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

Abstract

Two greenroofs were constructed for research and demonstration purposes in eastern 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 Nature Center Greenroof was a 27 m2 greenroof with a 3% pitch; this extensive greenroof was constructed with 102 mm deep media. The extensive, 70 m2 WCC Greenroof in Goldsboro was constructed with two different media depths of 102 mm and 51 mm to research the effect of soil media depth on plant growth. Each greenroof was compared to a reference roof, referred to as the control roof, on site.

Each greenroof site was monitored with a Sigma 900Max™ automatic sampler to measure the amount of runoff that drained from the greenroof and a control roof at each site as runoff flowed over a weir. A rain gage was also installed at each research site to accurately measure rainfall on location. Data was gathered at the WCC Greenroof for a consecutive nine month period from April 2003 to December 2003. Data was gathered at the Nature Center Greenroof during a portion of July 2003, August 2003, November 2003, and December 2003. Less data is available at the Nature Center Greenroof due to equipment malfunctions at the site.

Because the two greenroofs are located approximately 48 km apart, the two greenroofs could not be directly compared; therefore, each greenroof was analyzed separately. Over the entire monitoring period, the WCC Greenroof retained 62% of the total 901 mm of precipitation observed 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 dependant upon the rainfall pattern, how moist the soil was from a prior rain event, and the seasonal affects on evapotranspiration rates. More precipitation will be retained during months with higher 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.

Water quality samples were taken of the greenroof runoff, the rainfall, and the control roof runoff at each site for select rain events. There were no statistical trends relating the concentrations or the mass loadings of nitrogen in the greenroof runoff to the rainfall or to the control roof at each site. However, in general, higher concentrations and higher mass loadings of nitrogen were observed in the greenroof runoff. The concentration and mass loading of phosphorus was statistically higher in the greenroof runoff than in the rainfall and the control roof runoff. This was an indication that the soil media was serving as an additional source of phosphorus to the greenroof runoff, whereas, the loading of nitrogen was more dependant upon the concentration of nitrogen in the rainfall and the volume of runoff observed from the greenroof.

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 soil mix. The results of this study indicated that less nitrogen will leach from media with less compost present in the mix and less phosphorus will leach from greenroof soil media with less compost present in the soil 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.