Experiment And Stimulation Study On Efficiency Of Stormwater Disconnection Runoff Control

With the development of urbanization,the impact of urban heat island and rain island effect become more and more obvious,which leads to the increase in frequency and amount of precipitation in urban areas.In addition,the original pervious surfaces gradually turn into impervious surfaces.Furthermore,infiltration process is blocked,and surface’s detention storage function becomes more invalid,which lower the self-regulatory mechanism of the urban hydrologic system.Therefore,accelerated urbanization puts more pressure on urban drainage system,bringing on a crisis of the water logging.Stormwater disconnection is an important technical measurement to construct sponge city and develop rainwater system with low impact.The main purpose of stormwater disconnection is to break the contiguous impervious surfaces.So,it makes the rainfall runoff permeate into the pervious surfaces and disenables rainfall converge rapidly.In turn,it helps cut down the runoff volume and peak discharge and reduce the risks of urban waterlogging.Based on artificial rainfall experiment and SWMM simulation,this study analyzes impact on the runoff control efficiency of stormwater disconnection system from following aspects:five kinds of impervious surface ratio(0%,25%,50%,75%,100%),impervious surface space layout(distributed and centralized)and the concave measurement.In conclusions,(1)As decrease of the impervious surface ratio,the stormwater disconnection system has longer duration of runoff generation and higher runoff coefficient reduction rate.When the impervious surface ratio reaches 75%,50%and 25%,the runoff reduction rate is 0.41%,15.73%and 18.0%,respectively.The change pattern of stormwater disconnection system effect of peak discharge lag and volume reduction,with the impervious surface ratio changing,is not obvious.(2)With the same impervious surface ratio,the space layout of the impervious surface will have a big influence on the runoff generation and peak time.When impervious surface ratio is 50%,the surface layout are the PIPI,IIPP and IPIP.Runoff producing time of stormwater disconnection system is 1.9 min,12.6 min and 13.6 min respectively.The occurrence of peak time 43.0 min,38.0 min and 32.0 min,respectively.Runoff stops time is 66.7min,91.7tnin,and 70.6min,respectively(P represents pervious surface,I represents inpervious surface).(3)When the rate of impervious surface ratio and space layout keep constant,study sets concave depth as 5 cm for pervious surface which receives runoff from the impervious surface.This can delay runoff generation and peak time,also reduce the runoff coefficient and peak discharge effectively.For example,runoff generation time and peak time of IPIP are 13.6 min and 32.0 min respectively at the beginning.After setting concave depth as 5cm,the runoff generation time and peak time are delayed to 37.5 min and 38.0 min respectively.When the impervious surface ratio i5 25%,after setting eoncave depth as 5em,the peak discharge reduction rate of the stormwater disconnection system can be increased from 35.3%to 59.0%.(4)SWMM simulation results of stormwater disconnection system,s hydrologic process indicates that runoff depth is medium sensitive to pervious or impervious surface’s depression storage(degree of sensitivity is 0.19 and 0.10,respectively).And it is not sensitive to change in manning coefficient(close to 0.00).Since maximum rainfall intensity of artificial rainfall appears during 3 0min～40 min,and pervious or impervious surface have been completed process of depression detention and begin to generate runoff at that time,peak discharge is not sensitive to change in the depression storage for both of pervious or impervious surfaces.(5)Reducing the proportion of impervious surface under the conditions of designing heavy rainfall at different frequencies,the maximum runoff depth at the outlet of the study area,the peak flood discharge,and the total runoff reduction rate are different.The longer the design storm return period,the smaller the reduction rate,the maximum runoff depth,total peak discharge,and total runoff at the outlet of the district showed a trend of decreasing first and then increasing with increasing depth of the igTeen subsurface pit.The minimum depth of depression reaching this constant value increased with the increase of the design rainstorm return period.