No abstract

Rain gardens have recently been studied as important low-impact development (LID) facilities that play a critical role in runoff volume reduction and pollutant purification. Approximately 16-40 rainfall events were monitored from March 2011 to October 2017 in order to determine the running effect of three rain gardens with respect to runoff volume reduction and pollutant purification. In particular, running fate analysis of rain gardens is the key focus in this study. Combined analyses revealed three key points. Firstly, performance assessment demonstrated that rain gardens effectively cut inflow volumes through the filter media; when the confluence area ratio was 6:1-20:1 (confluence ratio = roof area or road/garden area) and the rainfall was approximately 2.8-39.9 mm, the runoff volume reduction rate ranged from 9.8% to 100.0%. However, the average annual runoff reduction rate presented an initially increasing and then gradually decreasing trend with monitoring time. Secondly, according to water quality data in 54 rainfall events, the annual average concentration removal rate of NH 4 + -N was relatively good, but generally decreased with monitoring time. The concentration removal rate of NO 3 − -N and total phosphorus (TP) is unstable; however, the removal rate of total suspended solids (TSS) is better than that of total nitrogen (TN). Combined with runoff reduction, the pollutant load reduction by rain gardens is greater than 50%, although this decreases with increasing monitoring time. Thirdly, through the study of 7-year running effect on runoff reduction and pollutant purification, the "three-stage purification (TSP) concept" (periods of purification growth, stability, and attenuation) with respect to pollutant load reduction processes was finally proposed, and a curve chart was drawn for pollutant load reduction and rain garden operating fate (the "P-F" curve chart).

The storm water management models were established at three spatial scales (large, medium, and small) based on a sponge city pilot area in China to explore the hydrological and environmental effects of rainfall conditions and development modes. Results showed the following. (1) Total runoff reduction rates increased from 26.7% to 53.9% for the rainfall event of a 2-year recurrence period as the scale increased. For 5-year and above recurrence periods, total runoff reduction rates were 19.5–49.4%. These rates increased from the small to medium scale and slightly decreased from the medium to large scale. (2) The runoff coefficients were 0.87–0.29, which decreased from the small to medium scale and were basically constant from the medium to large scale. (3) The peak flow reduction rates decreased with increased recurrence periods. The rates increased initially and then decreased at the small scale, whereas the opposite trend occurred at the medium scale. (4) The reduction rates of pollutants were negatively correlated with recurrence periods under the three spatial scales. The pollution load reduction rates were 19.5–54.7%, which increased from the small to medium scale and were basically constant from the medium to large scale.

Urban waterlogging and nonpoint source (NPS) pollution caused by urbanization have considerably increased, and their control effects are affected by many factors. This study established a distribution model to analyze the influences of different rainfall conditions and rain garden layouts on runoff output and NPS pollution. Simulation results showed that (a) the reduction rates of the runoff and pollution load decreased from 45% to 23% and from 57% to 23% with increasing rainfall duration (1-24 hr). The flow and concentration peaks, total discharge amount and load increased, and the reduction rates of the runoff and pollution load decreased from 47% to 32% and from 60% to 21%, respectively, with increasing recurrence period (1-50 annual). The changes in the rain peak coefficient had minimal reduction effect on water quantity and quality. (b) Under different rain garden layouts, the reduction rates of the runoff and pollution load were 37%-52% and 13%-57%, respectively. The reduction effects of the runoff and pollution load improved, whereas amplitudes decreased with increasing rain garden layouts (0%-4%). (c) Rainfall duration and rain garden proportion greatly influenced the runoff and load reductions, respectively. © 2019 Water Environment Federation • Practitioner points• The reduction effects of runoff and pollution load under different rainfall conditions were simulated and analyzed. • The reduction effects of runoff and pollution load under different setting proportions of rain garden were simulated and analyzed. • The influence degrees of different design conditions on water quantity and quality reduction rates were evaluated.• Key words analytic hierarchy process; rain garden layout; rainfall condition; reduction effect; Stormwater Management Model

Intensifying urban stormwater runoff and pollution caused by urbanization and climate change threatens property and lives in cities around the world. Low-impact development (LID) is an effective approach to improve urban flood resistance, but there is a lack of quantitative evaluation criteria to determine the best scenario in the newly built and built-up urban areas. In this study, based on the Fuzzy Comprehensive Evaluation method (FCE), a multi-index evaluation system is established to quantify the LID performance for the environment, technology, economy, and society. For overcoming subjective arbitrariness, the Criteria Importance Though Intercriteria Correlation (CRITIC) and Analytical Hierarchy Process (AHP) methods are used to evaluate the weight of each evaluation index. The proposed method is demonstrated for the Xi'an City, China. Results show that the integrated configuration of green roof, infiltration pavement and grass swale has the best comprehensive efficiency under the proposed method. More specifically, the allocation of different LID combinations in the newly built area can effectively manage the flooding in this area. However, the comprehensive efficiency of increasing the diameter of the storm sewer is optimal in the built-up area. The increased pipe diameter can discharge more rainwater through the urban storm sewer and mitigate overflow. A multi-index evaluation framework beyond the traditional subjective evaluation standard is established, which can quantitatively evaluate the effectiveness of LID facilities. The proposed quantification evaluation method can provide an efficient recommendation for sponge city construction and evaluation.