Evaluating Bioretention Cell and Green Roof Hydrologic Performance in northeastern Ohio

Graduate student Laura Sugano will also be presenting her green infrastructure research at the CUAHSI Biennial Symposium in July.

Evaluating Bioretention Cell and Green Roof Hydrologic Performance in northeastern Ohio

Laura L. Sugano*, Anne J. Jefferson, Lauren E. Kinsman-Costello, Pedro Avellaneda
Kent State University

Abstract

In urban areas, increased runoff from storm events is a significant concern due to flooding, erosion, ecosystem disturbance, and water quality problems. Green stormwater infrastructure is designed to ameliorate these effects by decreasing the flow rate and overall volume of runoff. We compared the effectiveness of a co-located green roof and bioretention cell in order to understand their relative capacities to decrease stormwater runoff, when subjected to the same weather conditions. Our field site was the Cleveland Metroparks’ Watershed Stewardship Center in Parma, Ohio. Beginning in June 2015, rainfall, underdrained outflow, groundwater levels, and soil moisture have been measured on 1-5 minute intervals during 84 storms. Event sizes spanned from 0.25 mm to 54 mm. The bioretention cell completely retained flow from 75% of the storm events, and the green roof retained 49% of storms. The bioretention cell completely retained all events smaller than 3.05 mm and the green roof completely retained all events smaller than 0.51 mm, though some larger events were also completely retained. For storms where underdrain outflow occurred, the average retention was 25% for the bioretention cell and 79% for the green roof. The bioretention cell completely retained 64% of the storm events in summer 2015, 90% in fall 2015, and 77% in winter 2015-2016. The green roof completely retained 37% of the storm events in summer 2015, 48% in fall 2015, and 89% in winter 2015-2016. The groundwater level in the bioretention cell increases in response to storm events and lowers between storms. The soil moisture in the green roof increases during storm events and slowly decreases between storms. My study suggests that bioretention cells can mitigate stormwater issues better than green roofs because they have the capacity to retain more stormwater due to their thicker substrate and their ground-location allows it to retain surface runoff as well as direct precipitation.