| Jiangsu is located in the plain river network area in the middle and lower reaches of the Yangtze River,which has a high level of urbanization.Jiangsu was greatly affected by rainstorms and floods,especially in Meiyu season,the frequency of urban waterlogging higher.In order to alleviate the negative impact of urbanization,China put forward the "sponge city"idea in 2013.Low impact development（LID）measures are the source control measures of the"sponge city" idea.However,the heavy rainfall,frequent rainfall and high groundwater levels of Meiyu season have a greater impact on the runoff reduction of LID measures.The study used a combination of field experiments and model simulations to analyze the differences of the runoff reduction effect of rain gardens in non-Meiyu season and Meiyu season.According to the long-series daily rainfall data in Yangzhou,the trend of rainfall in Meiyu season was analyzed.Based on the measured flow process of the rain garden,the factors of the Meiyu season and non-Meiyu season which affected the runoff reduction of rain gardens were analyzed.Matlab program was used to simulate the influence of each factor on the runoff reduction effect of rain gardens under quantitative conditions.The main conclusions obtained are as follows:（1）According to the 30-year daily rainfall data,the number of days before the rainfall more than 4 days was the highest proportion,accounting for 49.0%;while in the Meiyu season,the number of days before the rainfall being 1 day was the highest proportion,accounting for 34.0%,thus the rainfall was more frequency.With the reduction of the rainfall guarantee rate,the rainfall in the Meiyu season gradually sparsed,and the number of days before the rainfall increased significantly.From the high flow year to the low flow year,the proportion of the number of days before the rainfall in the Meiyu season decreased from 56.0%to 19.0%.（2）According to the measured flow process of rain garden from 2018 to 2021,69.23%of the situation of rain garden submersion and water retention occurred in the Meiyu season,during which the runoff reduction effect of rain garden significantly reduced.In Meiyu season,85.71%of the rainfall,which moderate rain,heavy rain and above,suffered submersion and water retention;the effective absorption rate of moderate and light rain in non-Meiyu season was high,reaching 92.59%,which was nearly 20%higher than that in Meiyu season.（3）The results of experiment from 2018 to 2021 showed that the volume reduction rate and peak reduction rate of rain gardens in non-Meiyu season were 17.21%and 18.59%higher than those in Meiyu season.The rainwater garden had a better capacity for the absorption of rainwater in Meiyu season.The average infiltration amounts of the light rain,moderate rain and heavy rain and above in non-Meiyu season were 203.74%,24.02%and 157.35%higher than that in Meiyu season respectively.（4）Excluding the impact of the process of construction,the average groundwater level in Meiyu season was 0.46m higher than that in non-Meiyu season,and the average infiltration rate in Meiyu season was 0.20mm/min lower than that in non-Meiyu season.When the groundwater level approached the bottom of the rain garden,the rain garden was prone to be invalid in runoff reduction.When the groundwater depth was less than 1m,the rate of being invalid in runoff reduction was 33.33%.（5）According to the actual parameters of the rain garden,using Matlab to simulate,the results showed that the rainfall and soil condition had a greater impact on the runoff reduction of the rain garden.Under the same parameters of rain garden,the impact of continuous rainfall of moderate rain and heavy rain on the runoff reduction rate of the rain garden was more than 20%,while the impact of rainfall distribution,rainfall duration and rain peak position on the runoff reduction on rain garden were less than 10%.The soil conditions and groundwater level had a significant impact on the runoff reduction of rain gardens.The model simulated the conditions from "dry+deep level" to "saturated+shallow level",while the runoff reduction rate of rain garden dropped by 35.46%. |
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