Stormwater retention by a Baltimore greenroof and watershed-scale implications

Background/Question/Methods: Greenroofs are rooftop gardens that are planted in cities in order to address a number of environmental concerns including excessive stormwater runoff.   In Maryland, for example, greenroofs are now considered an acceptable technology to achieve site design requirements by the Maryland Department of the Environment. However, more information is needed regarding how greenroofs respond to storms of varying size and frequency. Custom-constructed weirs were installed at the downspouts of both a control and a greenroof in the late summer of 2008 in the Herring Run watershed.  Results/Conclusions: Hydrographs depicting runoff from both roofs were compared.  Stormwater retention by the greenroof ranged from 75% to 30% for storms of increasing size compared to minimal retention by the control roof.  Implications for this variation at the watershed scale were assessed through a case study of the Glyndon watershed, a suburban watershed that is part of the network being monitored by the Baltimore Ecosystem study.  GIS imagery for Glyndon was used to identify roofs with the appropriate slopes for greening.  This roof area information, combined with the results of the monitoring study, suggest that 8 million gallons of stormwater could be retained by greenroofs in this watershed annually, representing a 10% decrease in total exports.