| With the rapid growth of economy in Shanghai, the demand in port depth increased. Because of Shanghai's disadvantage of natural conditions, the Yangshan deepwater port was built in Qiqu archipelago where lay between Shanghai and Hangzhou Bay with its water depth above 15m. The Yangshan deep water projects contained three fork channel closure projects and throw-filling land making project from Jun.2002 to Oct.2006. Because it had become a regular coast with 5.6km long, the north island chains boundary and hydrodynamic and sediment conditions changed which led to the variation of natural seabed in siltation and erosion and topographical pattern. During construction stage, each individual fork channel closure project resulted in siltation at respective sea area witch brought adverse effect on port depth at Yangshan sea area. It is very important to study the effects of changes in hydrodynamic and sediment conditions on Yangshan seabed siltation and erosion.Based on a mass of hydrological sediment and terrain survey datum, from the point of view on hydrodynamic condition, suspend sediment and siltation and erosion of seabed, the evolution law of Yangshan sea area is analyzed by the Yangshan deepwater port projects.The results show that (1) the Yangshan deepwater port projects weakened tidal dynamics at wharf apron waters with its average ebb velocity decreasing by 32% but increased it at the main channel area with its average ebb velocity growing by 13%. Channel Kezhushan- Xiaoyangshan became an important channel for draining and desilting, which meant the flood advantage reinforced by 30% at left end of it while the ebb advantage increased at its right by 4%;(2) The proportion of flood and ebb volume both went up 5 percentage points at main channel area which indicated the tidal dynamics enhanced after throw-filling land making project. The proportion of ebb volume was 2 percentage points higher while the proportion of flood one was 2 percentage points lower at Channel Kezhushan-Xiaoyangshan, so there was an ebb advantage at Channel Kezhushan-Xiaoyangshan. The tidal dynamics declined at southwest of main channel area with its flood and ebb volume both increased 4 percentage points;(3) Annual average suspend sediment concentration decreased by 4% at Yangshan sea area from 1998 to 2007 after Yangshan deepwater port projects, and monthly average suspend sediment concentration remained higher in winter but lower in summer. Besides that it kept higher at west and south Yangshan port sea area but lower at east and north, the suspend sediment concentration raised greatly by 155% at middle part of Yangshan port sea area;(4) During construction period, each individual fork channel closure project led to deposition. The first and second phase wharf waters deposited after Xiao yangshan-Huogaitang fork channel closure project. The channel Kezhushan sea area deposited after Dawugui-Kezhushan fork channel closure project. South of main channel area deposited at Yangshan sea area after Dawugui-Kezhushan fork channel closure project;(5) The Yangshan deep water port projects made an important impact in seabed siltation and erosion after changes of hydrodynamic and sediment conditions. When it is at high and low tide slack, week current points and interval, vertical circular and eddy current became more at wharf apron waters, and horizontal circulation occurred at main channel area. In such a case, weak current became longer and larger at wharf apron waters, south Yangshan sea area and north Dashantang, which brought deposition near fork channel. The tidal dynamics became stronger at main channel area after Yangshan deep water port projects. Channel Kezhushan-xiaoyangshan proved to be the only channel for draining and desilting at north Yangshan sea area. Because the ebb current could carry the sediment out of it, Channel Kezhushan-xiaoyangshan caused erosion at north yanshan sea area.
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