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The City of Fort Collins completed construction of the storm drainag~ improvements on Spring Creek from College Avenue through the BNRR in December 1990, Work on the reach from Stover Street to Stuart Street is underway and is planned for completion in 1993. The City of Fort Collins no longer has interest in Corps participation in construction of flood control improvements on Spring Creck bccauS<': the preliminary design for the remaining improvements upstream from Taft Hill Road has been com- pleted, and the Drake Road improvemems are to be eompkted by the City in a fairly short time frame. <ienifk.:>nr flOO'><"! h~7.arr1 exists along the lowermost 4 miles of the c~k. ThiS nood hazard may increase with continued development in the lo....ermost portion of the basin, Previous studies have examined the flood hazard, and various mitigation measures have been proposed, UYDROWGIC ANALYSIS To provide a solid basis for further studies on flood control alternative S,adetai1ed hydrologie analysis of the Dry Creek basin was carried out. A detailed rainfall-runoff model of the basin was developed to determine discharge-frequency rc1at ionshipsalong the detailed study reach, extending from the Larimer and Weld Canal to the confluence with the Poudre River. The model incorporates the interaction of the complex network of irrigation canals and reservoirs in the basin. The model was used to compute pcak f10ws under existing and fully developed conditions and to evaluate a ran geofdiversion alternat;ve.undcrfullydevclopcdconditions. Because the review of previous studies and flood problems indicates that flood control measures on Boxelder Creek and Spring Creek arc either economically inft\1sible, have been constructed, or are being constructed by the City of Fort Collins, the hydro- logieanalysis was limited to Dry Creek. Dry Cre<:k, lo.;atcd in Larimer County, Colorado, enters the ('-ache la I'oudre Rivcr 10 the vicinity of Fort Collins. [-and \ISC in the Dry Creek Basin is primarily agri- cultural. with urban d"vcl<'pment confined to the lower n1(lst portion of the basin. ^ system of irrigation canals and reservoirs makes the baSIl) hydrologically comple~. Beeallse of a low channel capadly and urbaninlion In tile lower portion ()f the ba.\in, a The V.S, Army Corps of Engineers HEC-l modd was used to simulatc the rainfalJ. runoff response of the hasin. Basin parameters were developed from topographic maps. aerial photographs. l.oningmaps, soil surveys, reservoir plans and operating criteria. and site visits. Data from adrainageway planning :;tlId)' covering a portlOn of the lower basin was "sed to include more detail in the model. A 6-hour duration storm was r1cvelofl..d forsimlllating the 2-, 5-, 10-, 50-,100-. and 500-year events. Starting reser- voir levds were set according to reservoir operating criteria. Irrigation canals were included in the model formulation as diversions with capacities set equal to the excess canal capadty over an assumed maximum irrigation flow (generally e\jllal to the decrt'c for a particular canal), The Larimer Cc>unty niteh and Tt,rry l.ake Inlet were :hS<tIllCJ to capture runoff from outside the basin and w~re explicitly included in the modd network. Larg~ :areas were assllmed to be non~'Onlribllting. wilh analyses perfurmed to verify the assumption. o..'crflo\\ to tile Cache La Poudre River bet\li~'\:n :-;mth College Severalconeernswereidentifi~dduringtherevie.....oftheprevioush)'df{llogicanaly- sh for Dry Creek, It was ddermincd that these oonccms warranted additional h}'drologic analysis at this time. A ne.... hydrologic analysis for Dry Creek was completed by Resource Consultants & Engineers, Inc. in Octo~r 1992 under contract with th~ Corps of Engincen, Omaha District. The hydrologic analysis report is cont.1.incd in Appc:n- di~ B. ^ summary of the hydrologic analysis is presented as follows: '~9 4.10