Stormwater September 2012 : Page 44
or seep from the bottom of the system. Generally, these enhanced tree pits and street-side infi ltration swales receive run-off from drainage areas between 2,000 and 6,000 square feet. Monitoring results indicated that these source controls were frequently able to retain runoff from storm events with 1 inch of rain or less (Figure 3). As there were no underdrain systems, all runoff that did not overfl ow or bypass along the curb was retained. Furthermore, re-sults from early monitoring efforts (not shown here), in conjunction with onsite evaluations, illustrated the importance of effective curb cut designs. Initial curb cuts had a closed frame structure to maximize consistency with existing curb lines; however, these curb cuts were frequently blocked by litter and other debris, hindering source control perfor-mance. Installation of open curb cuts greatly improved runoff capture perfor-mance. Subsequent bioswale designs in-corporated not only was an open curb cut but also a depressed concrete apron around the curb cut, further improving runoff capture effi ciency. North and South Conduit Avenues Bioretention. Another bioretention pilot site built in the fall of 2011 is a pair of bioretention areas within a highway me-dian between North and South Conduit Avenues in Queens, NY (Figure 4). These bioretention areas receive runoff from nearly 82,000 square feet of impervious area, in addition to pervious contribu-tions within the median. The vertical pro-fi le of the bioretention areas consists of a 6-inch surface depression, 2 feet of sandy engineered soil media, and a 1-foot stone drainage layer. An underdrain system connects the subsurface storage layers, while an overfl ow channel hydraulically connects the bioretention surface storage during large storm events. A single over-fl ow grate structure in the eastern area combines surface overfl ow with underd-rain outfl ow and conveys that water to Figure 4. Conceptual plan of North and South Conduit Avenues bioretention, with inset photo of curb cut flume 44 September 2012 www.stormh2o.com