| Suspended sediment is one of the important material compositions in the water of the estuaries, especially the turbid estuaries. Spatial distribution, temporal variation and transport of suspended sediment play a vital role to the ecology system and engineering in the estuaries and adjacent waters. Numerous researches have been conducted on the suspended sediment. The complexity of multi-factor and multi-scale (e.g. variations of the runoff, current, waves, coastal currents, and human activities) of the estuaries and their natural different characteristics, however, lead to further study of temporal and spatial variations of suspended sediment and transport mechanisms in the estuaries (especially the water column near estuary mouth). Based on data collected from observations of eighteen observation voyages at nine measuring sites (from the intertidal zone to twenty meters depth) located in offshore area in front of Chongming Island, the present study aims to:discuss the variation patterns of vertical profile of suspended sediment concentration and grain size; reveal single tidal cycle, neap-spring cycle, and seasonal periodicity of suspended sediment concentration and grain size, as well as their trends caused by decrease of sediment discharge; discuss the dynamic mechanism of sediment exchange between water column and seabed by comparison between the calculated bottom shear stress and critical bed shear stress for erosion, understand the horizontal net sediment transport flux through the analysis of net water and sediment transport rate per unit width. The main results and conclusion are:(1) Vertical variations of suspended sediment concentration. The402individual SSC profiles can be classified as nine basic types:L-shaped, stepwise, slanting concave-up, slanting concave-down, S-shaped, reverse S-shaped, slanting line-shaped, vertical stable, and arc-shaped.55%of the SSC profiles are belonging to the above nine types, while the rest45%are transitional type. The averaged SSC profile, however, shows a nonlinear downward (from the surface to the bottom) increasing trend with greater gradient as the depth (R2=0.99). The ratio between near-bed SSC and water surface SSC was about2.8, which is close to the value of3suggested by Whitehouse et al.(2000). A new approach has been provided to simulate SSC, which is closer to the measured SSC (the correlation coefficient between the simulated and measured SSC exceeds0.99), rather than the classical Rouse equation.(2) The vertical variations of suspended sediment grain size. The averaged profile of41single profiles showed a significant linear downward coarsening (R2=0.996), although each profile showed some different patterns. The averaged bottom suspended sediment grain size,7.2μm, was1.6times as the surface suspended sediment grain size. This suggests that the downward increasing of suspended sediment concentration contained contribution in the particle size downward Increases.(3) The periodicity and tendency of the SSC changes. The SSC changes significantly within the tidal cycle, although some differences exist in the tidal cycle of different sites and observation time. The maximum SSC appears in the peak ebb phase, and the minimum value appears at slack flood tide, indicating that the effects of tidal current, runoff-current interactions, and vertical mixing in different tidal stages. The SSC in the present study varies from0.05to2.80g/1, with an average value of0.36g/1. SSC of spring tides is usually greater than that of neap tide, probably because the current velocity of spring tide is significantly greater than the neap tide. The SSC of dry season was bigger than that of flood season (i.e., the ratio of dry season SSC and flood season SSC was1.62at site H). That’s mainly due to the dilution of the runoff, whose sediment concentration is lower than the water column of the estuary mouth, and which is smaller in dry season than in flood season, as well as stronger resuspension in dry season caused by the wind wave. The SSC of the present study area has decreased by about25%compared with thirty years ago, which reflects the response of estuarine SSC to the decline in riverine sediment supply due to human activities.(4) Dynamic mechanism of sedimentation and resuspension of sediment. In the present study area, the hydrodynamic condition was controlled by currents during calm weather as the contribution of τc to τcw was more than0.82. The frequency of τc greater than τcr (critical bed shear stress for erosion) was48%, while frequency of τc less than τcr was52%, which means that sediment exchange between water column and the seabed was frequent. Correlation between near-bed SSC and τc was not significant, suggesting the SSC change based on the observations once an hour is reflected by advection rather than the deposition and erosion processes caused by local hydrodynamic conditions.(5) Suspended sediment transport. Current velocity decreasing downward and SSC increasing downward lead to vary probability for the vertical profile of product them. The maximum product value can occur at the surface, the bottom, or the middle layer, whereas commonly occur at the middle layer. The residual current of each site and voyage varied from0.07to0.28m/s. Net transport direction of the water and sediment at the site near the estuarine mouth is eastward, namely seaward, reflecting sediment transport in this area was controlled by runoff. Net transport of water and sediment are greatly influenced by the wind direction. Wind-induced longshore currents cause southward or northwards net water and sediment transport at the sites40-50km seaward from the estuary mouth. |
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