| Probable Maximum Precipitation(PMP)is estimated for the design of important hydraulic engineering and flood design standard of coastal cities.PMP is not a purely theoretical research topic,but an engineering hydrology topic.More specifically,it is mainly an engineering topic and an applied discipline.It is very difficult to study and apply the extreme value.In theory,the scientific question of PMP is the extreme precipitation event in nature how to achieve"maximum" under "possible" conditions.In application,the purpose of the PMP estimation study is to summarize set of reasonable,effective,and operationally applicable technologies and methods,and to extend them into production practices to make up for the gaps in our research and application fields.The research idea of PMP is that under modem climate conditions,the reason and factors of the historical heavy rain are analyzed to infer the backgrounds and conditions leading to the rainfall more serious,and then it is estimated that it will reach the maximum at a given time and specific place.Further thinking,in the context of global climate change,what is the trend of this possible maximum precipitation event?The research method of PMP is mainly the cause analysis(hydrometeorological route),supplemented by statistical estimation.As we known,landside is the serious hazard in Hong Kong,and the precipitation that causes landslide can be either short-duration heavy rainfall or cumulative(effective)rainfall.Therefore,systematical 4hr PMP estimates representing short duration and 24hr PMP estimates representing long duration regarded as the highest standard are utilized in the design of project decreasing the landside and the landside risk assessment.In the context of climate change,extreme rainfall events happened frequently than before.Therefore,in spite of the PMP with no allowance made for long-term climatic trends,it is still necessary to discuss the probable impact of climate change on PMPIn this paper,the revised statistical method,moisture maximization,storm transposition based on Step Duration Orographic Intensification Factor(SDOIF)method and Duration-Area-Depth(DAD)method are used to analysis the historical rainfall,real-time storm and dewpoint data in Hong Kong and Taiwan to estimate the 24hr and 4hr PMP of Hong Kong systematically and comprehensively.Then the 24hr and 4hr PMP estimates passed reasonable comparison and analysis are used in landside risk assessment.Finally,the impact of climate change on PMP estimates are discussed by analyzing the variation of precipitable water,one of the factors effecting the PMP estimates,the Cumulation PMP(C-PMP)and the Slide PMP(S-PMP).Five primary conclusions are given below:(1)The statistical method is improved by the two constraint conditions related to the length of the site sequence,and it is renamed as the revised km-value method.This method can reduce the error of the statistical parameters Km,and make the PMP estimates more reasonable by excluding the sites that do not meet the conditions.The revised Km-value method is simple and convenient to quickly obtain point PMP estimates for reference.However,the result shows that it heavily relies on the size of the data itself and the length of the sequence.(2)The SDOIF method can be used to quantitatively divide the "Morakot" typhoon storm and the "Kalmaegi" typhoon storm into the convergence rain component and the topographic rain component and the spatial distribution.It found that the Alishan mountain reduced 45%24hr rainfall of "Morakot" typhoon storm while 24%4hr rainfall of "Kalmaegi" typhoon storm.The generalized 24hr and 4hr convergence rain pattern is transposed to Hong Kong to get the 24hr and 4hr PMP estimates,respectively.(3)The 24hr and 4hr PMP estimates obtained by the revisedKm-value method is 1,753mm and 558.5mm,respectively.The maximum 24hr and 4hr PMP estimates obtained by storm transposition based on the SDOIF method is 1,502mm and 718mm,respectively.The maximum 24hr and 4hr PMP estimates obtained by the DAD method is 1,358 and 534 mm,respectively.After comprehensively examining the data and the processing in the calculation,and rationality analysis and comparison,the 24hr and 4hr PMP estimates obtained by storm transposition based on the SDOIF method is determined as the final 24hr and 4hr PMP estimates result in Hong Kong.(4)When the 24hr and 4hr PMP estimates of Hong Kong are applied to landslide risk assessment in Hong Kong,the central location is selected on Hong Kong Island,the city center of Hong Kong,taking the 24hr and 4hr PMP estimates of 70%and 66%,respectively,with a maximum of 1051mm and 474mm.(5)In the past 50 years,there has been no significant change in the maximum persisting 12hr dew point in Hong Kong,resulting in no significant change in the amount of precipitable water calculated by the pseudoadiabatic method in Hong Kong.Since 1914,Hong Kong’s annual maximum 24-hour rainfall has not changed,but 24-hour C-PMP has been declining,while 24-hour S-PMP has been affected by extreme rainfall events in different periods caused by climate change.It is not yet clear that there is a statistically significant change of PMP with the climate change. |
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