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. . " Both the City of Fort Collins and Larimer Count)' participate in the National Flood Insurance Program and, as such, have adopted flood hazard area regulations in conformance ....ith the program. These regulations contain specific guidelines for development within the tOO-year flood plain and fOf the ICIO-year flood way. The flood plain regulation requires that the construction of a new structure or the major rehabilitation of an existing facility must be flood proofed or elevated to at least l'h feet above the estimated level of a lOO-year flood. This requirement includes basements. The floodway regulation further limits development in the floodway 10 those which will not result in an increase in flood levels during the occurrence of a lOO-year flood. Because the floodway is located within the lOO-year flood plain, all flood plain regulations also apply to tltis are<I. The City and county flood plain and f100dway regulations greatly limit the potential use orland for development purposes in the Dry Creek flood plain. The flooclway regulation is especially limiting bec.ause it requires that thc dcveloper show that proposed land use will not raise the estimated l()Q.year flood elevation. The Dry Creek flood way has an approximate width of from 600 to 1,200feet in the city and I,OOOto 1,500 feet in the area. under county jurisdiction neat the Fort Collins Ailpark. . Weld Canal to the Cachc La Poudre River, a culven improvement under severa] road crossings, and diversion of flood flows to Terry Lake. The City has no! initiated any of the recommended improvements; however, it requested the Corps review the 1980drainageway plan to determine the extent ofFederaJ interest in cost-sharing construction of flood control measures. For that reason, the previous plan is not discussed in detail here. It is discussed in the Plan Formulation and other sections of this report. EXISTING DRAlNAGEWA Y PLANS A!I."D lMPROVEME.1'\'TS BOXElDER CREEK A drninageway plan for the lower portion of BoxeJder Creek was prepared for the City of Fort Collins in August 1981 by Simons, Li & Associates, Inc. The plan evaluated the flood problem from Statc Highway I4 to the confluence with the Cache La Poudre River. Alternatives that were considered included flood plain regulation and channel improvements. It was determined that channel improvements on Boxelder Creek would not be economically feasible based on the levels of existing development in this reach, with a first cost of SI,340,00Q(in 1981 dollars) and a benefit-cost ratio of 0.22. However, the plan suggested some lOO-year channel improvements downstream in order 10 carry flows from Cooper Slough and Boxelder Creek with proposed channel improvements. The plan suggested thaI some channel improvements be made in order to reduce the flooded areas and the overtopping of roads and irrigation canals downstream from Larimer and Weld Canal. It recommended enlarging both the siphon under Larimer and Weld Canal and the flume under Lake Canal, enlarging Ihe channel downstream from the Lake Canal crossing to Prospect Street, and enlarging the Prospect Street bridge opening to convey 2,OOOcubie feet per second (e.f.s.). Downstream from Prospect Strtet, the Boxelder Creek channel would retain its natural stale. The proposed access roMs for the sludge application area which parallel the creek onei!her side would be used 10 confine the iiood ilows Wlthlll the Channel for aoout I mile. The last DRY CREEK A drainageway plan for Dry Creek was prepared for the City of Fort Collins in January 1980 by Gingery Associates, Inc. (Gingery). The plan evaluated seven! alternative methods of accommodating flood flows up to the loo.year flood event. Four economically feasih]e alternatives were identified in the plan. The feasible alternatives ine1uded channel improvements, including a channel bypass around Alta Vista, a channel diversion from IIpstream of the Larime-r and 4-' 4-6 .