A North Carolina Field Study to Evaluate Greenroof Runoff Quantity, Runoff Quality, and Plant Growth

MORAN, AMY CHRISTINE. A North Carolina Field Study to Evaluate Greenroof Runoff Quantity, Runoff Quality, and Plant Growth. (Under the direction of Gregory D. Jennings and William F. Hunt) Two greenroofs were constructed for research and demonstration purposes in North Carolina; the first was constructed at the Neuseway Nature Center in Kinston in April 2002 and the second greenroof was constructed at Wayne Community College (WCC) in Goldsboro in May 2002. The 27 m Nature Center Greenroof, with a 3% pitch, was constructed with 102 mm deep media. The relatively flat, 70 m WCC Greenroof was constructed with two different media depths of 102 mm and 51 mm to research the effect of media depth on plant growth. Each greenroof was compared to a reference roof, or a control roof, on site. Data was gathered at the WCC Greenroof for a consecutive nine month period from April 2003 to December 2003 and at the Nature Center Greenroof during a portion of July 2003, August 2003, November 2003, and December 2003. The two greenroofs were not directly compared because they are located approximately 48 km apart; therefore, each greenroof was analyzed separately. Over the entire monitoring period, the WCC Greenroof retained 62% of the total 901 mm of precipitation recorded at the site. The Nature Center Greenroof retained 63% of the 262 mm of precipitation measured. Variability of percent precipitation retained within each month of data was dependent upon the rainfall pattern and the seasonal effects on evapotranspiration rates. Percent flow reduction of the greenroof was determined by comparing the greenroof runoff peak flow and the peak rainfall rate. The Nature Center Greenroof observed an average 87% reduction in peak flow from the greenroof during the monitoring period. The WCC Greenroof observed an average peak flow reduction of 78% during the monitoring period. Average Rational runoff coefficients were developed from the runoff flows observed at the WCC Greenroof. The average greenroof Rational coefficients ranged from 0.25 for storms between 25 mm and 39 mm to 0.55 for rain events greater than 39 mm; typical rooftop Rational coefficients range between 0.75 and 0.95. Water quality samples were taken of the greenroof runoff, the rainfall, and the control roof runoff at each site for select rain events. While no statistical trends were observed, higher concentrations and higher mass loadings of nitrogen were generally observed in the greenroof runoff. The concentration and mass loading of phosphorus were statistically higher in the greenroof runoff than in the rainfall and the control roof runoff. This was an indication that the media was serving as an additional source of nitrogen and phosphorus from the greenroof runoff. If a media had been used which did not have high nitrogen and phosphorus loadings in the initial composition, fewer nutrients would have leached from the media into the greenroof runoff. A brief laboratory study was performed to determine the leaching effects of nitrogen and phosphorus from various greenroof soil media with varying percentages of compost in the media mix. This laboratory study provided vital information for greenroof media composition: less nitrogen and phosphorus will be present in the greenroof runoff when less compost is included in the mix. Results also indicated that concentrations of nitrogen leaching from the soil media will decrease to a minimum over time. Plant growth in 102 mm deep soil media was determined to be significantly higher than plant growth in 51 mm deep soil media. Extensive greenroof plant species recommended for growth in North Carolina are Delosperma nubigenum, Sedum album, Sedum album murale, Sedum floriferum, Sedum reflexum, Sedum sexangulare, and Sedum spurium fuldaglut.