Sediment Transport And Dynamic Mechanism In The Zhoushan Archipelago Sea Area

Sediment transport processes play an important role in the estuarine and nearshore environment, because the sediments are not only important carriers of various nutrients and pollutants, but also have a stong impact on the transfer and recycling of pollutants. At the same time, sediment transport will significantly influence changes of landform and physiognomy; meanwhile, such changes will correspondingly have great effect on human activity such as port development, seaway maintenance, marine structures construction and laying of submarine cable and so on. Therefore, it is vital to understand the sediment transport and dynamic mechanism in the river mouth and adjacent areas. For one thing, it has important practical significance for human activities such as the development of the port; for another, it has theoretical significance for discussing modern sedimentary processes and spread of pollutants.Zhoushan Archipelago sea area is located in the southeast waters offshore the Changjiang River Mouth and outer edge of the Hangzhou Bay. The study area is not only the only way to the south for Changjiang Diluted Water, but also the main channel for the water exchange between Hangzhou Bay and the East China Sea. It is characteristic of numerous islands, changeful water depth and topography and complicated hydrodynamic conditions. Therefore, the distribution and transport of sediments in the study area must have special properties. Based on the measured data, using different research methods and formulas, sediment distribution and transport characteristics are systematically analyzed, sediment transport mechanisms are revealed, and impacts of Archipelago on the sediment distribution and transport are discussed.Characteristics of ocean current, distribution of suspended sediment and surface sediment are systematically analyzed on the basis of measured data. Results show that the suspended sediment concentration sharply decreases from the Hangzhou Bay to the offshore area. Average suspended sediment concentration of ebb tide is generally higher than that of flood tide. Changes of suspended sediment concentration are significantly affected by fluctuations of water level in a tidal cycle. Impact on the suspended sediment concentration caused by changes of flow velocity is covered up sediment transport induced by fluctuations of water level. Silt is the dominant sediment type which is primarily distributed in the open sea area where it is less affected by Zhoushan Archipelago. Sandy silt, sand and gravel are locally distributed around the islands. Distribution of surface sediment type, grain size parameters and the different components of surface sediments grain size are greatly affected by the Archipelago.Flux, transport direction and mechanism of suspended sediments are analyzed using flux decomposition method. Settlement characteristics are studied based on formula of suspended sediment settling velocity and previous research results. Results show that the suspended sediment flux quickly decreases from the Hangzhou Bay to the offshore area. Although the suspended sediments are primarily transported towards the offshore area, net transport towards the Hangzhou Bay is also found in the study area. The study area is basically located in hydrodynamic balance belt, and the suspended sediment transport is characteristics of reciprocating movement and small amount of net sediment discharge. The net suspended sediment transport is mainly controlled by average current transport, Stokes drift-induced transport and gravitational circulation transport. The advection transport effect gradually weakens and gravitational circulation transport evidently intensifies from Hangzhou Bay to the offshore area. Settling velocity of single particles is far smaller than that of floccules, so, influence of flocculation of suspended sediments must be taken into the consideration.Incipient velocity and transport flux of surface sediments are analyzed using formulas of incipient velocity and transport flux. Transport trend and controlling factors of surface sediments are studied by the method of grain size trend analysis. Results show that surface sediments become more and more difficult to be resuspended from the Hangzhou Bay to the offshore area. The surface sediments are mainly transported from the Hangzhou Bay to the offshore area; meanwhile, the net transport fluxes per day rapidly decrease towards the offshore area. In the area of archipelago, the surface sediments are mainly transported along the trend of channel because of the restriction of archipelago. In the offshore area, there are two obvious sediments transport trends that correspond to the Changjiang Estuary mud area and Zhejiang Fujian coastal mud area. Tidal current is the major dynamic factor dominating in the sediment transport. Meanwhile, the East China Sea Coastal Current, Taiwan Warm Current, wind waves and topography also play important roles in the sediment transport.The impact on the sediment distribution and transport of the archipelago is discussed according to the research results of this article and the reference of theory about point bar of river. Results show that the changes of ocean currents, distribution and transport of sediments are significantly affected by the archipelago that has obvious"archipelago effect". The"archipelago effect"means processes of transport, deposition, resuspension and distribution of sediments controlled by the restriction of the archipelago and changes of hydrodynamic conditions. "Two-way circulation" is dominated by flood/ebb tide because of the existence of the archipelago. It controls the transverse distribution and transport of sediments, meanwhile, the flood/ebb tide masters the lengthways distribution and transport of sediments. The Zhoushan Archipelago has a dual role in the sediments derived from the Changjiang Estuary and the Hangzhou Bay that is"filter"process and"diffuser"process. Coarse particulate matter is filered and fine particulate matter is diffused by the archipelago.