Effect And Benefit Of LID Rain Flood Control Based On SWMM

The rapid development of urbanization has not only improved the living standards of residents,but also changed the original hydrological cycle of cities,resulting in frequent urban waterlogging disasters,and the urban rain-flood problem needs to be solved scientifically and effectively.Sponge city is a new stormwater management concept.The simulation of stormwater based on the concept of sponge city and the exploration of appropriate sponge reconstruction scheme are of certain value for promoting urban development,reducing urban waterlogging and solving water shortage.As an important part of the city,the campus district,due to its regional particularity and independence,It is very suitable for sponge city construction pilot zone and demonstration zone.Therefore,this paper takes the campus of Northwest Normal University as the research object,builds SWMM model by collecting and sorting meteorological data,pipe network data and underlying surface data in the study area,selects appropriate LID measures,designs four kinds of LID layout schemes,and evaluates the effect of different LID layout schemes from three aspects:surface runoff,pipe network overload and overflow water.Secondly,the economy of the scheme is evaluated by constructing the comprehensive benefit model and monetizing the benefit index.Finally,according to the improved cost-benefit formula,the economy-effectiveness analysis of the runoff control effect of each LID scheme is carried out,and LID measures which are most suitable for the study area are obtained.It is hoped that the study of this paper can provide some reference for the spongy transformation of campus area,and the specific conclusions are as follows:(1)According to the rainstorm intensity formula and the Chicago rain pattern,the recurrence period of this region is 1a,2a,5a,10 a,20a,the design duration is 180 min,and the design rainfall of 0.382 is 6.96 mm,15.16 mm,26.01 mm,34.21 mm,42.42 mm,respectively.The subcatchment area was divided by hydrologic analysis combined with manual division method,and deterministic parameters such as subcatchment area,feature length,slope,impermeability,pipe diameter and joint elevation were calculated and sorted out by basic data and Arc GIS.The catchment layer,pipeline layer and node layer are converted into the inp format file required by SWMM through the inp.PINS software.After the uncertainty parameters were initially set through the user manual and literature data,the comprehensive runoff coefficient method was used to calibrate and verify the model,and the final values of the remaining uncertainty parameters were obtained to complete the construction of the SWMM model in the study area.The final division result of the study area is 51sub-catchment areas,41 nodes,41 pipe sections and 3 discharge outlets.(2)The current situation is before LID is deployed.In terms of surface runoff,with the increase of return period,the total runoff and peak runoff are increasing,and the peak time is also advancing.The current runoff coefficients in 1a,2a,5a,10 a and 20 a were 0.597,0.661,0.690,0.723 and 0.754,respectively.In terms of pipe network overload,the proportion of pipe network overload increases with the increase of rainstorm recurrence period,and the proportion of overload node,overload pipe section and overflow node increases from 17.07%,24.39% and 4.88% in 2a to 92.68%,95.15% and 63.41% in 20 a.In the aspect of overflow water,the range of water also expands with the increase of return period.Overflow flow and overflow time increased from 0～50m3 and 12 min at 2a to 270～680m3 and 35 min at 20 a.LID measures of permeable pavement,rain garden and green roof are selected according to local conditions,and a total of four LID schemes are set up.Scheme one is rain garden,and the transformation ratio is 4.34%.Scheme 2 is permeable pavement,and the transformation ratio is 15.17%.Scheme 3 is green roof,18.11%;Scheme 4 is three LID combinations,and the transformation ratio is 37.62%.(3)After the LID is deployed.In terms of surface runoff,each scheme reduces the total runoff and peak runoff to different degrees,and the peak time is delayed to a certain extent.Among them,scheme 4 has the best runoff control effect,and the return period is 1a,2a,5a,10 a and 20 a.The reduction rates of peak runoff were 54.48%,53.79%,53.09%,52.40%,51.89%,and the reduction rates of total runoff were 58.12%,57.03%,56.23%,53.67%,51.33%,respectively.The peak time lag is 8min,5min,2min,1min and 0min respectively.After adding rain barrels on the basis of scheme 4,the contribution rates of rain barrels in the total runoff reduction process are 21.67%,10.24%,6.05%,4.67% and 3.97%,respectively.In terms of pipe network overload,different LID schemes can improve the overload situation to some extent.Among them,scheme 4 has the best effect and the smaller the return period is,the better the effect will be.When the return period is 2a,the reduction rates of pipe section overload,node overload and node overflow are all 100%.At 20 a,they were 49%,58% and65%,respectively.In terms of overflow water,the reconstruction effect of overflow water is analyzed by taking Scheme 4 as an example.When the recurrence period is 1a and 2a,no overflow occurs at any node;in 20 a,the overflow flow ranges from 24 to 104m3,with a reduction rate of 84.70%;and the overflow time is within 7min.After LID renovation,the campus water has been greatly improved.(4)The function and demand coupling method is adopted,and the benefit index is identified by referring to the Evaluation Criteria of Sponge City Construction and Assessment Methods.The comprehensive benefit system of sponge city construction in campus area is constructed by induction and integration according to the ecological,economic and social benefits.Then,it uses the principle of environmental economics to conduct a quantitative study on the monetization of each index in the comprehensive benefit model,and calculates that the annual comprehensive benefit brought by the "Scheme Four + rain barrel" scheme is6,745,800 yuan after the scheme is set up,which has a total cost of 97,636,100 yuan in present value within 30 years.The economic,ecological and social benefits were 70.04 million yuan,3.8893 million yuan and 2.1561 million yuan respectively.The ratio of dynamic benefit cost and static benefit cost were 1.06 and 1.54,respectively.The results showed that the designed LID scheme was reasonable and worth deploying.(5)Economic and effectiveness analysis of LID runoff control effect was carried out.The results show that the economy-effectiveness of runoff control effect of different LID schemes decreases with the increase of rainstorm return period.In scheme 4,when the recurrence period is 1a,the cost-benefit values of the total runoff and the peak runoff are0.0048 and 0.0045 respectively,indicating that the investment of 10,000 yuan in the whole life cycle cost can reduce the total runoff and the peak runoff by 0.0048 and 0.0045 percentage points respectively.In addition,among the individual measures of LID,the cost-benefit value of permeable pavement in different recurrence periods is higher than that of rain garden and green roof,indicating that the runoff control of permeable pavement has the best economic efficiency and the best investment effect.Therefore,the scale of investment and construction of permeable pavement can be appropriately increased.