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I I I I I I I I I m m I I I I I I I I . Lower Big Dry Greek Hydrologic Study I I In Water Resources Engineering (Mays 2001), detention and retention facilities are identified as approaches to mi{igate the negative impacts of urbanization on stormwater drainage, primarily inc~eases in both volumes and velocities. In the ASCE and WEF (1992) mLual of practice titled Design and Construction of I Urban Storm water Management Systems, design characteristics of detention facilities are identified as being dependent upon the local objectives, which may differ across the country. Larger regional facilities are generally identified as being more effeetive I than more numerous on-site facilities, but institutional arrangements for regional facilities are identified as being Jore challenging. To be most effective, the manual recommends control of multiple flow events that include both infrequently and frequently occurring storms. . . In the text Urban Hydrology, Hydraulics and Storm water Quality, Akan and Houghta1en (2003) note that urbah development results in increased runoff volumes and flow rates, which may ca~se more frequent flooding and severe erosion downstream. They note that man~ communities use "flow retardation" structures to minimize the adverse downstream effects of urban stormwater runoff and to enhance stormwater quality. They discus~ and provide design information for various flow retardation structures such as dete~tion basins, retention basins, infiltration basins and others. 7.2 Case Studies in Colorado Four watersheds in Colorado were selected as case studies of interest relative to Big Dry Creek: the Fountain Creek, Chatfield Reservoir, Bek Creek and Rock Creek watersheds. Many more cases exist, but these few were selected due to their Front Range location. Highlights of these case studies, including reasons that communities have chosen to invest in these watersheds, follow. 971-179.092 June 2005 Page 52 Wright Water Engineers, Inc. I I