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. relationships along thedetaiJed study reach extending from the LarimCT and Weld Canal to the confluence with the Poudre River. The model incorporates the interaction of the complelt network of irrigation canals and reservoirs in the basin. The model was used to oompute peak flows under e:dsting and fully developed conditions, and 10 evaluate a range ofdivcrsion alternatives under fully developed conditions. HYDROLOGIC ANALYSIS Because the review of previous studies and flood problems indicates that flood control measures on Boxelder Creek and Spring Creek are either economically infeasible, have been consltUcled, or are being constructed by the City of Fort Collins, the hydrologic analysis \\-'as limited to Dry Creek. Several concerns were identifled during the review of the previous hydrologic analysis for Dry Creek. It \\-'as determined that these concerns warranted additional hydrologic analySis at this time. A new hydrologic analysis for Dry Creek was completed by RCSQurce ConsullanU & Engineers, Inc. in October 1992 under contract with the Corps of Engineers, Omaha District. The hydrologic analysis report is contained in Appendix B. A summary of the hydrologic analysis is presented as follows: The U.S. Army Corps of Engineers HEC-l model was used to simulate the rainfall-runoff response of the basin. Basin parameters were developed from topographic maps, aerial photographs, roning maps, soil surveys, reservoir plans and operating criteria, and site visits. Data from a drainagcway planning study covering a portion of the lower basin was used to include more detail in the model. A 6-hour duration storm wasdeve!oped for simulating the 2-,5-, 10-,50-, loa-, and SOD-year events. Starting reservoir ]evels were set according to reservoir operating criteria. Irrigation canals were includoo in the model formulation as diversions with capacities set equal to the excess canal capacity over an assumoo maximum irrigation flow (generally equal to the decree for a particular canal). The Larimer Coullty Ditch and Teny Lake Inlet wcre assumed to capture runoff from outside the basin and were explicitly included in the model network. Largc areas were assumed non- contributing, with analyses performed to verify the assumption. Overflow to the Poudre River between Nonh College Avenue and the Lake Canal was quantified using a separate hydraulic analysis and wasineluded in the model as a diversion. Dry Creek, ]ocated in Larimer Count)' Colorado, enters the Poudre River in the vicinity of Fort Collins. Land use in the Dry Creek Basin is primarily agricultural, with lIrban development confined to the lower most portion of the basin. A s)'stem of irrisation canals and reservoirs makes the basin hydrologically complex. Due to a low channel capacity and urbanization in the lower portion of the basin, a sigr'lifi(,M! "('1{'01 hnar{l ('xi<l< a1nnz !h(' l(lw('rmn~r 4 mill'< nf thl' (':fl'f'_1.:. 11Ii. flnnI1 hazard may increase with continued development in the lowermost portion of the basin. Previous studies have examined the flood hautd, and various mitigation measures have been proposed. Due to a lack of data toea1ibrate the model,sensitivitya.'alyses were carried out to help develop best estimates for basin panmeters. Parameters included in the sensitivity analyses were uniform loss rates, the initial loss, channel Manning's n \'alues, and overland flow n values. Best estimates for these parameters were developed during a review meeting with "1lres.enllltives from the Omaha Dimiet, the To provide a solid basis for futther studies on flood control alternatives, a detailed hydrologic analysis of the nry (;r('t'k h~<in w;'!< r..::lml'd out. A detailed rainfall-runoff model of the basin wasdevelope(\ to determine discharge-frequency . 4-10 4-9 . .