to a net lateral sediment transport directed from the channel to the sand ridges, which cause erosion in channels and deposition on the sand ridges. This sediment transport pattern likely contributes to the evolution of the sand ridges under fair weather conditions (i.e., excluding storm events). • Coarse-particle concentrations positively correlated with bottom shear stress. • A simplified model explains sediment-flow phase relationship. • The tidal asymmetry plays an important role in the sorted bedforms. • Lateral sediment transport contributes from tidal channels to ridges. [ABSTRACT FROM AUTHOR]

Fine-grained sediments suspended in coastal waters play an important role in submarine topography evolution and associated environment changes. The convergence of suspended sediments concentrated near the seabed results in high sediment concentration, and contributes significantly to sediment transport. In order to investigate the mechanism triggering high suspended sediment concentration (SSC), we deployed a tripod to the seabed to obtain in situ bottom boundary layer measurements of sediment dynamics and a buoy to the sea surface to collect meteorological and wave data at the northern Changjiang River mouth from December 20, 2015 to January 20, 2016. The high SSC (e.g. >3 g/L) events were observed together with fluid mud (thicknesses of 4–16 cm) near the seabed during neap tides when cold air intrusion generated winter storms and strong waves. Further, we found that the high SSC event was mainly resulted from wind waves and sediment resuspension supported by local benthic fluid mud, which was associated with three stages. At the setting up stage, the winter storm brought long duration of strong-waves (e.g. significant wave height >1.5 m) more than 15 h, resulting in a maximum wave-current combined bottom shear stress of 3.2 Pa and the increase of SSC to >1 g/L. At the reinforcement stage, the strong waves and bottom shear stress lasted for several hours, and further increased the SSC to >3 g/L. At the final decay stage, wind waves and the maximum wave-current shear stresses decreased significantly, with the disappearance of high SSC. Thus, there was no high SSC event observed even with strong wind waves during spring tides because strong wave duration was too short for the reinforcement process. The sediment source for the high SSC events was mainly from bottom fluid mud resuspension, as well as the advection transport from the adjacent subaqueous Changjiang River delta. • Tripod deployment obtains bottom sediment dynamics under winter storm conditions. • Resuspension SSC peaks supported by wind waves and local benthic fluid mud. • High SSC events formed through the setting up to reinforcement and decay stages. • Reinforcement determined by winter storm induced long duration of strong waves. [ABSTRACT FROM AUTHOR]