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I I I I I I I I I I I I I I I I I I I In Reaches 4 and 5 floodplain development has been regulated. The channel in developed areas is adequate for floods up to the 100-year event. Except for several culverts between Sheridan and Wadsworth Boulevards, flood hazards in these reaches are less than in the lower reaches. Development in Tributaries Band C has, for the most part, also been regulated. With the exceptions of the inadequate culvert below Wadsworth on Tributary C and shallow flooding above Lake Arbor on Tributary B, the channel capacity is generally sufficient along these two tributaries to convey the 100-year flows without major flood damage. A detailed discussion of the existing flood hazards on Little Dry Creek is included in the Flood Hazard Area Delineation Report. Summary of the Phase 'B' Plan The flood control plan selected by the Sponsors is outlined by Reach in the following paragraphs and illustrated schematically on Figures 1-1 through 1-4 and on the drawings, Sheets 1 through 28. The directive memorandum is included in Appendix A for reference, but the selected plan is essentially that recommended by the Consultants in the Phase 'A' analysis. Implementation of the selected plan in reaches 1,2, and 3 (Clear Creek to Sheridan Boulevard) will prevent the extensive damage caused by floods equal to and less than the 25-year event (Reachs 1,2A) and the 10-year event (Reaches 2B,3) and will provide parks and recreational opportunities, enhance land values, and improve the visual impact of the floodplain. Implementing the plan will provide improved water quality and reduce potential public inconvenience as well. Because the benefit/cost ratio of the plan is greater than one (see Table 1-1 of the Phase 'A' report), the plan will be an effective utilization of public funds. In Reach 1, a concrete-lined open channel capable of safely conveying the 25-year flow is proposed from West 64th Avenue to the confluence at Clear Creek where it terminates in a riprapped channel section. Bridges whose spans are minimized by use of this concrete lined channel are required at West 64th Avenue and at the railroad sidings. Upstream from West 64th Avenue, through the remainder of Reach 1 and all of Reach 2A, in Adams County, sell:~ted structural and channel improvements designed for the 25-year storm are proposed, pr'ncipally: construct grass-lined channel from the West 64th Avenue crossing upstream approximately 6,500 feet to the West 69th Avenue crossing; build bridges at the Colorado and Southern Railroad tracks just downstream from Federal Boulevard and at the West 69th Avenue crossings; enlarge the culvert beneath Federal Boulevard; construct a low flow crossing at Irving Street; construct a continuous hiker/biker/maintenance trail for access; obtain necessary right-of-way and easements; and relocate utilities as required, see Sheets 15 through 18. 1-2 A 10-year grass-lined channel is proposed for Reaches 2B and 3 from West 69th Avenue to just west of Winona Court in the City of Westminster, Sheets 12 through 14. Due to R.O.W. limitations, a concrete walled 10-year flood channel with a sloping grass- lined bed is proposed from West 72nd Avenue to just west of Winona Court. Alternate cost estimates are included which reflect reduced construction costs attainable by eliminating crossings at Utica and Vrain Streets and the Del Villa Apartment parking lot. After evaluation during the Phase "B" Study, it was considered best if this alternative or some form thereof remained as an option until the final design. No channel improvements are required for the short section of channel between Winona Court and West 75th Avenue because the floodplain can be regulated and required improvements made at the time of development, however, a possible 100-year grass channel section has been included for reference for this channel reach. From West 75th Avenue to Sheridan Blvd., a a concrete lined 100-year flood channel is proposed at the request of Westminster. Improved crossings at W. 75th Ave., W. 76th Ave. and Sheridan Blvd. will be required and some possible cross sections are illustrated. It will be necessary to acquire right-of-way in part of this reach where easements do not now exist. Because the improved channel sections in Reaches 1, 2A, 2B, and 3 will contain either the 10 or 25-year flood flows, flood insurance and regulation of the remaining 100- year floodplain are inherent aspects of this plan necessary to offset residual flood damage from larger floods. The focal point of the flood control plan in the Shaw Heights Tributary, Figure 1-2 and Sheets 21 through 25, is improving the detention pond just east of U.S. 36. The capacity of the reservoir is to be increased to contain the 100-year runoff, about 33 acre- feet. The proposed outlet is a 54 inch diameter reinforced concrete pipe extending beneath U.S. 36, continuing under Apex Lane in Reach AlC and outfalling south of the Colorado and Southern Railroad tracks. A 10-year storm drainage system is proposed for the residential area east of the existing detention pond. This system is designed with flows reduced by a detention pond to be constructed east of Lowell Boulevard and capable of storing the entire 100-year runoff of approximately 18 acre feet from upstream lands yet to be developed. The Channel and related structures in the lower reaches of the Shaw Heights Tributary are sized for the 100-year event, with a reduction in design flows attributable to the improved detention pond. In Reach AlB, an enlarged concrete-lined open channel is proposed above West 80th Avenue for approximately 820 feet. A grass-lined channel extends upstream from this point, fOllowing the existing alignment for approximately 1,200 feet before transitioning to a concrete-lined channel that intercepts the 54 inch