Study On Tidal Current And Sediment Characteristics Of Zhujiajian Island Offshore Area

Tidal current and sediment characteristics of estuaries and coastal areas are the basis for human’s using of marine resources, Which are relative related with the construction of Port and Offshore Engineering, so it’s the transportation of nutrients and the migration of pollutants. Offshore area is one of most important area for human to develop and utilize the ocean in the coming21st century.In2011,the State Council set the Zhoushan Archipelago up as an National New Area, the forefront of China to enter the Blue Economy area, which has brought the marine development to a new height. As the development forefront of marine, Zhoushan Archipelago area will run into a new great planning and construction period. So it’s of great importance to participate in researching tidal current and sediment characteristics of this area. This area locates in the south of Yangtze River estuary and the outer edge of Hangzhou Bay, it also has important theoretical significance for the research of diffusion and transportation of the sediment.Based on three fixed stations’tidal period data of currents, suspended sediment,58stations of surficial seabed sediment and four core samples of east Zhujiajian island,October2011, this essay analyzed the tidal and residual current characteristics, the type and feature of suspended sediment concentration profiles, the characteristics of particle size, the distribution of grain size sediment and the deposition rate of some local area. The main results are as follows:1、The tidal type of Zhujiajian area is irregular semidiurnal tide, the mean tidal range is2.52meters, and the maximal range is4.32meters, the maximum velocity usually appears in the early floodtime, which flowing southward with the wind. The minimum velocity is generally in the late ebb flowing with northerly wind. In neap tidal period, the vertical average velocity is0.53m/s for the middle station, and about0.76m/s for the outside station; In spring tidal period, the vertical average velocity is0.83m/s for the middle station, and about0.78m/s for the outside station.2、The rotating flow in this area is evident, for it is in clockwise rotation, and the flow direction varies much between the surface and the bottom.3、During the observation, there is significant difference in the residual current between the surface and bottom. Surface residual is one order bigger than that of bottom residual, and there is also a difference of121degrees to161degrees of direction between them. The vertical average residual current is0.21-0.22m/s and the direction is199～204degrees, which basically the same as the north by east wind direction. The average unit water discharge of middle site through the whole tidal period is333.2×103m3, with a direction of202degrees. The average unit sediment discharge of middle station through the whole tidal period is92.7ton, with a direction of213degrees. The average unit water discharge of outward station through the whole tidal period is472X103m3, with a direction of204degrees. The average unit sediment discharge of outward station through the whole tidal period is21.5ton, with a direction of210degrees.4、There are three basic types of SSC profile:L accounts for about50%, anti-S-type accounts for about20%and slash is about23%. Suspended sediment concentration peak will appear during the spring tidal period. The average SSC of middle station is0.25kg/m3for spring tide and0.04kg/m3for neap tide. So the average SSC for spring tide is6.3times of neap tide in middle station. The average SSC of outward station is0.05kg/m3for spring tide and only0.02kg/m3for neap tide, and the SSC for spring tide is2.5times of neap tide for this station. Average median size of suspended sediment of these stations are2.6～5.1μmn, and settling velocity for a single particle suspended sediment is1.7～6.5mm/s.5、As the composition of seabed sediment for Zhujiajian area,the sand accounts8.7%on average, which is the least; and silt accounts to an average of65.5%, which is the most. The particle size tapers from west to east and it didn’t change significantly from north to south.6、The particle size change little in0.8meters’ vertical depth for Z2station area, the frequency curves have no significant double peak phenomenon, which refers to a stable depositional environment generally. And the deposition rate is0.5-0.8mm/a since1950s for Z2area.