Study On Calculation Method Of Critical Rainfall Of Flash Flood Based On Basin Hydrological Model

Flash flood is normal in China and warning of flash flood is a method to reduce the loss.The warning method of critical rainfall is one of the common warning methods.The research purpose of critical rainfall calculation is to make the calculation result as close as possible to the"true value"of critical rainfall.This paper mainly focuses on the influences of rainfall hyetographs and the unit hydrographs on the calculations of critical rainfall.Based on the Xin'anjiang hydrological model and the historical rainfall and flood data of Tanjiahe Basin,this paper presents a calculation method which makes the result of critical rainfall closer to the"true value",which makes the warning results of critical rainfall more safe and reliable.The research contents and results are as follows,(1)Influence of unit hydrographs on calculation of critical rainfall.This paper builds Xin'anjiang hydrological model with three water sources and ten historical floods in the Tanjiahe River were used to calibrate the model.The calibration results are as follows:the time-variant unit hydrograph is the best among all unit hydrographs,the relative errors of runoff and flood peak are both within 20%within the allowable range,the average peak time difference is 0.298hours and the average certainty coefficient is 0.954.By analyzing the minimum net rainfall intensity unit hydrograph's time base and historical rainfall data,the maximum calculation period of critical rainfall in Tanjiahe River Basin is chosen to be 7 hours.By the P-III frequency curve calculation of the historical flood,the critical discharge of the Tanjiahe River Basin is determined to be 350m~3/s.The selection of unit hydrograph has a significant effect on the calculation results of critical rainfall.For the time-invariant unit hydrographs,the critical rainfall decreases with the increase of the net rainfall intensity.For the time-variant unit hydrograph,the model adopts different unit hydrograph with the variation of antecedent rainfall.The critical rainfall decreases with the increase of the antecedent rainfall and when the antecedent rainfall increases to a certain extent,the influence on the calculation of the critical rainfall weakens.(2)Influence of rainfall hyetographs on calculation of critical rainfall.For linear increasing,linear decreasing and uniform rainfall hyetographs,no matter what the value of the antecedent rainfall is,no matter which unit hydrograph is used,the critical rainfall calculated by the linear increasing rainfall hyetograph is the minimum therefore the linear increasing rainfall is unfavorable.On the basis of increasing rainfall hyetograph,exponential increasing rainfall hyetograph is more consistent with the natural situation and the rainfall type in the basin,it needs further studied.Through the comparative analysis of rainfall-runoff simulation,it is found that the upconcave exponential increasing rainfall hyetograph is more unfavorable than the linear and upward-convex exponential increasing rainfall hyetographs and combined with the historical rainfall data of the basin,the rainfall hyetographs in different periods of the basin was determined through a large number of data screening,data processing,collocation and fitting of plots and integral calculation.(3)Calculation and application of critical rainfall.A combination of linear increasing hyetograph,uniform rainfall hyetograph and time-invariant unit hydrographs,time-variant unit hydrograph was adopted to compute the critical rainfall of 5 hours.The calculations are used to simulate early warning for three historical floods and it can be seen that only the combination of linear increasing hyetograph and time-variant unit hydrograph has no false alarm or missed alarm.Based on the Xin'anjiang model,the warning effect of 12 historical floods in the basin is analyzed by using the critical rainfall curves calculated by the linear increasing rainfall hyetograph and exponential increasing rainfall hyetograph.The warning results are as fllow,POD called hit rate is 0.91,FAR called flase alarm rate is 0,CSI is 0.91,the warning results of two critical rainfall curves are good.By comparative analysis,using exponential creasing critical rainfall curve can warning forward about 0.21 hours per flood than using linear creasing rainfall curve,so it is safer to use the exponential increasing critical rainfall curve for flash flood warning.There are 23 figures,12 tables and 104 references in this thesis.