System Evaluation And Numerical Simulation Optimization Of LID Technology Based On The First Flush Control

With the acceleration of urbanization,the effect on water environment and urban rainwater management is increasing because of the impact by first flush on quantity and quality.Sponge city is an important countermeasure to alleviate these problems.However,the design and construction of LID facilities in Sponge city are lack of systematicness and pertinence presently.In that case,the study area NO.1 located in SJ city and the study area NO.2 located in NJ county are taken as typical research cases.The rainwater quantity and quality reduction rate efficiency curves of different LID facilities are drawn,and the control effects of water quantity and water quality on LID facilities combination which were designed based on efficiency curves were evaluated by SWMM model.Finally,the influence factors of efficiency curves applied in LID facilities designing numerical simulation and practical application were obtained.The main achievements were made as follows:The influence of some factors on SWMM simulation results and numerical optimization effect in LID facilities were investigated when study area NO.1 and study area NO.2 were chosen as typical research cases.And the factors included catchment area size,topography,proportion of impervious area,land types and rainfall conditions.Furthermore,the initial effect of rainwater runoff in study areas were determined,as well as the first flush in study areas was defined as the first 30 minutes rainwater runoff at rainfall events through rainwater runoff sampling and monitoring.Based on the SWMM model,the water quantity and quality reduction rate efficiency curves of six common LID facilities were drawn.It showed that bioretention ponds is suitable for roads and residential land,especially for COD and SS reduction.Permeable pavement is most suitable for roads,especially for runoff and SS control.Vegetative swale can be used as rainwater transfer facilities or pretreatment facilities for roads,residential and commercial land.Infiltration trench is applicable to roads,residential land and industrial land,it mainly focuses on water quantity control and has some water quality control ability at the same time.Rain garden is good at runoff reduction of various land types,but the water quality control ability is relatively poor.Green roof is often used as supplementary LID facilities in roofs of residential and commercial buildings.In a word,the quantitative guidance of efficiency curve is necessary on LID facilities selection and area design which were used to achieve accurate and scientific rainwater management,because the applicable LID facilities vary obviously in different land types and different control objectives.SWMM simulation showed that using the LID facilities optimized design based on the reduction rate efficiency curves,the reduction rate of total runoff in study area NO.1 is between35.27%and 41.61%,the reduction rate of of total runoff in study area NO.2 is between 32.50%and 37.39%,when repetition period of rainfall was selected as 1 a,2 a,5 a and 10 a.In the meantime,the reduction rates of SS,COD,TN,TP and NH₄⁺-N in study area NO.1 were between 52.53～57.09%,50.10～54.78%,50.11～55.36%,42.85～49.64%and 51.97～57.96%respectively,the reduction rates in study area NO.2 were between 45.07～47.88%,51.40～54.25%,38.24～42.02%,48.29～51.37%and 48.24～52.71%respectively.The rainwater quantitative and qualitative control effect of two study areas is greatly improved compared with the current LID facilities design,but the improvement performance decreased with the increasing of rainfall repetition period in two study areas.In addition,the accuracy and applicability of reduction rate efficiency curves are further verified in study areas.It also shows that LID facilities optimized design based on efficiency curve optimization is more systematic and targeted.