江东大桥建设对钱塘江水动力的影响

应用二维水流数学模型模拟了江东大桥建设前后大范围的流场,比较了建桥前后水位、流速、流向等水动力因子的变化,评价了大桥建设对工程区域水动力的影响,并与物理模型试验的结果进行对比,分析了结果的合理性。研究成果为大桥的设计和管理提供了技术依据。

Impacts of Jiangdong Bridge construction on hydrodynamic conditions in the Qiantangjiang River Estuary

The river section where the Jiangdong Bridge is located was influenced by both the tide and runoff.At present standard seawalls have been built up at both sides of the river,thus the plain movement of main channel will be limited between the banks.Analysis of the topography data available now shows that main channel near Jiangdong Bridge was mainly to the northern of the bridge whose minimum altitude is-2.85 m corresponding to maximum one 2.11 m,with a riverbed change by scour and deposition up to 5 m.Using unstructured triangular grids based on the finite element method,a plane 2-D mathematical current model was set up in the reaches from Wenjiayan to Ganpu.Local denser grids were used so as to simulate bridge piers one by one.The whole computational zone includes 18 666 triangular grids and 9 789 nodes,with the minimum mesh size of 2 m.The calculation interval is 0.5 second and the block-water area ratio of the lower piers at medium tidal level is about 5%.Based on the verification using field data,the flow fields before and after building the bridge were simulated under two hydrographic conditions of tidal range and flood flux of 1% frequency rate.Variations of the hydrodynamic factors such as the flood and tidal level,and the velocity and direction of the flow before and after the construction of the bridge were analyzed.The impact of bridge construction on the hydrodynamic conditions of the project region was assessed and compare with the results of physical model test under the same hydrodynamic conditions.The results show that under hydrographic conditions of flood flux with 1% frequency rate,flood water level at the upper reaches of the bridge increased but decreased at the lower reaches.Within 200 m from the bridge at the upper reaches it increased by about 5 cm and the farther away from the bridge the less the increase.The influence of the construction on the water flow was mainly in the area near the bridge.The flood velocity in the main span increased about 4.9%~8.1% which is corresponding to 0.3 m/s.The flood flow direction changes 1 degree within 500 m around the bridge.After the bridge construction the high tidal level at the lower reaches of the bridge was lifted while that at the upper reaches was lowered and it is quite the contrary for the low tidal level.Under hydrograph condition of tidal range of 1% frequency rate,high tidal level about 200 m away from the bridge at the lower reaches was 5 cm higher and the influence zone reached Yanguan,while it was 6 cm lower at the upper reaches 200 m away from the bridge and the influence zone reached Qibao.The low tidal level increased by 3 cm at the upper reaches and decreased by 3 cm at the lower reaches and the influence zone was from Laoyancang to Qibao.The velocity changes caused by the bridge construction occurred near the bridge.The water flow converged between piers and the velocity of flood tide and ebb tide increased by 4.6%~6.2% and 3.7%~5.7% separately.They decreased by less than 5% 1.5 km away from the bridge axis and no changes farther than 5 km.The experimental results of mathematical model and physical model were qualitatively consistent and quantitatively close to each other,which provides evidence for each other and also the technology basis for the design and management of the bridge.