Revisited urban rainfall intensity formula: An example of the main urban area of Hangzhou
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Abstract:
Extreme rainstorms in many cities have broken historical records repeatedly, thus urban flooding disasters occur frequently. Under the background of global warming, extreme rainstorm events showed an increasing trend. In the past few decades, China's urbanization process advanced rapidly, and the rain island effect caused by rapid urbanization also led to the frequent occurrence of extreme rainstorm events to some extent. The characteristics of many urban rainstorms changed significantly, and the consistency of rainfall series no longer existed, so the current rainfall intensity formula may no longer be applicable. The rainfall intensity formula is one of the basic bases for the construction of urban drainage and waterlogging prevention infrastructure. To objectively reflect the characteristics and rules of urban rainfall and avoid underestimating the intensity of design rainstorms, the current formula needed to be reviewed and updated.The main urban area of Hangzhou was taken as an example, based on the rainfall extreme data from 1982 to 2022 at Zhakou station, the Mann-Kendall trend analysis method and the Pettitt test method were selected to analyze the trend and abrupt change of the annual rainfall extreme and selected Pearson type III distribution and Gumbel distribution curve for model fitting, respectively. The particle swarm optimization algorithm was used for parameter estimation of the rainfall intensity formula, and the newly derived rainfall intensity formulas were reviewed and analyzed. The rationality of the newly derived rainfall intensity formulas was indirectly analyzed using the rainfall extreme data of Gongchenqiao, Zhongcun, and Qibao rainfall stations and the extended historical rainfall extreme data of Zhakou station.The following results were obtained: (a) At the significance level of 0.05, the maximum rainfall series of 10, 20, 30, 45, 90, 360, 540 and 720 min at Zhakou station showed an insignificant downward trend. The annual maximum rainfall series of 60, 120, 180, 240 and 1 440 min showed an insignificant upward trend, and there were no significant abrupt change points in different duration rainfall series. (b) The average absolute mean square deviations of the new rainfall intensity formulas obtained from Pearson type III distribution and Gumbel distribution were less than 0.05 mm/min, and their average relative mean square deviations were less than 5%. (c) The rainfall intensities of the two new formulas were significantly larger than that of the current formula for considering different durations except for the 10-minute duration. (d) Based on the rainfall extreme data of Gongchenqiao, Zhongcun, and Qibao rainfall stations, the design rainfall intensities of the current formula were indirectly verified to be small. (e) It was shown that the difference between the new formulas and the current formula was due to the change in sampling stations and the change in the data years. Trend analysis results of the rainfall series showed that extreme rainfall in the main city of Hangzhou showed an insignificant increase, so the new formulas were more reasonable and safer than the current formula.The rainfall intensity formulas of Pearson type III distribution and Gumbel distribution were deduced and compared with the current formula based on the rainfall extreme data from 1982 to 2022 at Zhakou station. The new rainfall intensity formulas derived from the latest rainstorm data could be used as a reference for the design of drainage and flood prevention projects and the construction of related facilities in the main urban area of Hangzhou.
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