| The estuarine turbidity maximum is a special region of estuary,which is the product of the concentration of the sediment from different directions influenced by river flux and tidal dynamic. Under the background of the decrease in the sediment load of the Yangtze River, the research on bed sediment resuspension of the Yangtze River estuarine maximum turbidity as a new research has important scientific significance, especially to explain the high suspended sediment concentration in the estuarine turbidity maximum, and the effect to the aggradations of the channel. As well, the research on bed sediment resuspension of the Yangtze River estuarine maximum turbidity has important practical significance, such as the maintaining of channel, the construction of port engineering, the protection of water environment, the development of fishery resources, even the comprehensive development and governance of the whole Yangtze River estuary.Based on the observation of flow and suspended sediment concentration by myself in North Branch in April 2011, South Passage in October 2012, North Passage in October 2013, and the observed data of North and South Channel in January 2014 as a comparison, this research analyses the characteristics of the tide and suspended sediment in the estuarine turbidity maximum zone channel. In this paper, main works as follow:spatial and temporal characteristics of the current and of the suspended sediment concentration in estuary turbidity maximum are analyzed according to the latest multi-point continuous measured data; Using the measured data of suspended sediment and bed sediment size, composition and temporal variation characteristics of the suspended sediment size and the bed sediment size are analyzed, and also the exchange rules of the suspended sediment and the bed sediment; the main emphasis of this paper is the dynamic mechanism of the sediment resuspension in the turbidity maximum, and the suspended sediment transport of the estuarine turbidity maximum. Through analysis, the conclusions are as following:(1) The latest observed date show that the channels in the Turbidity Maximum have higher current velocity. The current velocities vary obviously from spring tide to neap tide, spring tides are associated with higher velocity, whereas neap tides are associated with smaller velocity. The maximum current velocities always observed in the surface layers and the minimum current velocities always observed in the bottom layers. Generally speaking, the maximum observed current velocities in spring tides are twice than these in neap tides. The current velocities also vary obviously between flood tides and ebb tides. The tidal currents in the ebb tides are greater than these in flood tides in the upper North Branch but in reversal condition in the lower North Branch The tidal currents in the ebb tides are greater than these in flood tides in the South Passage and North Passage, and both of them are ebb tide dominated. This shows that the recent characteristic of tidal flow has not change.Recently, the channels in the Turbidity Maximum have greater Suspended Sediment Concentrations(SSC). The SSC vary obviously from spring tide to neap tide, the SSC in spring tides are greater than these in neap tides. The SSC are also different in each channel. In the North Branch, the average SSC during the spring tide is 2.88～3.34 kg/m3, up to 4.68 kg/m3. In the North Passage and South Passage, the average SSC during the spring tide is 0.80～1.24 kg/m3, up to 2.34 kg/m3. The SSC in the South Channel and the North Channel which are not in the Turbidity Maximum is 0.33～0.52 kg/m3, up to 0.71 kg/m3, meaning a small variation between neap tide and spring tide is not obvious. The total results show that in channels with Turbidity Maximum, sediment from different directions converges by lengthways and vertical circulations under the interaction of runoff and tidal current, which results in high SSC.(2) Most of the suspended sediments are fine with a median size of 5.98～10.78μm in the Turbidity Maximum of Yangtze River Estuary. The temporal and spatial distribution characteristics of suspended sediment grain size show that:The median sizes in spring tide are greater than these in neap tide. The grain size varies unobviously between flood tide and ebb tide in neap tide. The grain size varies obviously from flood tide to the neap tide with a temporal distribution character--"big-small-big-small". In general, the grain sizes in flood tides are greater than these in ebb tides. The characters of spatial distribution of suspended sediment particle size characteristics show that:North Passage> South Passage> North Branch; upper layer<lower layer.The characteristics of channel sediments show that:shoal sediments in North Branch are coarse with a median size of 8.57～138.17μm, mainly consisted of silty sand and sandy silt with poor or very poor sorting. Sediments in the main channel of North Branch are fine with a median size of 6.1～27.11μm, mainly consisted of clay, silt and sandy silt with poor sorting. Channel sediments in North Passage are fine with a median size of 4.8～31.31μm, mainly consisted of silt, clay, sandy clay and sandy silt with poor or very poor sorting. Channel sediments in South Passage are fine with a median size of 12.26～20.45μm, mainly consisted of sandy silt and silt with poor sorting.In channels with Turbidity Maximum, the grain sizes and the composition of suspended sediment and riverbed sediment are similar. It means that there is frequent exchange between suspended sediment and riverbed sediment and sediment resuspension process is significant, which is one of the sources for sediment in Turbidity Maximum.(3) The SSC near the bottom shows the same change trend with periodical variation of the tidal velocity and there is a significant correlation between SSC and tidal velocity which means tidal velocity is the major driving force of riverbed sediment resuspension process. Bed shear stress changes from neap tide to spring tide regularly, which means in neap tide, there is small bed shear stress and weak sediment resuspension in the surface of riverbed. And in spring tide, the tidal velocity is significantly larger than that in neap tide and min tide, which results in maximum bed shear stress and strengthens the sediment resuspension in the surface of riverbed and raises sediment resuspension height. The exchange direction of suspended sediment and bed material is bottom-up. Moreover the suspended sediment concentration near the bottom reaches the maximum during the tidal cycle with increased mean grain size. In neap tide, with small bed shear stress and weak riverbed sediment resuspension, the settlement of upper suspended sediment plays a significant role and the exchange direction of suspended sediment and bed material is mainly top-down.Resuspended flux of sediment near the bottom in the turbidity maximum shows a significant neap-spring tidal cycle and flood-ebb fluctuations.Take BC2 in North Passage as an example, sediment resuspension is much more frequent in spring tide and resuspended flux of sediment is larger than that in neap tide.During neap tide, frequency of sediment resuspension and resuspended flux of sediment in ebb tide are both-larger than those in flood tide.It is not quite the same during spring tide, which shows that the average resuspended flux of sediment in flood tide is larger than that in ebb tide.(5) The field data shows that the magnitude of sediment transport at every station Turbidity Maximum is increased significantly from neap tide to spring tide. The magnitude of net sediment transport is decreased in North Branch, while it is increased in South Passage and North Passage. The direction of advective sediment transport is seaward. The magnitude and direction of tidal pumping transport change frequently. The magnitude of net vertical circulation transport term is small and the transport direction is landward in the neap tide, and turns into seaward in the spring tide.The difference of suspended sediment transport mechanism between channels sometimes is remarkable, and so it is in an identical channel from neap tide to spring tide. The results show that in neap tide, the advection transport term is the principal constituent of the suspended sediment transport at BZ1 in North Branch, the tidal pumping transport term is the secondary constituent and the magnitude of net vertical circulation transport term is very small, while at NC1 in South Passage, BC1 and BC2 in North Passage, the magnitude of net vertical circulation transport term is the principal constituent of the suspended sediment transport, the advection transport term is the secondary and the tidal pumping transports term is small. In spring tide, the tidal pumping transport term is the principal constituent of the suspended sediment transport at BZ1 in North Branch and NCI in South Passage, then the advection transport term and last is the net vertical circulation transport term. While at BC1 and BC2 in North Passage, the advection transport term is the principal, and then the tidal pumping transport term and last is net vertical circulation transport term. |
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