As urbanization has expanded, so have impervious surfaces such as paved roads, parking lots, and sidewalks, increasing stormwater runoff. These types of surfaces prevent rainwater from naturally absorbing into the ground. Some communities use combined sewer systems, where both wastewater and stormwater drain into a single system that then flows to a wastewater treatment plant. When heavy rains occur, the stormwater volume can exceed sewer hydraulic capacity resulting in overflow that pollutes surface water, also called combined sewer overflow. CSO is a major problem for municipalities across the U.S. Implementing stormwater control management, like green infrastructures, can help reduce the volume, flow, and pollutants of stormwater. To date, there has been limited field research specific to applying green infrastructures alongside CSS and in studying the impact it has on stormwater volume.
Could green infrastructures minimize the risk of CSO and protect surface waters? Researchers Isam Alyaseri, Jianpeng Zhou, Azadeh Bloorchian-Verschuyl, and Susan Morgan wanted to learn how GIs affect stormwater in sewers. The objectives for their paper “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” were twofold: first, to investigate the impact of field-scale GIs (rain garden and planter box) on stormwater volume reduction in combined sewers at paired sites in an urban area; and second, to develop a methodology for analyzing field-based data and addressing the variations of field-collected data. They selected two urban sites for a paired evaluation, using both rain gardens and planter boxes, along with a control site with no GIs installed. To learn more about applying green infrastructures as part of a stormwater control management solution, see their paper in the Journal of Sustainable Water in the Built Environment: https://doi.org/10.1061/JSWBAY.SWENG-416. The abstract is below.
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±0.356 m3/m2-m (0.255±0.222 gal./sqft-in.) at the test site and 0.704±0.979 m3/m2-m (0.439±0.610 gal./sqft-in.) at the control site, revealing a 42% reduction of stormwater; the medians were 0.321 m3/m2-m (0.200 gal./sqft-in.) at the test site and 0.428 m3/m2-m (0.267 gal./sqft-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.
Learn more about how you can mitigate CSOs with green infrastructures in the ASCE Library: https://doi.org/10.1061/JSWBAY.SWENG-416.