Bioretention can be an effective measure for stormwater treatment. However, there is a lack of systematic analysis of the impact of bioretention design parameters on hydrologic performance.Herein, SWMM and RECARGA models were applied to generate the typical annual rainfall runoff and simulate the water balance of the bioretention system in an expressway service area. The purpose of the investigation was to identify key design parameters for the bioretention system and delineate the priorities in developing the design. Results showed that the average groundwater recharge ratios for bioretention basins with and without an underdrain were 58.29% and 92.27%, respectively, the average overflow ratios were 4.13% and 4.19%, the average evapotranspiration ratios were 4.48% and 4.47%, and the average outflow ratio for bioretention with an underdrain was 33.94%. The ratio of the bioretention area to drainage area, and the saturated infiltration rates of planting soil and native soil were the main factors influencing water balance, while the underdrain diameter and gravel layer depth exerted little effect. Based on the impact analysis, multivariate nonlinear regression models of runoff reduction rate for two types of bioretention basin were established, which both exhibited high determination coefficients and acceptable Nash-Sutcliffe coefficients.

Bioretention is an important technology for ecological control of runoff. The purpose of this study was to investigate the coupling effect of in-situ soil and groundwater level on the hydrological performance of bioretention. VADOSE/W was used to simulate the water transport processes during bioretention under a single rainfall event. The effects of four in-situ soil types and two groundwater levels on the surface ponding, underdrain outflow, exfiltration, and runoff regulation effects of bioretention were studied. Under eight geological situations and the rainfall of 0.17 mm/h (6.0 h), the ponding duration and overflow volume of bioretention were 556-649 min and 24.71-39.61 mm/m 2 , respectively; the underdrain outflow peak value and duration were 0.549-0.804 mm/min and 380-730 min, respectively; the exfiltration volume per unit area from the bottom and lateral of bioretention were 106.79-396.10 mm/m 2 and 50.60-147.45 mm/m 2 , respectively; and the runoff reduction rate, runoff peak reduction rate, and runoff delay time of bioretention were 53.46%-96.19%, 18.43%-68.08%, and 288-318 min, respectively. These results suggest that bioretention without an underdrain and with a relatively smaller K s (saturated permeability coefficient) of in-situ soil might result in longer ponding times and larger overflow volumes. With an increase in K s of in-situ soil, the underdrain outflow weakens, the exfiltration volume increases, and the runoff control effects improve. Although the groundwater level has little effect on surface ponding, it can cause a stronger underdrain outflow. The shallower groundwater level leads to a larger exfiltration volume when the K s of in-soil is much smaller than that of the planting layer and leads to a reduced runoff regulation effect for bioretention without an underdrain. Therefore, when locating and designing bioretention systems, the in-situ soil type and groundwater level should be comprehensively considered to ensure that the runoff control target is achieved.

No abstract

Taking an expressway service area as the study object, based on the SWMM and VADOSE/W models, the long-term water balance of exfiltration type bioretention (ZR) and anti-seepage type bioretention (ZF) were quantified, and the performance of bioretention on the long-term hydrological effects of the service area was studied. The results show that the ZR can significantly increase the annual underground discharge ratio compared with ZF, thereby reducing the underdrain outflow ratio, but has little effect on the overflow ratio and evapotranspiration ratio. ZR can significantly improve the hydrological balance of the service area, and increase the annual runoff control rate by 48.21%, and the utilization rate of rainwater resources in the service area can reach 73.47% by the combination of ZF and reservoir. For typical annual rainfall events, the average values of runoff reduction rate, runoff peak reduction rate and runoff delay of traditional service area were 59.45%, 57.17% and 5.19 h respectively, those of service area with ZR were 91.56%, 95.22% and 20.55 h respectively, and those of service area with ZF were 68.59%, 86.67% and 15.22 h respectively. ZR and ZF can both effectively improve the rainfall regulation ability of the service area, and the performance of ZR is relatively better. Furthermore, the correlation between runoff characteristics and rainfall characteristics for three types of service areas was analyzed, which can provide a reference for forecasting and controlling rainwater runoff in the service area.

Exfiltration type bioretention can collect rainwater runoff to recharge groundwater, but the water diffusion in the in-situ soil can have an impact on the foundation of adjacent structures (such as roads). Generally, the existing studies are primarily focused on the exfiltration of bioretention as an index of runoff control or the water balance. However, research is lacking on the diffusion characteristics of soil water content in different in-situ soils. Therefore, in this study, the VADOSE/W model was used to simulate the water transport process of bioretention ponds and in-situ soils, under long-term rainfall. The water diffusion characteristics of four in-situ soils were studied: silt loam (SL), loam (L), sandy clay loam (SCL), and sandy loam (SaL). The results showed that under 12 rainfall events, with a monthly maximum rainfall of 268 mm in the study area, for four in-situ soil types, the bioretention pond's bottom exfiltration volume per unit area reached 3.93-7.91 times that of the lateral. The order of bottom exfiltration volume was SaL>SCL>L>SL. Over time, the in-situ soil water content fluctuated with rainfall events. The order water content was usually SL>L>SCL>SaL, and the water diffused into the in-situ soil was distributed in a symmetrical arc along the horizontal direction. After rainfall events, at depths of 1, 3 and 5 m, for SL, L, SCL, and SaL soils, the lateral water diffusion ranges were ~1. 25-1.47 m, ~1.23-1.45 m, ~1.22-1.77 m, and ~1.46-1.60 m, respectively. With a continuous supply of water, the horizontal diffusion distance of each in-situ soil tended to be the same, although the water diffusion range of SCL was relatively larger. Therefore, when an exfiltration type bioretention area is designed, the distance between the bioretention edge and the adjacent structures should be