Revision Of Chongqing Rainstorm Intensity Formulas Based On The New Rainstorm Characteristics

The rainfall intensity formula is the important basis for rainwater drainage systemdesign, so the accuracy of the formula poses a great impact on the safety of urbandrainage system. With the speeding of Chongqing urbanization, the rainstormcharacteristics of Chongqing has been greatly changed, which reflects in the significantincrease of single rainfall depth and rainstorm events frequency. The changed rainstrormcharacteristics have resulted in frequent urban flooding disasters, which slow the urbandevelopment for Chongqing. The current rainfall intensity formula for Chongqing hasso many problems, such as the old-dated source data for deriving the formula and lowaccuracy of deriving methods. Thus, it is quite urgent for deriving the new formula forChongqing based on the new rainstorm characteristics.The study collects the continuous data of1980to2009from Meteorology Bureauof Shapingba District in Chongqing; whch is measured by automatic rain gauge. Threesampling methods, annual exceedance method, annual maximum method and partialduration methods, are utilized to screen samples from source data. The samplesscreened by annual maximum method are adjusted by Gumble Distribution Curve; theparameters for Gumble Distribution Curve are solved by Least Squares Method. Thesamples screened by annual exceedance method are adjusted by Gumble DistributionCurve, Pearson-Ⅲ Distribution Curve and Negative Exponential Distribution Curve.The parameters for Negative Exponential Distribution Curve are solved by MomentMethod and Least Squares Method; the ones for Pearson-Ⅲ Distribution Curve aresolved by curve fitting method with Excel and softwares; The parameters for GumbleDistribution Curve are also solved by Least Squares Method. The samples screened bypartial duration method are adjusted by Gumble Distribution Curve, Pearson-ⅢDistribution Curve. The parameters are solved by the same method as adjusting annualexceedance samples. According to the double log curve pattern ofintensity-duration-return period data, the calculation function for rainfall intensityformula is finalized, and the parameters for rainfall intensity formula are solved byGolden Section Method and SPSS software. Among all these derived formulas, thederived one with the smallest error, infinitely close to the measured data, is the bestformula for Chongqing.The study results show that annual exceedance sampling method is better than annual maximum method and partial duration method, whose samples reflects therainstorm characteristics better, and the method is quite simple. In the condition ofannual exceeedance sampling method, the results after being adjusted by NegativeExponential Distribution Curve and Gumble Distribution Curve are better than byPearson-Ⅲ Distribution Curve, which are close to the measured data in the weatherstation. Basically, the solving method with SPSS software is better than Golden SectionMethod, so SPSS software can be widely applied in solving the formula parameters. Bythe comparison of formula fitting errors under different situation, the best formula isachieved by annual exceedance sampling method, Gumble Distribution Curve, GoldenSection Method or SPSS software solving method, the smallest error is0.045, lowerthan the provision of specification. The calculation value of the best formula is slightlybigger than the measured value, which is quite safe as an important design basis.Based on the new characteristics of rainstorm, the newly derived rainfall instensityformula isi=34.543(1+0.616lgP)i=42.794(1+0.623lgP)(t+26.095)0.870or(t+28.585)0.914, the accuracy ofwhich is greatly improved comparing with the current formula. Therefore, the formulacan be utilized to guide the rainwater drainage system design.