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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />SECTIINTHREE <br /> <br />Conceptual AlternaUVes <br /> <br />The conceptual alternatives for flood control in the Dry Creek basin were selected based on <br />previous recommendations from the GAl 1980 report and newly selected alternatives. The <br />conceptual alternatives ranged from diverting the peak flow to storing the entire flood volume <br />above the Larimer and Weld Canal. The objective for the flood control alternatives selected was <br />the elimination of the flood threat downstream of the Larimer and Weld Canal during storms up <br />to the 100-year event. Conceptually, the alternatives considered were to I) pass the peak flow of <br />approximately 5,000 cfs at the Larimer and Weld Canal around the damage area to the Cache la <br />Poudre River, 2) store the 100-year flood and reduce the peak flow through the damage area to a <br />safe discharge or 3) a combination of storage and diversion to alleviate the damage area. <br /> <br />Three locations were considered for diverting flows from the confluence of Dry Creek with the <br />Larimer and Weld Canal to the Cache la Poudre River. The diversion locations are Willox Lane, <br />East Vine and Boxelder Creek Diversions. The locations of the diversions are shown in Figure <br />3-1. The East Vine and Boxelder Creek Diversions require using the Larimer and Weld Canal to <br />pass flood flows originating to the north of Fort Collins. <br /> <br />3.1 PEAK FLOW DIVERSION (1980 RECOMMENDED PLAN) <br /> <br />A diversion to pass the peak flood flows from the 100-year event would require a capacity of <br />approximately 5,000 cfs. The East Vine and Boxelder Creek Diversions were discounted from <br />additional consideration due to the potential size of the structure needed to convey the flow from <br />Dry Creek to the East Vine or Boxelder locations. The highly developed areas surrounding the <br />Larimer and Weld Canal provide little room to expand the canal and normal operation of the <br />canal would be affected. The Willox Lane diversion was analyzed in detail and the results are <br />presented in Section 4 of this report. The Willox Lane Diversion produced a relatively short <br />distance between Dry Creek and the Cache la Poudre River while minimizing impacts to the <br />Larimer and Weld Canal. <br /> <br />3.2 COMPLETE FLOOD STORAGE <br /> <br />The complete storage of flood flows at the Larimer and Weld Canal was excluded as a potential <br />solution to the Dry Creek flood control plan. The 100-year event generates a volume of <br />approximately 2,200 acre feet of runoff above the Larimer and Weld Canal. No potential dam <br />sites with a sufficient volume of detention in the lower portion of the Dry Creek watershed could <br />be identified. <br /> <br />3.3 FLOOD DETENTION AND DIVERSION <br /> <br />A combination detention/storage and downstream diversion was considered. The size of the <br />residual peak flood flows eliminate a large diversion. The flood storage component of the <br />alternative was intended to reduce the peak flow in Dry Creek at the Larimer and Weld Canal to <br />a more manageable discharge, A peak discharge of approximately 500 cfs was assumed <br />reasonable as the target for the design discharge at the Dry Creek confluence with the Larimer <br />and Weld Canal to pass around the damaged area. All diversion locations were evaluated for <br />500 cfs. This alternative reduced peak flows from upstream ofthe damage area and potentially <br />reduces the size and cost of upgrading road crossings for County Road 19 and Highway 287. <br /> <br />T:\Projectsl6824660Dry CrkDiversionLarimerCountylSub_00I12.0 Word ProclFlood Control P1an-Rpt.doc 04/24100(11 :14 AM) 3-1 <br /> <br />