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I I I I I I I - I I I I I m I I I I I Lower Big Dry <:preek Hydrologic Study . Palhegyi, Mangarella, Strecker, BicJell and Sen (2004): controlling the amount of post, I development "work" done on the stream. Other general themes present in adVanCld stormwaler management programs are also summarized. 7.3.1 DesiqninQ Stormwater Facilities to Better Match Flow FreQuencv Curve In 2004, Nehrke and Roesner published theirl findings related to the effectiveness of the state-of, practice for urban storm runoff control. They began with the widely used concept of peak flow attenuation (i.e., reducing the peaks of posLdevelopment flows to predevelopment peak flow levels) for given return interval storms. The~ noted that in most cases, flow controls target low frequency events (storms equal to or 1argdr than the 2-year event); but not smaller, higher frequency events. They noted that the more recent emphasis on removal of pollutants in urban runoff has targeted control of high frequency rvents (smaller than the 2-year storm). From this starting point, Nehrke and Roesner examined how well post-development flow controls performed using various combiJations of design events compared to the pre- development flow regime. Performance wls judged on how well the post-development flow frequency curve matched the pre-developme~t curve. Continuous simulation, using 50 years of hourly rainfall records, was performed on a iite in Atlanta and a site in Fort Collins. Detention I ponds with a variety of orifices sized for ~eak flow control using traditional and innovative methods were examined alone and in conAll1ction with extended detention best management practices (BMPs) for water quality. I I Roesner and Nehrke found that the water q~ality BMP exerted at least some flow control in a I majority of storm events at both sites. Everi greater effectiveness was observed when the BMP was paired with orifices designed to control ~he 1- or 2-year storm. More specifically, as shown in Figure 9 for Fort Collins, when the BMP las combined with the 100/2-year peak flow control, it performed better than the 100/10/2-year or1100/10/BMP systems for controlling events smaller I than the 2-year flow, but for events betwebn the 2-year and 4-year storm, the 100/10/2-year I combination provided significantly better control. Going a step further, adding a fourth orifice to the flow control structure allowed for more domp1ete control of the flow frequency curve, which I Wright wrr Engineers, Inc. I 971-179.092 June 2005 Page 57