Research On Surface Runoff Characteristics And Low Impact Design Method Of Mountain Parks In Chongqing Main Urban Area Based On SWMM

In the context of the rapid development of Chongqing’s urban development,significant mountain features and relatively concentrated rainy season have brought severe challenges to the urban stormwater management.Under the background of Chongqing’s vigorous construction of mountain sponge city,defining the characteristics of surface runoff and controlling regional stormwater runoff based on low impact development concept have become the important means of building a healthy and sustainable mountain water environment.As an important part of the green open space in the main urban area of Chongqing,the mountain parks are facing the problems related to stormwater runoff management.However,their good underlying surface basement is conducive to the realization of stormwater runoff control relying on natural hydrological processes.Based on the complex topography,fragile ecological environment and sensitive construction conditions,the objectives and strategies of low impact development system construction were proposed through the analysis of hydrological processes in mountain parks.It is of great significance to study the basic research of stormwater runoff process in mountainous watersheds and to guide the concrete practice of mountain sponge construction.Based on the differences in mountain conditions,this paper selected mountaintop,hillside,mountainfoot and valley parks with representative mountain features as research objects.Through data query,field survey and on-site sampling,a comprehensive analysis of the climate characteristics and the site conditions including underlying surface composition,vegetation cover,soil infiltration,slope and drainage in each mountain park was completed.Through investigation,it was found that the large green ground of parks provided a good foundation for stormwater management based on natural hydrological processes.However,due to the constraints of site and construction conditions,the stormwater management of parks needs to be improved in terms of infiltration capacity improvement,runoff erosion suppression,soil and water conservation and drainage system optimization.Through the division and analysis of the basic research unit of runoff characteristics(subcatchments),the dismantling of four types of mountain parks were realized.According to the phase characteristics of the runoff process,they were divided into three types: no peripheral flow type,runoff crossing type and confluence exit type;according to the different conditions of the underlying surface,they were divided into four types: paved square type,mixed cover type,vegetation type and water cover type.Based on the statistical analysis of the survey results,the key parameters of each subcatchment were obtained,and the surface runoff analysis models of mountain parks were established by relying on the SWMM.This paper simulated the runoff under four kinds of precipitation events of 25 mm,50mm,127 mm and 166 mm.Quantitative results showed: 1)The runoff of the paved square type,water cover type,mixed cover type and type subcatchments show a decreasing trend;2)Vegetation ground as a permeable surface can effectively delay the arrival time of the peak runoff.In addition,the comprehensive underlying surface condition of mountain parks is one of the key factors affecting the runoff.Among them,imperviousness and average slope have significant effects on hydrological processes.The three conclusions were clearly defined by the correlation between the results of the runoff and the underlying surface factor(imperviousness and average slope): 1)Under the frequent occurrence of small-intensity precipitation events(less than 50mm),the imperviousness plays a key role in controlling the runoff of the mountain parks,and this effect gradually decreases as the precipitation intensity increases;2)Under the same precipitation event,the runoff coefficient increases with the increase of slope,but under the effect of composite underlying surface condition,the influence is much less than the impact of imperviousness on runoff;3)With the increase of precipitation intensity,the effect of underlying surface factors on the process of production and convergence is gradually weakened;4)The comprehensive runoff coefficients of the mountain parks in Chongqing’s main urban areas under the four precipitation events are revised to 0.313,0.343,0.526 and 0.614.Finally,based on the underlying surface conditions and the simulation results,the stormwater runoff control objectives were clarified,and the method of constructing the low impact development system in mountain area based on the principles of suitability,systematicness,ecology,security and aesthetics was put forward.According to the stage characteristics of the runoff process,the key points of low impact development control of no peripheral flow type,runoff crossing type and confluence exit type subcatchments were summarized.Combining with the hydrological characteristics of mountain parks,the landscape practical strategies of realizing runoff control,erosion suppression,utilization of rainwater resources and expansion of storage space were put forward.In addition,relying on the results of the runoff simulation,the general scale of the single low impact development facilities in the mountainous layout was further explored in order to provide reference for the construction practice of the mountain sponge.