Study On The Risk Evaluation Of The Reservoir Flood Control And The Influence Of The Changes In Hydrological Series

Floods disaster，are one of the most serious forms of natural disasters in the worldoccurring with increasing frequency and scale in China. Reservoirs can storefloodwater and cut down the rate of peak flow of the flood. They are frequently builton the upstream of protected areas to promote the beneficial effects while minimizingthe loss. However, flood sequences have changed due to global warming and theeffects of human activities. The operational rules of reservoirs no longer adapts to theoccurrence regularity of floods. To reduce losses resulting from flood disastersthrough engineering measures once the emergency event like catastrophic flood andearthquake occurs, Reservoirs will be unsafe and may even burst their banks whichwill not only threaten lives but also public security, economic development andenvironmental protection. Taking the Ziya river system of Haihe River Basin as anexample, the flood encounter risk of the main stream and tributary has been analyzed;design floods of the main stream and tributary were modified. The methods forreservoirs emergency regulation was studied for reducing the flood losses when oneof the cascade reservoirs is in danger. The main research contents and conclusions areas follows:⑴Peak over threshold model(POT) was used to calculate the flood frequency.Under the hypothesis that the annual occurrence number of the partial duration floodseries obey the Poisson distribution, the partial duration flood series estimated by thegeneralized Pareto distribution (GPD). The calculation of the peak flow frequency ofXiaojue station showed that, there were some slight differences in the design floodpeak between the GPD and the P-Ⅲ distribution, but the differences were all in theerror range, the POT model could be applied in the flood frequency analysis of theHaihe River Basin.⑵The Von Mises distribution was applied to estimate the flood occurrence timeseries and the flood occurrence interval series for the Xiaojue and Pingshan stations.The3-D Copula function was used to establish the joint distribution of the floodoccurrence time interval and the frequency. The flood encounter risk of the mainstream and tributary for the Gangnan and Huangbizhuang cascade reservoirs showthat, under the condition of same scale of the main stream and tributary floods, theencounter risk of flood peaks, maximum3d flood volumes, maximum6d flood volumes increase gradually. When both the return periods of the main stream andtributary flood volume are less than5years, or one of them is less than5years andthe other is in the range of5~20years, the encounter risks of flood volumes are higher;while, for the case in which the return periods of the main stream and tributary floodvolume are both more than50years，the risks are quite lower. The risk and the lossesof the flood encounter should be reduced by the operation of the cascade reservoirs.The studies of the flood encounter could give scientific points for making the floodcontrol operation decision of the cascade reservoirs and for flood control planning of acertain river basin.⑶Based on composition and resolution of the time series, the design floods ofthe main stream and tributary were corrected through the statistical method under theenvironment changes. Through the comprehensive analysis of variation, the floodseries of Xiaojue and Pingshan stations were changed in1982and1987respectively.The design flood correction results showed that: the mean and the design values of thepeak, maximum3d volumes and maximum6d volumes of the flood are all smallerthan the previous values. The changing of the design flood will reduce the risk of thereservoir, but the water resource crisis will get even more serious.⑷Seasonal AR (1) model was used to simulate the Gangnan and Huangbizhuangreservoirs inflow flood before and after the design flood modified. Risks are bothevaluated in the case of Gangnan reservoir dam is in danger and Huangbizhuangreservoir is in danger. The results show that by applying the appropriate emergencyoperation models, the probability of dam overtopping caused by unexpected accidentis low which indicates that emergency operation is one of the effectivenon-engineering measures to reduce flood disaster loss. Since large populations and alot of engineering structures are located in the down stream of reservoirs, thevulnerabilities of cascade reservoirs’ flood overtopping are high, and effectiveemergency operation models can reduce flood damage in a large degree. The risk ofthe cascade reservoirs’ flood overtopping was reduced in different degrees, whichindicates the trends of the flood series could decrease the risk of the dam overtopping.