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. CHAPTER 6 . EV ALVA TJOX OF WTIH-PROJECTC01'\'DmON PLANS and Weld Canal for the loa-year channel diversion plan, and 10 allow events greater than the 3D-year event to pass into Larimer and Weld Canal for the 30-year channel diversion plan. In addition, low f\ows up to the S-yearevenl would be allowed to pass into the canal to {acilitate downstream diversion of irrigation water for both the lOO.year and the 3D-year channel diversion plans. The diversion weir would consist ofa 160-foot-long, rock-lined overflow section with a maximum height ofS feet. The backslope would be IV on ]OH. A small, l.S-foot.high rock sill would be placed at the entrance to the channel diversion to force flows of less than the5-yearevent to pass directly into Larimer and Weld Canal. . The de~ription and analysis of the ....ith-project conditIon plans (lOQ-year plan and3o-yearplan) that were studied during the reconnaissanee extension,thelreffects on flooding, their potential impacts, plan cost estimates, and economic summaries are containedinthis.section. DESCRIPTION OF PLANS A lOO-year channel diversion and a 3o-year channel diversion, both along alignment B were evaluated. The alignment B channel diversion would begin in the lower portion of reach A, about 500 feet upstream from Larimer and Weld Canal, and flow southwest, crossing the canal about 300 feet west of where the existing Dry Creek channel erosses the canal. The channel would cross Willox Lane and the UPRR tracks, extend south for about 2,500 feel through an open area west of Hickory Village Mobile Home Park, then extend southeast and discharge into the Cache La Poudre River. Both plans would require relocation of the homestead south of Willox Lane and west of the UPRR. Larimer and Weld Canal would pass under the Dry Creelr. channel diversion through a 12-fooHliameteI siphon sized to pass I,OOOe.f.s. Larimer and Weld Canal has a decreed water right of approximately 600 e.f.s., but has a maximum capacity of about I,OOOe.f.s. The siphon was designed for this maximum capacity. Over the siphon, the channel diversion would consist ofa 70-foot-long rectangular concrete channel that would be 100 feet wide for the 100-YC&Tchanncl diversion plan and 40 feet wide for the 3D-year channel diversion plan. Upstream from the Larimer and Weld Canal crossing, training levees would he constructed to direct or t1Msition Dry Creek flows into the channel diversion over a distance of about 120 feet. The channel diversion for alignment B is shown on plates S, 6,and 7. The actual layout shown on these plates is for the 3G-year channel diversion. The alignment for the lOO-year channel rliversion is basically the same with some minor adjustments made to accommodate the wider channel. The channel diversion would be a grass-lined channel with a 12D-foot bottom width for the lOO-yearchannel diversion plan, and SO-footbonom "idth for the 3D- year channel diversion plan. Both plans wouid have 1 vertical on 4 horizontal side slopes. The lOO-year channel diversion would carry a lOO-year discharge of S020 c.f.s_ The 3D-year channel diversion would carry a 3D-year discharge of 2830 c.f.s. The average depth of the channel diversion would be about S ftel for both of the plans. The average velocity in the channel diversion would he about ?8feet per second for the lOO-year flood event. The average velocity in the channel diversion would be aoout ?.5fect per s<<:ond for the 3D-year flood event. After a design 100- year or 30-year flood e,'en! as applicablc to cach plan, wme maintenance of the Downstream from the training levees, the channel diversion would have a dlVCrsior. wcir:o allow evcnts grater than the lClO-)'carcvcnt to pass into Larimer . 6-1 6-2 CD