Operational Technologies And Practice For Three Gorges Reservoir

The preliminary design of the Three Gorges Reservoir (TGR) stipulates that the reservoir shall be capable of ensuring the safety of the Jingjiang River reach by flood retaining operations in the case of large floods of the Yangtze River and specifically when the flow measured at Zhicheng Station exceeds56700m3/s. Whilst in the cases of medium or small floods and when the flow at Zhicheng is lower than56700m3/s, no flood retaining operation would be necessary by TGR. In the past decade, it has become possible to make the maximum utilization of the reservoir thanks to improvement in hydro-meteorological forecasting technologies. Under the circumstances that there will be no large floods in the forecast period, the TGR can start retaining the medium and/or small floods in a proper way to reduce water spills, which will not impose significant impact on the flood storage capacity reserved for future big floods, and at the same time keep the downstream below the warning water levels. In order to increase the possibility of full storage of the TGR and thus maximize its multiple benefits, the starting date of refilling water during post flooding season is set to be October1according to the preliminary design, but the optimized operation rules as approved by the State Council brings the date forward to September15. There has been a tendency of decreasing inflow in September in recent years, but in the same period water demand downstream has seen a big increase compared to that projected in the preliminary design. In such cases, meeting downstream water demands will compromise the possibility of full impoundment of the reservoir, as demonstrated by the reality experienced in2009. In the light of the present inflow variations in September and October as well as water demands downstream during the period of reservoir refilling, the starting date for reservoir refilling for the year2010was further brought forward to September10. Consequently, medium and small floods may raise the reservoir water levels, and earlier refilling may also increase flood control risk and create sedimentation problems. Real-time operation of the reservoir must meet the premises of flood control safety and allowed level of sedimentation and risks, and integrate a wide range of considerations and demands. This thesis presents a summary of progress in utilizing medium and small floods and their dynamic scheduling, sedimentation operation and risk analysis, and addresses such critical issues of TGR operation as utilization of medium and small flood inflows, reservoir refilling operations, as well as sedimentation reduction operation, with main findings presented as follows:(1) The hydro-meteorological forecasting methods and its applications in the Yangtze River basin are introduced. The accuracies of the Yangtze River basin rainfall forecasts and TGR inflow forecasts issued in recent years are statistically analyzed. The mean relative errors of forecast reservoir inflows (with actual flow measured at Yichang Station) since2003for lead time1～3d are4.7%.6.9%and8.5%, respectively. As flood flows above35000m3/s will trigger consideration of taking flood safety actions, statistical analysis of flow forecast errors is done for flow levels up to35,000m3/s and55000m3/s. Exceptional analysis is conducted for the cases of forecast values smaller than measured ones, which represent the unfavorable situation for flood control. Results indicate that the current forecast accuracy is fairly high, and such forecasts are reliable basis for flood operation of the reservoir.(2) A dynamic control and scheduling method and prerequisites for medium and small flood, together with procedures and risk control measures are proposed. Based on flood characteristics, flood period from June10to September10is divided into4phases, and for each phase the indexes controlling medium and small floods scheduling are identified in light of the constraints for flood control and capacity for forecasting and pre-releasing. Risk control model is employed to simulate the reservoir scheduling process given the typical medium and small floods. Simulation results show that the actual scheduling on middle and small floods in recent years is reasonable and practicable, and has achieved significant benefits in flood control, power generation etc. Increasing sediment flushing from the reservoir is also investigated in conjunction with scheduling medium and small floods.(3) Long series of hydrological observations are used to study the full-reservoir possibility under different reservoir refilling schemes during post flood season. If the starting level of the reservoir is set to155m, and controlled at165m by the end of September, the full-reservoir possibility will reach96%toward the end of October, and98%by November. Further raising the reservoir levels will not be significant to increase the full storage possibility of the reservoir. Flood risk modeling is developed for post flood season period, showing that if the reservoir water level is controlled below170m at the end of September, the risk in flood control is basically minimal.(4) Utilization of medium and small floods and earlier refilling of the reservoir will inevitably raise the water levels in the reservoir and increase sedimentation especially in the locality of the reservoir tail. The sedimentation in turn has impact on flood control and navigation. The thesis presents analysis on water-sediment characteristics, volume and distribution of sedimentation at the reservoir tail during falling period. A collection of sedimentation reduction scheduling schemes are worked out for the reservoir tail (Fuling-TongluoXia river reach) under different scenarios of inflow and reservoir level with the hydraulic conditions and sediment starting flow velocity derived from mathematical modeling. This provides a useful guide for testing sedimentation reduction operation during falling period, and the preliminary application demonstrates good effect in sedimentation reduction.