Hydrological And Water Quality Effect Of Urban Raingarden And Its Influences By Rainfall Distribution

In this paper,field monitoring and simulation analysis of two different types of rain garden facilities were carried out.Firstly,two rain gardens’ inflow and outflow process were monitored for 4 months,and their effects on rainwater runoff were analyzed.Secondly,several rainfall samples and water quality tests were conducted to study the effect of rain garden on pollutant removal.Thirdly,the effects of runoff reduction in study area under different rainfall conditions were simulated based on the measured data.Finally,the error analysis of rainfall characteristics in different areas is predicted.The main research results are as follows:(1)The western infiltrated raingarden’s backfill soil was local soil,and the design-depth of aquifer was 0.2m.10 overflow occurred during the monitoring period.The mean runoff reduction rate was 64.0%,the mean runoff coefficient was 0.644,and the mean variance was 0.022.The eastern drainage raingarden’s backfill soil is(powder)sandy loam,the design-depth of aquifer is 0.3m,and the bottom is laid with geotechnical film and perforated pipe.Two overflow flows occurred during the monitoring period.The mean runoff reduction rate is 100.0%,the mean runoff coefficient is 0.545,and the mean variance is 0.021.In general,the runoff reduction effect of eastern drainage raingarden is better.(2)The water quality monitoring results showed that the average removal rate of nitrate and nitrogen in west-east rain gardens was 11.3%and 67.6%,the average removal rate of ammonia nitrogen was 88.9%and 59.9%,the average removal rate of total phosphorus was-62.1%and 53.9%,respectively.In general,the removal efficiency of pollutants in the western infiltrated raingarden was better.(3)Aimed at the error caused by average rainfall intensity may be adopted in the current design method,the calculation results of the design depth of the aquifer by mean distribution method and normal distribution method are basically the same under the same rainfall standard,and the maximum difference is only 1 cm in different reproduction periods.When the recurrence period was 5a,20a and 50a,the runoff reduction rate distribution was 82.5%,65.1%and 57.5%calculated by mean distribution method.The runoff reduction rate calculated by normal distribution method was larger than the average algorithm,but the relative difference was only 2.3%to 6.9%.The recurrence period is 50a,and the runoff reduction rate at the standard design depth can still reach 57.2%to 65.1%,which shows that the hydrological effect of rainwater garden prediction is good.(4)The error caused by average rainfall intensity was analyzed in some provincial capitals of China.The IDF method is used to calculate the average rainfall intensity in provincial capitals,and then the average distribution method and normal distribution method are used to calculate the depth range of rainwater garden design,the range of design depth is 3.2cm to 140cm and 6cm to 139cm,the maximum absolute difference is only 2.8cm,the design depth of Xinjiang,Tibet and Gansu is less than the recommended depth in The technical guide of sponge city construction.The relative difference of runoff reduction rate calculated by the two methods is positively correlated with regional rainfall intensity,and the maximum value of relative difference is 21.7%in Hainan province.Runoff reduction rate is positively correlated with infiltration rate.When K=O.lm/d,the change range of runoff reduction rate is 22.7%to 100%;When K=1.5m/d,the change range of runoff reduction rate is 29.9%to 100%;When K=8m/d,the change range of runoff reduction rate is 61.8%to 100%.In these three cases,the lowest runoff reduction rate occurs in Hainan where the average rainfall intensity is the largest,and in dry regions such as Xinjiang and Tibet,the decrease rate is as high as 100%.