Simulation Of Hydrodynamics Of Yulin River In The Backwater Area Based On EFDC

The Yangtze River Three Gorges project was the largest hydro-junction project in the world today. It play a huge role on the flood control, irrigation, shipping, power and tourism, but may causes some environmental and ecological impact which also is get more and more the attention of people. With the operation of The Three Gorges reservoir, a cyclical storage and release process brought about great changes on hydrological regime for the upstream water storage. Tributaries of Reservoir were completely influenced by backwater and they ecological environment had great changes. Tributaries of Reservoir which The were affected by backwater have appeared different degree of eutrophication and hydrodynamic characteristics was one of the important reasons for the change. In order to understand how hydrodynamic of Tributaries of Reservoir were influenced storage and release process in the Three Gorges Reservoir. This paper takes the Yulin river which located in Chongqing,s Jiangbei district as the research object, and The Yulin river dynamic model was established based on EFDC(Environmental Fluid Dynamics Code) model. The observed hydrological monitoring data were used in the calibration and verification. The main research conclusions were as follows:①In order to get a better simulation results, the Delft3 D software was used to draw orthogonal curvilinear grid for the Yulin river,s hydrodynamic model. The river was too long, so the backwater area of the river was divided into three sections, namely the downstream section was the "Hekou" to the "Paihua", the middle section was "Paihua" to the "Shujia" and the upstream section was the "Shujia" to the "Meixi". Horizonal grids were respectively set to 1820, 2820 and 2070 and the vertical grids were five layers. The three-dimensional underwater topography of the Yulin river was constructed based on the 32 river section and 31 middle thread of stream hydrological datas which obtained from ADCP.②The Yulin river backwater area hydrodynamic model was built by using EFDC and the two years dates were used to simulate the hydrodynamic of the river and for calibration and validation of the model. Comparing the measured dates with simulated results, the average relative error of the velocity and level respectively were 9.20% and 8.90% in the "Hekou". The average relative error of the velocity and level respectively were 18.0% and 3.51% in the "Paihua" and the average relative error of the velocity and level respectively were-1.62% and-13.65% in the "Shujia" and the average relative error of the velocity and level respectively were 21.28% and-17.75% in the "Meixi".③ The Yulin river was influenced by the Three Gorges Reservoir. In mid-September, in the early backwater stage, the Three Gorges Reservoir began filling. The river was located in the rear section of the reservoir area, so there was appear some backwater in the estuarine of river and without the presence of the backwater of in other sections. In the mid-November, namely the middle backwater stage, the "Hekou" to Shujia was in the backwater zone and the top of the backwater zone was located between "Shujia" and "Meixi".In the beginning of December, the late backwater, the backwater area was maximum from the "Meixi" to the "Hekou". The simulation results of "Meixi" section show that the flow field direction was changed from negative to positive on December 5th to 7th and the velocity varied from 0.12 m/s to 0.25 m/s. Simulated results show the beginning of December was the late backwater stage and the end of December was the early water discharge stage. The flow field was mixed and disorderly at this point but the direction began to change to positive and the velocity became highly gradually and varied from 0.17m/s-0.32m/s. In the mid-January, the middle water discharge stage, the flow field direction began to change to positive in large areas of the Yulin river backwater area with the velocity changed high and the velocity varied from 0.24m/s-0.35m/s. At the middle and late of February, the late of water discharge stage, the velocity was all changed to positive and varied from 0.30m/s-0.55m/s. The Yulin river showed normal river hydrodynamic features and The Three Gorges Reservoir had minimize impact on the river and could not affect the river,s hydrology. Until to April, the process of storage and release of The Three Gorges Reservoir was completed.④For the estuarine of Yulin river, its hydrodynamic was simulated particularly. The simulation result show that there was circulation in the horizontal and vertical direction due to the influence of tidal current, runoff, coriolis force, centrifugal force, density and estuarine topography. In the middle backwater stage, the circulation phenomenon is most obvious. In the late backwater stage, the surface velocity direction is positive but the bottom velocity direction is negative in the estuary of the Yulin river in the early and middle backwater stage. In the late backwater stage, the surface and bottom velocity direction were both positive. In the storage period, the surface and bottom velocity respectively were 0.026m/s and 0.076m/s. In the water discharge period, the surface and bottom velocity respectively were 0.026m/s and 0.061m/s.