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- . Tal>I.4.1 Dry eu.k Oischarge Frequency Rrlationship City of Fort Collins, and Larimer County. The efflX:ts of irrig.ation canals on peak flows were also ct.arnined during the sensitivity analysis. The completed model was used to develop discharge-frequency relationships at specific locations along the detailed study reach from the Larimer and Weld Canal to the confluence with the Poudre River for both existing and developed conditions. The modeled flood peaks were not smoothed graphically due to the effects of upstream diversions which create the situation where more frequent flood peaks are caused by local runoff downstream of the Larimer and Weld Canal and larger less frequent peaks by runoff from the entire basin. Resulting discharge frequency relationships for existing and developed conditions are shown in Table 4-1. Location FT<'queney ~ D/SWelctCanaJ , , >0 " >0, '" Mouth , , >0 " 100 '00 The model for developed conditions was used to evaluate various diversion altematives for protccting the detailed study reach from the Larimer and Weld Canal to lheconflucnn: with the Poudre River. Diversions withca=cities for the 10- 5Q.. ,-' , , 1QCl., and 500-ycM peak flows were e....aluated at two locations: (1) at the Larimer and Weld Canal and (2) just downstream of fut Vine Drive. Diversions at each location were cvalualed separately and in conjunction with diversions at the other location. A lotal of twelve diversion alternatives were examined. Diversions were modeled with n constant flow rate once the incoming flow reached the diversion cap""iiy. Fvii,w..i::;;co:n;o!~';"n of the hydrolog.ic studies. a variable diversion rating curve was developed for the 30-year ilml 'lao-year diversion channels and those projects were re_cvaluatcd with the HEC-l model. With the variable diversion rating curve, which more aa:uratcly reflects the project operation, mor~. discharge would be pass<<l down the diversion channel for events which exceed the design capadty. This results in lower residual flows passing down Dry Creek. With and without prujc.;( Ili:.eharf;e f~'.lenGY 1"f'1"t;on~hiF~ for the ]OO-yearand 3G-veardiversion are shown in Tablc5 6-1 and 6-2. . 4-]2 4.11 Peak Diocharge in d... Existing Condition. , , '" 3110 4520 8910 '" '" ." 2170 "'" 7M' . D.""lol'ed Condinonl , , 700 3140 4550 8950 '00 '" '00 "" 3350 1160 .