Numerical Study Of Effects Of A Bay Bridge On Tidal Flow And Sediment Transport In Xiangshan Bay

The Xiangshan Bay, located in northern Zhejiang province, is a typicalsemi-closed bay. It is well protected by a number of islands at the bay mouth.There is no big river flowing into the bay and catchments area of the bay isalso small. Sediment supply from the land is thus limited. Although a smallamount of the suspended load from Changjiang river mouth would betransported to the bay and finally settled down, sedimentary processes in thebay is relatively intensive. Consequently, the bottom topography and thecoastline of the bay are fairly stable.This study is to investigate the effects of the Xiangshan Bay Bridge that is underplanning on the stability of the bottom topography, the main channel of flow and thecoastline of Xiangshan Bay. There are two routes, Xiaoweizhuang route and Qinglairoute, and three kinds of designs, namely, cable-stayed with two towers, cable-stayedwith three towers, and cable-suspension with three towers, for comparison at thefeasibility study stage.Tidal current is considered as the main factor that causes sediment transport andthus seabed evolution in the Xiangshan Bay. FESWMS and RMA2 are employed inthis study. Both of these models are based on shallow water assumptions. Theirhydrodynamic module can be used to compute the water surface elevation andvertically mean velocity. Mean concentration of the sediment is solved from adiffusion-convection equation with source term. Temporal variation of the seabed isobtained considering the conservation of sediment mass. Computational results arecompared with measured data at several stations in the site, and the agreement isshown to be very good.Effects of the bridge on the tidal flow are evaluated by adding the resistance ofthe bridge piers to slow into the numerical model. Based on such disturbed flow,seabed evolution and local scour are predicted. Numerical results show that the Xiaoweizhuang route is slightly better thanQinglai route;cable-stayed with two towers is the preferred design, if consideringonly the effects of bridge on tidal flow and its resulting sedimentary processes. Theinfluence of the bridge on the flow is found to be limited within an area of 4 km awayfrom the bridge location. Change of the tidal flow velocity due to construction of thebridge is no larger than 5%. The flow direction keeps almost unchanged, and thelargest deviation is smaller than 1.5°. After the construction of the bridge, the tidalvelocity in the main channel of the bay is strengthened, the high tidal level slightlydecreases while the low tidal level slightly increases, and the maximal change of thetide level is less than 3cm. The largest scour depth of the cable-stayed bridge withdouble towers bridge is found to be 12.39m, and that of the cable-stayed bridge withthree towers be 11.21m.