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An understanding of the dynamics and behaviors of suspended sediments is vital in analysis of morphological, environmental, and ecological processes occurring in coastal marine environments. To study the mechanisms of maintaining high suspended sediment concentrations (SSCs) on a tide-dominated offshore shoal, we measured water depths, current velocities, SSCs, wave parameters and bottom sediment compositions in the southern Yellow Sea. These data were then used to calculate bottom shear stresses generated by currents ( τ c ), waves ( τ w ), and wave–current interactions ( τ cw ). SSCs time series exhibited strong quarter-diurnal peaks during spring tides, in contrast to the semidiurnal signal during neap tides. A Fourier analysis showed that suspended sediment variations within tidal cycles was mainly controlled by resuspension in most stations. There existed relatively stable background SSCs (maintaining high SSCs among tidal cycles) values at all four stations during both windy (wind speed > 9.0 m/s) and normal weather conditions (wind speed < 3.0 m/s). The background SSCs had strong relationship with spring/neap-averaged τ cw , indicating background SSCs were mainly controlled by mean bottom shear stress, with a minimum value of 0.21 N/m 2 . On account of the strong tidal currents, background SSCs of spring tides were greater than that of neap tides. In addition, on the base of wavelet, statistics analyses and turbulence dissipation parameter, background SSCs during slack tide in the study area may be maintained by intermittent turbulence events induced by a combined tidal current and wave action. [ABSTRACT FROM AUTHOR]

Water exchange, featured by bottom inflow and surface outflow in a typical estuary, determines the transport and redistribution of salt, nutrients, pollutants, and suspended sediments and organisms. Water exchange in Chesapeake Bay, the largest estuary in the US, has been extensively studied, yet its long-term interannual variability and its relationship with the external forcings are not fully understood. Based on a long-term (1980–2011) numerical model simulation, this study examines the water exchanges between Chesapeake Bay and the adjacent coastal shelf, between different regions within the bay, as well as their relationships with river discharge, wind, and residence time. Through an EOF analysis of the bottom inflow and surface outflow at seven selected cross-bay sections, we found that over 90% of the spatiotemporal variations of water exchange can be explained by the first two EOF modes, which are highly correlated with the freshwater discharge and northwesterly wind, respectively. Unlike the outflow that increases linearly with river discharge as commonly expected, the inflow responds non-monotonically to river discharge. The relationship between the river discharge and inflow can be described by a combination of the Michaelis–Menten/Monod equation and a linearly decreasing function, i.e., the inflow initially increases with river discharge due to enhanced gravitational circulation and then levels, and gradually declines due to overwhelming seaward barotropic current. We found a locally enhanced water exchange in the lower-middle bay, which can be attributed to the persistent reflux of surface outflow due to the irregular geometry and rapid shoaling in the channel bathymetry. The water exchange and the mean residence time can be connected reciprocally through the bay volume, yet the validation of this relationship depends on the timescale to be considered since the residence time at a given time is controlled by the future hydrodynamics. A delay effect should be considered when using the relationship to estimate outflow interchangeably with the direct computation of the outflow. • Reflux contributes significantly to the water exchange in the lower bay. • River discharge and down-estuary wind dominate the water exchange in the bay. • Bottom inflow responds non-monotonically to the river discharge. • Water exchanges in the lower/upper bay respond oppositely to the down-estuary wind. • The time-lag effect should be considered to estimate the residence time by V/ Q out. [ABSTRACT FROM AUTHOR]

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]

The definition of the threshold of sediment motion is critical for continental shelf sediment dynamics. The work by A. Shields laid the foundation for this research direction, leading to the well-known Shields curve. Here we review the most widely used threshold curves that have followed from the original Shields curve over the last 80 years, and propose that in terms of physical processes the threshold (critical Shields parameter) is a function of at least six variables, i.e. grain Reynolds number, grain size distribution, sphericity, roundness, particle cohesiveness and the scale effects of turbulence. Identifying these key factors, we paid a special attention to the role of the scale effects of turbulence. Turbulence was thought to be a random process, but the improvement of measurement techniques revealed that it has both temporal and spatial structures: the magnitude of instantaneous velocity fluctuations varies in time and in location, which can cause the deviation between in situ measurements and flume experiments. In coastal and shelf waters, in situ measurements of tidal currents and suspended sediment concentrations have revealed that resuspension takes place even though the bed shear stress is well below the Shields curve. Further process and mechanism studies are required to improve the theoretical framework regarding the turbulence structures and their interplay with sediment threshold. The scientific problems for future studies include the establishment of laboratory experiments, in situ measurements and process-based modelling under different water depths and hydrodynamic conditions to quantify the scale effects of turbulence; the development of new observation techniques for higher resolution and for extreme environments; development of new data processing methods, including big data methods to analyse turbulence structures; and the quantification of the effects of biological contributions and non-particle components on the family of Shields curves. • Measured threshold for initial sediment motion is well below the Shields curve. • The scale effects of turbulence cause the deviation between in situ measurements and flume experiments. • The threshold (critical Shields parameter) is now a function of at least six variables. [ABSTRACT FROM AUTHOR]