Stormwater September 2012 : Page 46
bioretention areas were generally installed with a 6-inch sur-face depression, 12-inch engineered soil layer, and 8-inch stone drainage layer. Runoff is diverted into the bioretention areas using curb cuts, which incorporate fl agstone fl umes and sumps immediately downstream to reduce erosion potential and fa-cilitate litter and debris collection. These bioretention areas include underdrain pipes within the stone drainage layer that drain to existing catch basins onsite. Most bioretention areas exhibited little or no measurable outfl ow for smaller storm events during the 2011 monitoring period. While there were variations in the performance of in-dividual bioretention areas, likely attributed to differences in contributing drainage areas, these source controls generally re-tained 80 to 100% of storms smaller than 1 inch, even with an unrestricted underdrain system (Figure 6). Detailed evaluations of pilot performance suggest that runoff was detained at the surface and within the bioretention soil, subsequently seeping into the underlying soil. Even with surface detention, the biore-tention areas drained relatively rapidly. With a median surface drawdown duration of fi ve minutes, the bioretention surface frequently drained before a storm event ended. Bronx River Houses: Subsurface Systems. Subsurface detention and infi ltration systems were also installed under existing parking lots at the Bronx River Houses complex. A stormwater chamber system receives runoff from nearly 4,000 square feet of impervious parking area, while a per-forated pipe system receives runoff from more than 13,000 square feet of impervious parking area and surrounding sidewalks. Both of these systems incorporate a baffl e struc-ture for pretreatment and contain a restrictive orifi ce plate on the downstream end. These systems were designed to comply with DEP’s new stormwater performance standard, which limits peak discharges to 0.25 cubic foot per second during a 10-year storm at these sites. Although runoff deten-tion is the primary intended role of these facilities, they were designed and constructed with an open bottom in contact with in-situ soil to encourage infi ltration losses. Despite a design that focused on detention, the open-bottom stormwater chamber system was able to fully retain stormwa-ter runoff for most storm events. Measurable outfl ow only oc-curred during large or intense storms. During events that gener-ated outfl ow, measurable outfl ow generally ceased while runoff infl ow was still being measured, indicating seepage losses af-fected overall source control performance. Because runoff was diverted from a retrofi tted existing catch basin, there was no evidence of fl ow bypass similar to what was observed at some of the source controls with curb cuts. Preliminary observations were not available for the 2011 monitoring period at the perfo-rated pipe system due to later construction and ongoing evalu-ations at that facility. Far Rockaway Permeable Pavement. Within the Far Rock-away section of Queens, permeable pavement was installed within a DOT park-and-ride facility. This source control pilot consists of a porous asphalt section, a permeable pavement sec-tion constructed from crushed glass, and a standard asphalt sec-tion for comparison purposes (Figure 7). The porous asphalt and crushed glass pavement cover approximately 6,400 square feet and 4,200 square feet, respectively. The permeable pave-ment sections did not generate measureable underdrain out-fl ow during the 2011 monitoring period, suggesting that sub-surface stone storage and underlying soil infi ltration rates were signifi cant contributors to retention. During 2011, surface run-off from the permeable pavement sections was not monitored; however, onsite testing suggested small amounts of surface runoff may have been generated by the porous asphalt section during intense storm events. Additional monitoring equipment has been installed since to quantify any surface runoff during the remainder of monitoring activities at this site. Additional Monitoring Activities. Additional implemen-tation and monitoring of source control pilots is underway throughout New York City. Beyond the pilots discussed here, blue and green roof facilities throughout the city, a series of bioretention areas installed within an existing park and DOT park-and-ride facility, and a stormwater wetland installed along the edge of a parking lot are being monitored. Furthermore, widespread implementation of green infrastructure within lo-calized priority sewersheds is underway to evaluate the effect of green infrastructure implementation on overall sewer fl ows. Preliminary Conclusions Within New York City, there are opportunities to manage stormwater runoff by implementing a range of green infra-structure source controls within various types of public prop-erties. The New York City green infrastructure pilot program has illustrated that green infrastructure can provide an array of benefi ts, but implementation is not without challenges. Within an ultra-urban area like NYC, careful planning, design, and im-plementation efforts are needed to ensure green infrastructure implementation is effective, addressing everything from site constraints and maintenance considerations to public percep-tion and involvement. Quantitative pilot monitoring activities illustrate that substan-tial retention of stormwater runoff is possible, even in one of the largest and most densely developed urban areas in the Unites States. In fact, many of the green infrastructure source controls monitored to date only discharged fl ow to the sewer system during large or intense storm events, alleviating pressure on the Figure 6. Stormwater retention provided by the Bronx River Houses bioretention areas during the 2011 monitoring period 46 September 2012 www.stormh2o.com
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