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I I I I m I I I I I m m I - I I I I I Lower Big Dry C:;:;reek Hydrologic Study 7.3.2 Full Spectrum Detention Concept Wulliman and Urbonas (2005) developed the full spectrum detention concept because they recognized that even though stormwater deJention has been used on individual sites for many years to control peak stormwater runoff flow II rates in urbanized areas, the effects of multiple detention basins in urban areas were not we!] understood. They built upon work by Urbonas and Glidden (1983) that suggested that storrh releases to pre-development levels could be accomplished just downstream of each basJn, but as the watershed increased in size, and the numbers of on-site basins also increased, thJ peak flows in the receiving waters were not being controlled to pre-development levels. In sJrnary, Urbonas and Glidden (1983) found that when hydro graphs are individually released from detention basins at low flow rates over extended periods of time, their flows can add to each ither as they travel downstream, potentially resulting in increasing peak flows along the receiving, waters as the urbanized watershed sizes served by on-site detention basins get bigger. Urbonas and Glidden (1983) concluded that something more needed to be done to control post-development flows. Two basic approaches were identified to LIP counterbalance this problem. First, UDFCD published Volume 3 of the Urban Drainage and Flood Control District Manual in 1992. This manual officially recognized the need reduce stormwater surface runoff volumes and flow rates I that occur during a large number of small I storms to the maximum extent practicable. This resulted in the metro Denver area moving beyond the practice of just controlling the peak flow rates from the larger events such as the 5- ttkoUgh 100-year storms and additionally controlling I the "Water Quality Capture Volume" (WQCV) to release it over an extended period of time. (Most of the municipalities in the Big D& Creek watershed now implement this practice, gradually phasing it in over the last five year~ or so.) Some ofthe practices recommended in the manual reduce runoff volumes, as well as Jeak flow rates, while some only control flow rate, I leading to the need for additional steps to im~rove detention sizing practices, which is the subject of the Wulliman and Urbonas (2005) paper. The second issue was the need to improve detention sizing practices based on the observation of significant geomorphic changes (e.g., downc~tting and erosion) observed in gulches and stn::ams, . d" h fr 1 . I m lcatmg t at equent y occumng smallyr runoff events needed to be better controlled. I 971-179.092 Wright Water Engineers, Inc. Page 60 June 2005 I I I ,