| Mass transport is an important and basic topic in study of Ocean-environment problems, especially in nearshore areas. Mass transport itself covers two distinct layers of significations. On one hand, mass transport not only means the transports of sediment, pollutants and dissolved particles, but also does to temperature, salinity and density. On the other hand, the points we eager to know are not each tache in its processes likes movement of mass positions or quantity of transports etc, but to exposes the impacts of processes on remodel of landform, diffuse of contamination, water quality, and hydrodynamic factors and circumfluence. Each movement, in course of mass transport, should base on a kind of momentum, that is hydrodynamics. It is an indispensable factor to the origin of mass transport and its processes and, be effected by their behavior nevertheless. Tidal current plays the most importance role in the course of mass transport, and residual current also has its contribution to longtime transporting and various forms of distribution.Zhoushan Archipelago (ZSA) is the topmost one in its size of the archipelagoes in our country. It is located in East China Sea (ECS), continuous with Changjiang (Yangtze) River Estuary and linked the Hangzhou Bay (HZB) together. ZSA, enchased by a large number of islands in shapes, has its vast scale of sea area and much too complicated condition in hydrodynamics. Its sea area, among islands and islets, is the main flow path to the south for Changjiang Diluted Water (CDW), and also the route to HZB for its water exchange with ECS. HZB is a typical sea area by its strong tidal and, accordingly has singularly strong tidal currents in the world. From points of view mentioned above, the hydrodynamics condition in area cared about affected by many factors such as CDW, tidal currents from ECS and complicated landform etc, has its tarnal particularity.Based on induction and summing-up of predecessor's productions for tidal currents field in ZSA sea area, by analyses the field survey data, the comprehensive study show that shallow water tides such as M4 and MS4 has remarkable impact on hydrodynamic condition. The tidal style in the sea area presents anomalous semi-diurnal shallow water tidal currents. The main movement modality of the currents in this area put up to be a kind of to-and-fro, and the trend of this trait weakened gradually from nearshore water to offshore water, along with strengthen of characteristics of circumrotation. And the primary pattern of circumrotation in the sea area is clockwise, but turns up to right-about in some local region where currents diffracting and overlapping with each other. As a whole, ebb current velocity is bigger than flood current velocity in numerical at both of surface layer (2.0metres below surface) and middle layer, but adverse at bottom layer (1.5metres above seabed). Along with the increase of water depth, the directions of maximal flow velocity of lower layers deflected right relative to that of upper ones, but to the directions of minimum flow velocity in the same layer, things are reverse. This paper takes the flow of Yangtze River and Qiantangjiang and baroclinicity into consideration, and then based on that, simulates three-dimensional hydrodynamic field by using Estuary and Coast Ocean Model (ECOM) in HZB-ZSA sea area. The 3-D model contains a 2.5-order turbulence closure sub-model, and uses a sigma coordinate system in the vertical direction. The comprehensive study show as follows:(1) The phenomena of unequaled of two high-tide or low-tide is so notable in ZSA sea area, and the trend buildup gradually from ECS to HZB. That is also fits the currents but just not as intense as that of tide.(2) The difference of two hing-tide increase by degrees from ECS to HZB, and the difference between two low-tide have a mutative trend of rightabout to the foregoing.(3) high-tide occurred to southeast of ZSA is nearly 1 hour earlier in temporal than the stations among islands, and about 2.5 hours earlier than that of Sheshan and Tanhu station which located at Yangtze River Estuary and HZB. That is to say the tide wave from ECS enters into HZB-ZSA sea area in direction of southeast-northwest.(4) Owing to aggregation and diffusion effect engendered by water channel, to the east of ZSA the flow velocity of flood currents is greater than ebb currents and just opposite to west area.The simulated result has been verified well by the field survey data, thus offers a solidate foundation for further study on residual currents and mass transport.The direction of residual currents in ZSA sea area is towards southeast. Velocity of Euler residual flow at surface layer is about 5~30cm/s, and can reach up to 40cm/s. It appears 3~18cm/s and 2~15cm/s at middle and bottom layer respectively, and is bigger than Lagrange flow velocity at each of the layers. But the complexion turns great changeful due to the complexity of landform in research area.Study shows that the patterns of mass transport and postures of its distributing in the research area are have much more relationships with Lagrange flow than that with Euler flow. The hydrodynamics is so complex as too many factors impact on it in ZSA sea area, and thus, results in a multiformity of patterns of mass transport and distributing in this area.
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