金塘大桥桥轴线优化及建桥对水流和航道的影响

Optimization of the Jintang Bridge's axis and the influence of the bridge on the water flow and navigation channel

金塘大桥跨越灰鳖洋，连接金塘岛沥港镇与宁波镇海新泓口，全长18．5km，为舟山连岛工程的主体。该大桥工程所在海域潮汐为非正规浅海半日潮，历年最大湖差为3．67m，多年平均潮差为1．91m；潮流呈往复流特征，且基本平行于峡道走向。金塘大桥水域东侧微冲，西侧微淤，深泓摆幅较小，深槽稳定。在海床演变分析的基础上，运用数学模型进行了桥轴线的优化，由实体定床与动床模型探讨了建桥对水动力的影响。数学模型选用2002年3月和11月实测水文测验资料对模型进行率定和验证。物理模型选取中值粒径为0．2mm的塑料沙作为动床试验的模型沙，选用2002年11月实测水文测验资料对定床模型进行验证，选用2002年11月和2003年4月两次水下地形资料对动床模型进行河床冲淤验证，验证精度均满足相关规范要求。研究结果表明，以推荐的方案建桥对桥位近区的湖位、流速与潮量的影响均较小。建桥后，下游高潮位有所抬高，上游高潮位有所降低；上游低潮位有所抬高，下游低潮位略有降低。由于桥墩的阻水作用，使该桥所在海域海流流速整体上略有减小，减小量一般在5％以内；桥墩间因海流集中使流速有所增加。建桥使主航道航深有所增加，桥位近区与远区分别冲刷0．6～0．8m和0．1～0．2m；西航道桥轴线近区冲刷0．4～0．5m，远区淤积0．1～0．2m。建桥对甬江口、金塘锚地、七里屿锚地均无明显的不良影响，对北仑港区基本没有影响。

The Jintang Major Bridge from Ligang, Jintang Island to Xinhongkou, Zhenhai is 18.5 km in length and it is the main body to link islands of Zhoushan Archipelago. The tide in the project area is shallow sea irregular semidiurnal tide with annual-mean and maximum tidal range of 1.91m and 3.67 m respectively. The tidal current is of reversing characteristics running parallel to the flume. There are micro-deposition on the west and micro-erosion on the east of the bridge and the deep channel is stable with small oscillation. Based on the analyses of seabed evolution, the bridge axis was optimized by using mathematical model and the influence of the bridge the hydrodynamics of the area is studied with physical model tests including fixed bed and movable bed models. The simulated area includes the whole Jintang waterway and Cezi waterway to the south of Haiwangshan to Dayushan and the finite element method of explicit current drift with minimal space step length of 15 m and time step length of 3 s is adopted. The model was calibrated and verified with in situ hydrologic measurements in March and November 2002. The principles of bridge axis optimization is to make each bridge section as orthogonal as possible with the main current during flood peak and ebb limit so as to minimize the impact of the bridge on the hydrodynamics such as tidal level, current speed and direction, and tidal discharge as well as the near by ports and channels. Horizontal scale 2.l =800 and vertical scale λh = 125 are chosen in the physical model and simulation water area is 580 km2. The fixed bed model was verified with in situ hydrologic measurements in November, 2002, the plastic bead with median particle diameter d50 = 0.2 mm was selected as the model sand and the erosion and deposition was verified with underwater topographic maps of November 2002 and April 2003. The test indicates that the influence of the Jintang Bridge on the tidal level, the velocity and the tidal volume is small and it occurs near the bridge. The high tidal level will increase a little downstream and decrease upstream of the bridge and vice versa for the low-tidal level. Because of the resistance of the piers to the water, the velocity will decrease by less than 5 %, while that between two piers will increase because of the concentration of the water. After constructing the bridge, erosion near the bridge will be about 0.6-0.8 m and 0.1-0.2 m farther away from the bridge. And there will be an erosion of about 0.4-0.5 m near the western channel and deposition of 0. 14-0.2 m farther away. The construction of the bridge has no obvious negative influence on the Yongjiang River Estuary, the Jintang roadstead and the Qiliyu roadstead and no influence on the Beilun harbor in essence.