- Hawley, Gary J
University of Vermont
Large areas of impervious surfaces found in urban ecosystems often result in uncontrolled rapid runoff of stormwater. During runoff events in urban ecosystems, the high volume of water being deposited into nearby waterways carries pollutants that have washed off of impervious surfaces, as well as silt from erosion, and contributes to elevated water temperatures. Some of the newest methods to reduce stormwater impacts in the built environment include treating stormwater on-site before clean water is either reused or discharged in a controlled manner. This green roof research/demonstration project includes 8 rooftop watersheds on the Green Aiken Center at the University of Vermont that are monitored separately to provide water quality and water flow rates for each watershed. Four green roof treatments (control, standard plants with standard soil, standard plants with alternative soil, and alternative plants and standard soil), replicated twice, were used in this study. To assess the thermal benefits of green roofs, we monitored 56 temperature probes at different heights. In 2013, we assessed green roof treatment performance during 5 storm events and in 2014, we used continuous measurement of water quantity and quality. Two years of results confirm that green roof treatments had significantly less discharge than the control. Standard plants and soil had half as much discharge than the control, and the other treatments had about 25% less than the control. Peak flows were substantially less in vegetative watersheds than in the controls. Summer temperatures were lower under vegetative watersheds and winter temperatures were higher under vegetative watersheds than under non-vegetative control watersheds. Discharge concentrations were less than other green roof studies. Nitrate leaving the vegetative watersheds was higher than the controls while ammonium leaving vegetative watersheds was lower than the controls. Phosphorus leaving vegetative watersheds was greater than the controls while the standard plants and soil watershed had over twice as much phosphorus discharge as the biochar and alternative plants watersheds. Biochar as a soil additive appears to have some promise of reducing discharge of nitrogen and phosphorus.