The Effect Of Construction Of Yangshan Deepwater Harbor On Morphological Change And Mechanism Analysis

Yangshan Deepwater Harbor, located in the Qiqu Archipelago adjecnt to Hangzhou Bay, is the new deepwater harbor of the Port of Shanghai. The study area features a series of islands with complex hydrodynamics and high sediment concentrations。Its construction since 2002 entails three types of engineering projects:closing a series of inlets, i.e., inlet between Xiaoyangshan and Huogaitang Island, inlet between Dawugui and Kezhushan Island and inlet between Jiangjunmao and Dazhitou Island; land reclamation and dredging. After the construction of 1,2 and 3, these engineering projects caused a series of morphological changes in the area. Because of the serious sedimentation in the harbor area further construction of the planned harbors has been stopped for the time being. Research on the sedimentation and its causes is urgently needed for the decision concerning further construction of more harbors.In this paper we analyze the morphological changes in the harbor area using bathymetric data collected from 1998 to 2010. Especially since 2004 bathymetry in the area has been surveyed every year, making it possible to analyze the spatial and temporal variation of sedimentation-erosion in detail. The analyses provide a good insight into how the morphological changes are related to the various projects of the harbor construction, and how the changes develop in time. It is shown that in the year immediately after an inlet was closed accretion occurred on both sides of the closure. The sedimentation rates decrease significantly in the following years. The SE zone of the inner harbor area continuously accreted. Accretion accelerated from 2007 to 2009 but decreased thereafter. The results of the analyses indicate that routine dredging will be necessary to maintain the requisite 15-m depth requirement for berths, but a regular dredging routine is feasible.It is shown that suspended sediment concentration (SSC) is maximum at ebb slack during spring based on observed data. The obvious difference exists between water and suspended sediment residual transport, and it is more accurate and reasonable that calculating residual sediment transport directly. Euler transport is predominant role of water long-period transport, horizontal advection and tidal pumping take primary role in suspended sediment transport in the study area, the contribution of vertical transport is small due to strong mixed tide.The net suspended sediment transport for several stations in middle and south inner harbor area are oriented to north of Dashantang Island in 2006, which may be one of the reasons of deposition.It is significant for evaluating trend of harbor morphological change and planned harbor layout by measuring and calculating water and sediment fluxes of key transects, after the pause time of engineering and a longer time of adaption and adjustment of the harbor area itself. In this paper, the instruments of ADCP (Acoustic Doppler Current Profile) and OBS (Optical Backward Scatter) were applied to the five key cross-sections (CS) in the Yangshan Harbor to observe profiles of water depths, flow velocities and SSCs, respectively. Based on the flux conservation principle, the fluxes of water and sediment were calculated by the temporal and spatial interpolation on a horizontal-uniform-and-vertical-varying mesh. The net flux for sediment moving into the harbor area is about 54 400m3/day in August,2008. This value can be taken as the daily-averaged net sediment transportation rate for flood seasons, and based upon which, the net inflow sediment for a flood season (three months) is about 489 400m3, and the harbor area is in the state of deposition locally. The Kezhushan inlet is the main outflow passage for sediments, through which about 231 300m3 of sediments move out of the harbor in a flood season.For a better understanding of the mechanisms responsible for the observed morphological changes a detailed Yangshan waters hydrodynamic and suspended sediment model is developed based on Delft3D. The detailed model can simulate the morphological change induced by inlet closures basically after verified bed level changes induced by the three inlet closures based on parameters calibration and verification. The model results provide insights into how the morphological changes are related to the changes in hydrodynamics induced by the closures of the inlets. The paper predicts morphological change induced by planned inlet closure. Not only deposition belts occur near the inlet closure, but also large areas of deposition occur in western part of the inner harbor if the south two inlets are closed.