Impact of Green Infrastructures for Stormwater Volume Reduction in Combined Sewers: A Statistical Approach for Handling Field Data from Paired Sites Containing Rain Gardens and Planter Boxes

Combined sewer overflow (CSO) is one of the major water pollution problems faced by many municipalities that are on combined sewer systems. Green infrastructures (GIs) can help mitigate urban stormwater problems including CSOs. A field-based study over 3 years investigated the effectiveness of rain gardens and planter boxes as GIs for stormwater control in residential urban areas. The objectives of this study were to (1) investigate the impact of field-scale GIs on stormwater volume reduction in combined sewers at paired sites in an urban area, and (2) develop a methodology, including the application of a series of statistical methods, for analyzing field-based data and addressing the variations of field-collected data. The paired sites were both located in the City of St. Louis, Missouri. The test site had 12 rain gardens and six planter boxes, whereas the control site had no GIs. The stormwater was separated from the combined flows measured in sewers using the antecedent subsequent dry-days estimation method and were then normalized by drainage area and rainfall amount. The statistical methods deployed for this study included the Anderson Darling normality test, the Mann-Whitney U-test, and the Monte Carlo randomization test. This study revealed that, based on the normalized stormwater volume in the sewers from all of acceptable rainfall events, the means were ${0.409\pm 0.356\mathrm{m}}^3/{\mathrm{m}}^2\text{-}\mathrm{m}$ ($0.255\pm 0.222\mathrm{gal.}/\mathrm{sqft}\text{-}\mathrm{in.}$) at the test site and ${0.704\pm 0.979\mathrm{m}}^3/{\mathrm{m}}^2\text{-}\mathrm{m}$ ($0.439\pm 0.610\mathrm{gal.}/\mathrm{sqft}\text{-}\mathrm{in.}$) at the control site, revealing a 42% reduction of stormwater; the medians were ${0.321\mathrm{m}}^3/{\mathrm{m}}^2\text{-}\mathrm{m}$ ($0.200\mathrm{gal.}/\mathrm{sqft}\text{-}\mathrm{in.}$) at the test site and ${0.428\mathrm{m}}^3/{\mathrm{m}}^2\text{-}\mathrm{m}$ ($0.267\mathrm{gal.}/\mathrm{sqft}\text{-}\mathrm{in.}$) at the control site, revealing a 25% reduction of stormwater. For small rainfall events, the reduction was higher, at 62%. The methodology developed in this study can be used for other field-based studies. The findings provide needed information for the application of GIs to manage stormwater to help reduce CSOs.

Green infrastructure provides numerous benefits for managing stormwater. However, field-scale studies that use continuously operating sewer systems to investigate their effect on stormwater in combined sewers are limited due to many hard-to-control factors, such as complex hydraulic conditions and flow variations in sewers and limitations in flow monitoring devices. This study developed a methodology to analyze flow data in combined sewers to quantify the effect of green infrastructure in reducing flow and, thus, combined sewer overflows. Stormwater volumes at a test site and a control site were separated from the measured combined flows using the antecedent subsequent dry-days estimation method and were normalized by the drainage area and rainfall amounts of the site. The results showed that residential rain gardens and planter boxes are a tool to help meet requirements to reduce combined sewer overflows. They reduced runoff within the sewer by 25% for the median of the measurements over the 3-year study and reduced runoff from small rainfall events by 62%.