The Characteristics Of Suspended Fine Sediment Flocs In Changjiang Estuary

In the present study, temporal changes in floes size (Dm) and volume concentration (VC) of suspended fine sediment were measured at Xuliujin station andHensha station of the Yangtze Estuary in 2003, using an in situ laser diffraction particle sizer, the Laser In Situ Scattering and Transmissomerty-100 (LISST-100). At the same time, the profile current velocity was obtained by a 500k acoustic Doppler profiler (ADP) and suspended sediment concentration (SSC) and salinity by an optical backscatter (OBS), respectively. Based on the value of Dm, VC and SSC, in situ mean effective density (Ap) and mean settling velocity (ωs) of suspended fine sediment floes can be computed.Analyses of these measured data showed that 1) floes diameter is 2-5 times larger than separate diameter. 2) Average Dm in the surface water during the spring tide and neap tide are 39.8μm, and 64.4pm at Xuliujin station, respectively. That in the surface water is 48.1μm during the neap tide at Hensha station. The Dm in the surface water is smaller than that in the bottom water. The vertical average Dm is 63.9μm and 70.0μm during the spring tide and neap tide at Xuliujin station, respectively. 3) Average VC in the surface water during the spring tide and neap tide at Xuliujin station are 98.0μL-L-1 and 70.8μL.L-1, respectively. 4) Average Ap are 1173 kg.m-3 and 919 kg.m-3 in the surface water during the spring tide and neap tide, respectively. Ap decrease with the increasing of water depth, ranging from 532~1456kg.m-3. 5) Average ωs in the surface water during the spring tide and neap tide are 0.80 mm.s-1 and 1.62mm.s-1 at Xuliujin station, respectively. It in the bottom water during the spring tide is 1.68mm.s-1. The vertical average ωs is 1.71 mm.s-1 and 1.97 mm.s-1 during the spring tide and neap tide at Xuliujin station, respectively.1)The analysis of Dm and Ap show that they are controlled by turbulent shear generated within the water column, which has a controlling influence over both the flocculation of fine grained cohesive sediments within estuarine waters, and their respective floes break-up. During the tidal period, Dm and Ap vary with the current velocity. The variation trend of Dm is opposite to that of tide current, and the variation trend of Ap is same with that of tide current. The analysis of the main oscillation periods of Dm shows that those periods are responded to the periods of the current velocity, which is fluctuating through the tidal periods. Maybe the salinity has little effecton suspended fine sediment flocculation at Changjiang estuary. The temperature and organic compound might affect the flocculation process importantly at Changjiang estuary. 2) Floes VC is closely related to the current velocity. The measurement indicated the phenomenon that the peaks of VC lag those of the current velocity. The lag variation during ebb tide and flood tide are from 10 to 30 minutes and 30 to 50 minutes, respectively. 3) The variation trend of Ap is opposite to that of Dm, larger floes have much lower Ap than smaller floes, and vice versa. 4) Floes ω3 vary evidently under different hydrodynamics. The stronger hydrodynamics, the smaller ωs is, weaker hydrodynamics, the higher ωs is. Floes ωS during the spring tide is smaller than that during the neap tide. Floes cos in the surface water is smaller than that in the bottom water. Both Dm and Ap of the floc affect its ωs. From the study, we found that Ap and Dm, ωs and Dm have good relationships. Their exponential relationships can be described as Δp∝ Dma and ωs∝Dma, respectively.