| In recent years,with the rapid development of China’s economy,the urbanization process has accelerated.A large number of new hardened grounds have replaced the original natural land cover.The hardening of the underlying surface effectively blocks the natural infiltration of stormwater.During the rainfall,a large amount of storm runoff overflows the ground,and it is easy to accumulated in low-lying areas,which threaten the lives and property of citizens.Chongqing is a mountainous city with a lot of underpass,and the development of Chongqing’ s regions is not balanced.In addition,the urban drainage system is imperfect,and some pipes are damaged due to various reasons.Once a heavy rain occurs,the drainage network is not draining in time,which will lead to waterlogging to endangering the city’s traffic operation and pedestrian safety.In recent years,due to the impact of global climate change,heavy rainfalls in Chongqing have caused frequent waterlogging.To ensure the safety of urban drainage and flood control,to evaluate the drainage capacity of the drainage nerwork and to reform the drainage system are essential.The project takes the drainage network of an overpass area in Chongqing as the study object,it analyzes the problems of the drainage network and establishes the simulation model of the drainage network in the study area based on MIKE URBAN.It evaluates the drainage network by standard rainfall with different return periods and durations.The conclusions are as follows:(1)Evaluation results of drainage network’s return period(in the order of rainfall duration: 1h,2h,3h,24h): 38.85%,36.99%,34.99% and 23.90% of the pipes with return period less than 2-year in the study area,respectively.This part of the drainage network does not meet the minimum standards of the latest " Code for design of outdoor wastewater engineering ",which is in urgent need of reconstruction;42.39%,41.54%,41.04% and 35.31% of the pipes with return period less than 5-year,respectively;(2)Through the comparison of evaluations between the present drainage network and the previous drainage network,it is found that the evaluation return period of the downstream outlet pipe is more than 20-year.During all the design rainfall,the pipe’s fullness did not exceed 0.3.At the design level,the design standard of the upstream drainage pipes is low.It means,the upstream drainage pipes severely limit the overall drainage capacity of the study area;(3)The reconstruction of the underlying surface in the study area includes two ways: the common roof changed into green roof,and the common pedestrian road paved with permeable pavement.After the reconstruction,the comprehensive runoff coefficient of the study area reduced from 0.603 to 0.518,down 14.06% year-on-year,which can effectively reduce stormwater runoff peak and total stormwater runoff;(4)The reconstruction goal of the drainage network in the study area is to smoothly drain during the rainfall once in a decade.Two reconstruction schemes are set up: 1)Retain the combined sewer system with a total length of 10730.438m;2)Changing drainage system to the separate system with a total length of 11431.155 m,which builds 700.717 m sewage pipes more;(5)During the short duration rainfall with a return period of 20-50 years,the full flow occurred in part of the reconstructed drainage network,but no leakage occurred in the manholes.;With a 100-year return period of short duration rainfall,There are only 3,3,4 rainwater manholes overflowing in a short time(in the order of rainfall duration: 1h,2h,3h);(6)After the reconstruction,during the long duration rainfall with a return period of 20--100 years,there is no manholes overflowing.During the long duration rainfall with return period of 100-years,there are only a few pipes has a full flow state at a certain period of time;(7)In the simulation of the real rainfall happened August 11,2018,after the reconstruction,the full flow rate of the drainage network decreased from 40.53% to 0,and the overflow manholes decreased from 57 to 0. |
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