Analysis Of Accretion And Erosion Of The Donghai Bridge And Its Adjacent Sea Bed

The Donghai Bridge sea area locates in the north part of Hangzhou Bay mouth. In this sea area, the South Passage of the Changjiang estuary is to the northern, Qiqu Archipelago is to the southern, the sea area between Shensi and Daqushan is to the eastern. The Donghai Bridge sea area locates among bay mouth, estuary and islands, so the flow current and suspended condition here is comparatively complex. Recently, the quick decrease of the SSC from the Changjiang river have affected the flow and SSC conditions, the topography of the area along the Donghai Bridge. After the beginning of this century, many big projects have been built here, this causes much changes of the boundary condition, and so the conditions of water flow and SSC, these changes certainly will causes more or less the changes of topography. In this paper, we first analyze the characteristics of current, suspended sediment, surface bed sediment and topography, and then discuss the topography changes under the background of the quick decrease of the SSC from the Changjiang river and the building of many big projects, and we also predict the topography change in future, at last we draw the flowing main conclusions:(1) The tidal current of the area along and around the Donghai bridge is controlled by the East China Sea tidal wave. The characteristics of the current velocity's distribution is: the middle part of the area along the Donghai bridge is low current velocity area, its current velocity is less than that of the west and east part along the Donghai bridge; The characteristics of the SSC's distribution is: because of the great affects of tidal current and wave, the north part of the area along the Donghai bridge and the area nearby Qiqu Archipelago are high SSC area, their SSC are greater than that of the middle part of this area, so the SSC's saddle-shape distribution of "North and South part is high, Middle area is low" is formed. Both the distribution of current velocity and SSC will affect the topography change of this area.(2) The topography of the area along and around the Donghai bridge experienced alternately siltation-erosion changes after the year 1959-1960. The topography of that area was mainly silted between the years 1960 and 1989,while it was mainlyeroded between the years 1989 and 2002, then it was both silted and eroded between 2003 and 2005. The above topography changes have evidently reflected the effects of the decrease of the SSC from the Changjiang river and the building of many big projects nearby.(3) The effect of the building of many big projects nearby is the dominating factor which affected the topography of the area along and around the Donghai bridge in recent years, and in which, the extensive reclamation of the shoal above 0～-2m is the most important factor of all. The effect of coastal engineering under the condition of the duratively decreasing of the SSC from the Changjiang river is like this: the holding up and digging up of large amount of suspended sediment by reclamation is the most direct and forceful factor. The current changes caused by the building of the Donghai bridge is also an important but just local factor.(4) The decreasing of the SSC from the Changjiang river is the durative and macroscopical factor which affects the topography of the research area; while the building of many big projects nearby is the local and short term factor. After the building of many big projects, the topography of the research area will come to a relative steady state under the present tidal current and SSC condition, if the boundary has no great change.