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I I I I I I I I I I I I I I I I I I I Lower Big Dry Creek Hydrologic Study 6. Given the downstream complaints about high water and flooding, local governments in the urbanizing areas should consider use of existing provisions in their drainage criteria to require stricter stormwater management in areas with known drainage problems. In many cases, this would mean more rigorous implementation of principles already in the UDFCD's Urban Storm Drainage Criteria Manual and adopted by the cities. Examples of the types of existing provisions that could be emphasized include: a. Require that developers mmlmlze directly connected impervious area to the maximum extent practicable and implement other "low impact development" techniques such as porous landscape detention, grass lined swales, porous pavement and other te:chniques that promote infiltration of runoff, particularly for frequently occurring storms. Because of maintenance issues and limited benefits of underground detention relative to other stormwater management strategies, underground detention should be discouraged and used only as a last resort. b. Require that developers evaluate and consider the downstream impact of their development and implement appropriate stream stabilization techniques. c. Strongly consider requiring outlet structures in new detention facilities that more closely approximate the pre-development flow-frequency curve based on the concepts developed by Nehrke and Roesner (2004) and Wulliman and Urbonas (2005). 7. From an engineering and regional perspective, the BDCW A should aggressively support regional open space planning along Big Dry Creek in the near-term because of the current rapid pace of development. The City of Thornton Open Space Master Plan has targeted the Big Dry Cn;:ek floodplain in Adams County for high-priority land acquisition. Open space taxes in Adams County and Thornton provide a realistic funding mechanism for open space acquisition. Many options exist for landowners to remain on their property and dedicate easements through a variety of tools described in the City of Thornton Open Space Master Plan. Aside from engineering issues, however, it should be noted that complex social, economic and cultural issues must be taken into account when considering private property acquisition. In the event that conservation easements or 971-179.092 June 2005 Page 8 Wright Water Engineers, Inc.