Numerical Simulation And Optimization Design For The Rain System In Mountain City

The process of urbanization changed the under laying surface characteristics of thecity, leading to the hydrological environment of the city changed, runoff coefficientincreased, confluence speeded and efficiency also increased, totally runoff amount,runoff peak increased, the runoff peak time advanced, increasing the potential risk ofcity water logging. Because of the terrain gradient of the mountain city is up and downobviously, runoff properties changed more significantly, runoff process, designparameters are also different to plain area.In order to optimize the urban drainage system, both domestic and foreign scholarsconducted many researches of rain runoff numerical simulation. A lot of models arebuilt to simulate the variation of water quantity, these models made a wide range ofapplications, but the model which is used to simulate the runoff process of mountaincity is rarely seen. Therefore, research on mountain city is need immediately, the workhas the important practical significance to improve the flood control ability mountaincity flood, increase the rain scientific rationality of rain design system.This paper mainly studies on runoff numerical simulation and optimization designof mountain city, such as ChongQing as a representative.①Based on the rainfall data, this paper confirm the typically rainfall type,established the rainfall son module. Give an example of the rainfall process line whichdesign time is120min, recurrence period is two years.②Based on the physical process and theory of runoff produce (runoff is equal torainfall interception deduct plants interception storage, depression storage andinfiltration amount), simulate the comprehensive runoff on three types of underlyingsurface (pervious, impervious, comprehensive), use variable unit hydrograph methodwhich based on the nonlinear reservoir to simulation the process of confluence,established a runoff son module. Rated parameters of the module, results is as follows,maximum depression storage, maximum plants interception storage, initial watercontent, maximum water content, vary coefficient of water content, stability infiltrationrate, attenuation coefficient of infiltration, the value of the parameters is3.5mm,7.0mm,44mm,136mm,0.476mm/min,0.330mm/min, these can provided reference to researchrunoff characteristics and runoff simulation of mountain city.③Based on the St.Venant equation which used to describe the one D flow of rain water in pipes, generalized the concentration process in pipes of the mountain city, andestablish the pipe flow concentration son module. The rainfall son module, runoff sonmodule, pipe flow concentration son module make up the numerical simulation modelon rain runoff system of mountain city, test the model parameters, parametric test resultof two experiment is as follows, the error of runoff total amount between simulation andtest data is respectively7.5%and8.3%, the error of the runoff peak amount betweensimulation and test data is respectively5.56%and9.4%, the error was acceptable,proves the rationality of the model.④Concentration time is optimized based on the experiment of rain runoff, refiningthe concentration time in many classes which rule by norms is5~15min, and getconcentration time of the cement road is all1~5min corresponding slope is respectively0°~5°,5°~10°,10°~15°, concentration time of the nature green areas is respectively35~85min,30~70min,20~60min corresponding slope is respectively0°~5°,5°~10°,10°~15°, concentration time of the asphalt roofing is4~10min corresponding slope is0°~5°, concentration time of the green roof is20~40min corresponding slope is0°~5°,⑤Runoff coefficient is optimized based on the experiment of rain runoff, refiningthe runoff coefficient in many classes which rule by norms is0.85~0.95min, and getrunoff coefficient of the cement road is respectively0.88~0.99,0.84~0.99,0.85~0.99corresponding slope is respectively0°~5°,5°~10°,10°~15°, runoff coefficient thenature green areas is respectively0.08~0.2,0.09~0.26,0.12~0.42corresponding slope isrespectively0°~5°,5°~10°,10°~15°, runoff coefficient of the asphalt roofing is0.89~0.92corresponding slope is0°~5°, runoff coefficient of the green roof is0.54~0.59corresponding slope is0°~5°,⑥Reduction factor is optimized based on the experiment of rain runoff, rainwaterdesign flow is reduced10%~40.5%because of the use and the value of reduction factor.Use the two times experiment data of small watershed, deduced and validated the valueof reduction factor, and get the value of reduction is1.05and1.1, this can providedreference for rainwater system design of ChongQing.Under the research of this paper, established the numerical simulation model formountain city drainage system, rated and tested the parameters of the model, proves therationality of the model, and optimized the rain system design parameters of mountaincity, these works has positive significance to guide the rain system planning and design.