QUANTIFYING THE EFFECTS OF RAIN BARRELS ON URBAN STORMWATER HYDROLOGY

In 2007, the Nine Mile Run Watershed Association launched the second phase of a large-scale rain barrel program in Pittsburgh, PA. Although primarily a demonstration project whose purpose was to raise citizen awareness on stormwater related issues, this ‘Rain Barrel Initiative’ was designed as an experiment to determine if rain barrel use would result in a measurable reduction in stormwater runoff. Rain barrels and ‘green’ infrastructure in general act as runoff speed bumps. They delay initial hydrograph response and lag time, prolong the duration of storm flow, and increase water storage within the catchment. However, the manifestation and magnitude of these changes is poorly understood.

Two small (9.0 – 12.6 ha), urban, and residential sub-sewersheds within the larger Nine Mile Run Watershed were selected for flow monitoring. Rain barrels contributed over 40,000 liters of additional water storage in each sub-catchment, or approximately 10% of total stormwater runoff volume for a 1.0” rain event. Stormwater hydrographs of pre (2001 – 2002) and post (2009 – 2010) rain barrel installation were compared for each study area. Preliminary results indicate that observed stormwater runoff volumes are lower than models predict for urban and sub-urban catchments. The degree to which rain barrels contribute to this difference will be discussed.