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Avenue and the Lake Canal was quantified using a separatc hydraulic analysis and was included in thc model as a diversion. constant !low rate once the incoming flow reached the diversion capacity. Following completion of the hydrologic studies, avatiablediversion ratingc urve wasdevcloped for the 30-year and lOO.year diversion channels, and those projects were reevaluated with the HEC.l model. With the variable diversion rating curve, which more accurately retlects the project operation, more discharge would bc passed down the diversionchan- nel for events which e,;ceed the design capacity, This resul., in lower residual flows passing down Dry Creek. With- and without-project discharge frequency relationships for the lOO.yearand 30.yeardiversion are shown in tables 6.1 and 6-2. Bccauseofalackofdatatocalibratethemodel,sensitivityanalyseswcrecarriedout to help devclop best estimates fllrbasin parameters, Parameters included in thescnsiliv- ityana.lyses were uniform loss rates, the initial loss, channel Manning's nvalues, anl.! overland flow nvalues. Best estimates for these parameters were developed during a review meeting with representatives from the Omaha District, the City of Fort Collins, anl.! Larimer County. The effects of irrigation canals on peak flows were also e,;amim:d during the sensitivity analysis. Thecomplctcd model was used to develop discharge-frequency relationships at spe- cific loc;uions along the detailed study rt:ach from the Larimer and Well.! Canal to the conflllence with the Cache La Poudre River for both c,;isting and developed conditions. The modele(] flood peah wcre not ~moothed graphically due tu the effects of IIp,trcam diversions which create the situation where more frequent 1100d peaks arecausedhylocal runoff downstream of the Larimer and Weld Canal and larger less frequ<:nt peak> by rUl)- off from the entire b..,in. Resulting discharge t'rcque:lc)' relationships [or existing ~"rl developcdconditions arc shown in Table 4-1. loc~\iQn DISWetdCanal The model for developed conditions was uscd to evaluate various diversion alter- natives for protecting the detailed study rcach from the Latimer and Weld Canal to the conl1L.tcnce with the Cache La Poudre River. Diver.lions with capacities for the 10-, 50., 100-, and 500-yC<lr peak flows were evaluated al two locations: (J) the Larimer and Weld Canal. and (2)just downstream from Ea~t Vine Drive. Diversions at each locat\\"1 were evaluated separately and in conjunction with diversion; at the other location. ^ total of 12 divCr\ion alternatives were cXilmincd. Diversions were rnodelco:l witll a MOU1h 4-11 Table 4.1 Dry Creek Discha,ge Frequency Relationship Peak Discha,ge Inc.f.s_ Frequency Existing Developed [Yearsl Conditions !dlnditions 2 0 0 5 0 0 " 620 200 " 3110 3140 100 4520 4550 500 8S10 8950 2 ,eo 200 5 200 330 10 <30 500 50 2170 2140 '" 3290 3350 500 7080 7160 4.12