WRRC/Ike Wai Seminar

Stormwater Management Using Green Roofs: Predicting Runoff Volume Reduction, Peak Flow Reduction and Peak Delay Dr. Roger Babcock

Urban areas typically consist of 50 percent impervious rooftop surfaces. Green roofs hold the potential to reduce urban runoff by allowing for onsite retention/detention/evapotranspiration of stormwater. A recent review found that green roofs can reduce stormwater runoff volume by 30 to 86 percent, reduce peak flow rate by 22 to 93 percent, and delay the peak flow by 0 to 30 min. Consequently, green roofs can help decrease pollution, flooding, and erosion during precipitation events. We have created a pilot green roof and used it to calibrate different models to extrapolate green roof performance and create a design tool type model.

First, a HYDRUS-2D model was developed for a modular green roof system. Physical hydraulic properties of the media were acquired through laboratory testing of cores extracted from a pilot modular green roof and then the model was calibrated with field measurement data. The model was then used to simulate performance of a full-scale green roof in response to a range of statistical recurrence interval storms from 1 to 100 years and durations from 30 min to 24 hr. The resulting simulated media water content profiles were used to develop regression equations to predict system performance for any combination of future design and precipitation event. The HYDRUS-2D model numerically solves the Richards equation for unsaturated flow and is computationally dense. Second, a simplified MATLAB-based model was created that incorporates the VanGenuchten equation for predicting water content, the Mualem equation for predicting unsaturated hydraulic conductivity, and the Penman-Monteith equation for evapotranspiration. This model was also calibrated using the field and lab data and used to simulate green roof performance for extrapolated scenarios. Third, an Excel-based VBA model was created that performs similarly to the MATLAB model and is user friendly for engineer-designers to use for design or predicting the performance of existing green roofs, and is available online. Additional pilot green roof experiments and simulation development are currently underway and will be described here.

Roger Babcock is a Professor with the Department of Civil and Environmental Engineering and Water Resources Research Center at the University of Hawai‘i at Manoa. He received his PhD in Civil Engineering from the University of California at Los Angeles.

Event Sponsor
Water Resources Research Center/Ike Wai, Mānoa Campus

More Information
Niels Grobbe, (808) 956-5857, ngrobbe@hawaii.edu

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