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I I I I I I I I I I I I I I I I I I I Alternative F: Detention and/or Retention Facilities Reservoirs for the control of floods are a means of supplying flood protection when the natural reservoir storage potential of a watershed may not provide as much storage as is needed. The amount of storage required depends upon the degree of protection needed and the non-damaging capacity of the channel. The effect of storage is to decrease the peak of the flood in the reaches of channel immediately below the dam. Stored water is then released at a rate compatible with downstream conditions. ~ Flood detention already exists on DFA 0054, both directly and indirectly. Further containment of storm waters may be accomplished by enlarging the Medema Ponds and constructing a new facility just upstream of 112th Avenue. The primary benefits would be realized in Reaches 1, 2, and 3, in the form of reduced flow peaks, as is shown in the peak flow diagram, Figure V-3. Because the channel is greatly inadequate in the lower reaches, some additional flood control measures may need to be implemented in conjunction with construction of additional detention facili ties. Alternative 1: Relocating Channel to Routes Other than Historic Channel The channel relocation alternative refers to structural methods of rerouting the flood channel, possibly even out of the damage basin, in order to reduce construction and operation and maintenance costs. It entails the use of lined flood channels and possibly the renovation or ,alteration of road or ditch crossings. There are only two places on DFA 0054 where relocating the channel is economically feasible. This alternative was considered in Reach 3 for conveying low or trickle flows down 112th and turning a right angle at Holly Street. An interceptor ditch was also considered west of Riverdale Road to convey runoff from the laterals of Reaches 2B and 2C. On all other reaches of DFA 0054, flood control improvements should follow the general course of the existing channel. Water Quality Together with the concern about the quantity of urban runoff is concern over the quality of storm water and its impact on the natural ecology of DFA 0054 and other urban waterways. The nationwide significance of pollution caused by storm-generated discharges was first identified in the 1964 U.S. Public Health Service's publication on the "Pollutional Effects of Stormwater and Overflows from Combined Sewer Systems." Congress, in recognizing this problem, authorized funds under the Federal Water Pollution Control Act of 1965 and following legislation for the research, development, and demonstration of techniques for controlling this source of pollution. The 1972 Amendments place new and stronger emphasis on urban runoff as a source of pollution. "An accelerated effort. . ." is stressed ". . . to develop, refine, and achieve practical application of waste management methods applicable to nonpoint sources of pollutants to eliminate the discharge of pollutants including, but not limited to, elimination of runoff of pollutants . . ." There is no doubt that pollutants are entering DFA 0054 in stormwater, but there is presently limited data with which to assess the severity of the problem. Some storm water pollution data has been collected as part of a study being carried out in the Denver Metropolitan Area by the U.S.G.S. under the joint sponsorship of the Urban Drainage and Flood Control District, the Denver Regional Council of Governments and the Denver Water Board. Runoff quantity and quality measurements were taken on three local drainageways during 1976 and 1977. Initial sampling results indicate that the major components of local storm water pollution are organic compounds, heavy metals, oils and greases. Concentrations of these pollutants vary, but preliminary indications are that organics are approximately equivalent to secondary sewage treatment effluent and, the heavy metals, oils and greases are predomimately in theform of suspended solids. V-7