Stormwater September 2012 : Page 41
was not targeted, as the prevalence of subsurface structures, including utilities, vaults, and subway tunnels, complicates the feasibility of using green infrastruc-ture source controls reliant upon infi ltra-tion. Rooftop controls piloted in the other boroughs will likely be more suitable for application in Manhattan. Pilot locations were selected to rep-resent a range of potential future imple-mentation sites within the city, including along streets, on rooftops, within parks, within public housing facilities, and within parking lots. These public prop-erties contain various confi gurations of impervious surfaces, are controlled and maintained by different agencies within the city, and provide unique challenges and benefi ts. As most of NYC has been developed, all of the pilots were con-structed as retrofi ts. Streets and adjacent sidewalks are of interest for stormwater management, as these surfaces cover more than one-quarter of the total land area within some neighborhoods of New York City. Furthermore, these impervious surfaces are often contiguous, with runoff fl ow-ing along curb lines until it is collected by catch basins, often located at street intersections. This confi guration, plus public ownership, presents advantageous opportunities for green infrastructure im-plementation within street rights of way. Street-side bioswales are currently the predominant source control used to man-age stormwater runoff from these areas. Rooftops are another practical loca-tion for green infrastructure. On some blocks within the city, rooftops can make up the majority of impervious surfaces, presenting opportunities for stormwater management directly at the source. Roof-tops are generally unused space with limited public access and are especially valuable for stormwater management in areas where ground-level space for source controls is limited. Blue roofs and green roofs were both implemented on rooftops as part of the green infrastruc-ture pilot program. Blue roofs consist of structures that detain water directly on the rooftop. Various confi gurations, in-cluding modifi ed inlets, check dams, and trays, are being evaluated through the pilot program. Green roofs also manage runoff directly on the rooftop, but incor-porate a greater focus on retention and evapotranspiration through the use of an engineered soil media and vegetation. Parks, public housing facilities, and parking areas distributed throughout the boroughs also present opportunities for stormwater management on pub-lic property. These properties offered chances for implementation of a variety of green infrastructure source controls including bioswales, larger bioretention installations, rooftop practices, permeable pavement, and subsurface detention and infi ltration systems. Implementing stormwater retrofi ts can present obstacles in any area; however, when implementing green infrastructure source control retrofi ts in an ultra-urban area like New York City, there are some unique challenges and benefi ts. As previ-ously mentioned, pilot green infrastruc-ture implementation focused on public properties within the city. These locations were selected to facilitate coordination September 2012 www.stormh2o.com 41