GLENCOE RAIN GARDEN: SUCCESSFUL MANAGEMENT OF COMBINED SEWER FLOWS

Because Green Solution facilities reduce both peak flows and flow volume, they are attractive options to manage stormwater issues in the combined sewer system such as combined sewer overflows and basement sewer backups.The Glencoe Rain Garden is a vegetated infiltration basin constructed in 2003 to protect a residential area suffering from severe and frequent basement sewer backups and to reduce combined sewer volumes downstream. The facility is approximately 2,000 ft2 and manages roughly 35,000 ft2 of impervious drainage. Monitoring began before construction in August 2003. Peak flow and flow volumes are compared before and after Rain Garden construction.In the 9 months prior to construction, peak flows in the combined sewer exceeded 0.5 cfs – the design threshold to protect downstream properties from basement sewer backups – during 4 events. In the 48 months after construction, no events have exceeded 0.5 cfs. This was despite the occurrence of storms of greater intensity since the facility was completed. Two flow tests were performed to simulate 25-year peak flows with a peak outflow was 0.6 cfs – slightly above the benchmark but within the error tolerance.Flow volume reduction was significant, with an average annual retention of 88% over the first four years. Retentions for storm events closest to the CSO design storm appeared to be around 80% retention. One conservative flow test was also done that retained 56%.While there are some uncertainties – performance over time and performance for naturally occurring, large storm events – Rain Gardens and other infiltration facilities appear to be very useful and economical options for managing runoff peak flow and volume. Monitoring will continue to improve the accuracy of the results and to track changes in results over time.