| The Dongting Lake is the second largest freshwater lake in China, located in the south bank of the middle reaches of the Yangtze River. It is not only an important China production base of commodity grain, fish, cotton and hemp, but also one of the regions suffered the worst floods in the history in the middle&lower reaches of the Yangtze River. After operation of the Three Gorges Project, the flood control in the middle&lower reaches of the Yangtze River has been greatly improved. So does it in Jingjiang and Dongting Lake areas. But when the flood of Dongting Lake comes from Xiang River, Zi River, Yuan River and Li River, the Three Gorges Reservoir is unable to play the role of flood control in Dongting Lake area and its flood control situation is still very serious. The Lake area is featured with large river and lake covering, complex river networks, connected parts between lakes and rivers, and uncertain flow directions. Also, there are various hydraulic structures to control flood, which makes Dongting Lake area become a complex flood control system, including rivers, lakes, reservoirs, flood diversion&storage area, and hydraulic structures.In order to accurately evaluate impacts of the water exchange among rivers, lakes and other basic elements in flood control system, the paper firstly analyzes the flow characteristics of the basic elements in flood control, and then presents the general structure of numerical model in such complex system. Combined flood diversion area model and hydraulic structures operation model in MIKE, a new numerical model named complex flood control system model was developed to achieve global flood control simulation with appropriate information transfer among basic elements, including flood routing of river, flood control of lakes and reservoirs, flood storage of flood diversion&storage area, and flood control of hydraulic structures.On the other hand, according to the complexity of the flow and the simulation requirements in the system, the new model expands the simulation scope to Datong station, which covers Yangtze River, Dongting Lake area, Poyang Lake area, and the main branches in the middle&lower reaches of the Yangtze River. In the process of parameter calibration, initial bed roughness is set according to different river reach, and the bed roughness of specific river reach in various water levers according to its cross-section composition is also set. Overall calibration of roughness is processing with slightly adjusting local bed roughness until the calculated values of water level and discharge meet required accuracy. The agreement between the calculated and measured flood process is identified by calculating the Nash-Sutcliffe and deterministic coefficient. The model’s accuracy is evaluated based on the relative error between the calculated and measured flood peaks and the absolute error between the calculated and measured highest water levels. The test results indicate that the model is capable of simulating flood process and its peak with good accuracy.On the analysis of flood formation and transformation, the new model is employed to simulate the water level-discharge relation in Jingjiang and Dongting Lake areas in the following three cases:no storage regulation of the Three Gorges, operation of the Three Gorges Reservoir considering the Jingjiang River flood control, and operation of the Three Gorges Reservoir considering the Chenglingji flood control, which all encountered the flood in1954and1998respectively. According to the simulated results, the flood controls in Jiangjiang and Dongting Lake areas are assessed, and the distribution of extra flood discharge is calculated in the middle&lower reaches of the Yangtze River. The flood control after taking diverting flood in each diversion area is also assessed in the two areas.Quantitative analysis of Dongting Lake’s flood control after the operation of the Three Gorges Dam indicates key zones of the frequent flooding in the Lake and its formation. Effective measures were suggested based on the analysis and requirements of flood control. The calculation using the new model quantitatively evaluates that the suggested measures are able to reduce flood risk to the Lake. This study introduces two measures:the joint operation reservoir system and flood peak alteration using gates, and recommends flood peak alteration schemes for the Songzi Gates, and also analyzes effects from joint operation of Jingya and Zaoshi reservoirs and effects from Songzi Gates’ alteration. Using the model, quantitative studies are conducted on effects from flood control in Jingjiang and Dongting Lake areas after adopting the proposed measures, and risk to Dongting Lake without changing Jinjiang flood control schemes.Simulation results indicate that the joint operation of Jiangya and Zaoshi reservoirs can effectively reduce the flood peak for the flooding from branches of the Sishui River, but it fails to control the peak flow under the safety discharge for the flooding from the Sishui River. The flood stage in the Songsi Area can be effectively reduced after joint operation of the two reservoirs. Since the flooding peak in this area is mainly from the Sishui River, the flood stage in the area is still above the safety stage even after Three Gorges Dam’s operation. In case of the flood peak in Songsi area above control discharge, implementation of flood peak alteration using the Songzi Gates can effectively alter the flood discharge from the Yangtze River to Songzi River, and reduce the flood stage in the area. But after taking joint operation of the two reservoirs, the flood peak is still beyond safety discharge even though no water flows through the Songzi Gates. On the other hand, peak alteration using the Songzi Gates can increase the water lever in Jingjiang river reach a little. But the increased highest water lever is still below the safety stage at Shashi station. Therefore, implementations of both dredging the Songzi River and altering flood peak with Songzi Gates can ensure the flood control in the Songzi area without changing Jingjiang flood control schemes. |
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