Alternative Treatment of Flow Monitoring Data to Evaluate the Impact of Green Infrastructure on Stormwater Volume Reduction in Combined Sewers

Abstract

Green infrastructure (GI) can capture stormwater onsite for volume reduction and peak flow attenuation in combined sewers. A pilot study in St. Louis evaluated how the use of rain garden and planter box to collect and mitigate stormwater runoff from roof drain can benefit the volume reduction of the flows in combined sewers. The study included two sites: a test site where GI (i.e. rain gardens and planter boxes) was installed; and a control site where no GI was installed. Flow and rainfall data were collected for a period of one and half year. Because many factors can affect the measured flow data from combined sewers, different methods for analyzing the same set of data could lead to difference in reported effectiveness of GI performance. This paper reports a study that examined two treatment methods of the flow monitoring data and the resulting effectiveness of GI performance for volume reduction. One treatment of the data considered flow data from same 24-hour periods between the test and control sites for evaluating GI’s performance. A second treatment of the data analyzed all of acceptable flow data collected from the test and control sites. The GI performance was assessed for percentage reduction of stormwater volumes in the combined sewers. The study found that the different treatments of a same set of data led to difference in the reported GI performance: 55% of volume reduction from treatment one vs. 65% of the volume reduction from treatment two. The analysis methods, results, and implications of how data analysis methods affect the evaluation of GI performance were discussed.