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<br />College Avenue to the Lari~cr-Weld Canal (Sheets 5 and ~) <br /> <br />City of Fort Collins and Larimer County. This prod~ced a floodway <br />with an average width of 1100-feet belm! the Larimer-Weld Car.al, <br />even though overbank flood depths are very shallow and flood <br />velocities are very low. <br /> <br />The IOO-year flood plain is again wide and shallow and includes <br />most of the area between College Avenue and the Colorado and <br />Southern Railroad. The flood plain crosses College Avenue over <br />a dist(l,nc<l of (l,pproxim(l,tely 3,900-fect. Below the Larimer-Neld <br />Canal, an extensive sheet flow area is the result of the spill <br />of Dry Creek flood flows over the canal embankment. <br /> <br />Because of the shallow depths and low velocity flow, much of this <br />floodway would not be considered a hazardous flood plain area. <br />Therefore, thc State af Colorado floodway criterion of a maximum <br />depth of 18" and velocities less than 3-fect per second was <br />identified as a means of better defini~g hazarcous flood plain <br />areas. In this case, a very narrOw floadway, gc~crally the width <br />of the low flow channel, was identified. In sOTl'e cases, no floodway <br />at all was obtaino;d where depths ,,-ere less than la-inches aCrOSS the <br />flood plain. <br /> <br />Extensive cOThmercial and residential development is located <br />in this reach of flood plain. <br /> <br />Larimer-Keld Canal to the ypstream Study Limit (Sheets 6 and 1) <br /> <br />In this reach, topography slopes toward a sizeable Dry Creek <br />cl1annel and flood plain ".idths are greatly reduced. '!:he <br />flood plain width ilveragcs about 700-feet. The majc.- cause <br />of residential floo<Unq in this reach is the backwat"r caused <br />by the Larimer-Held Ca'1al (Eaton Ditch). The canal directly <br />intercepts all of the Dry Creek drainage flows. Further <br />residential flooding is caused by undersized culverts at the <br />U. S. I:ighway 287 crossing and at a local residential sheet <br />crossing downstream of u. s. Highway 287. Upstream of <br />1.;. ~. Highway 287, tho flood plain continues to narrow. <br /> <br />1\either of these floo6ways WdS acceptable for the flooc plain <br />conditions existing for Dry Crock. The floodway, based upon a ~ foot rise, <br />woule unneccss"rily linit CQVQloplr.en~ in aver)' ';'i"l.' an,"', (I, much <br />~idcr "rea t~ar. ic ~orm"lly rQq~irQd to convey a laO-year discharge <br />of this ]~agnitude. The floodway, based upon a maximul'l depth of <br />18", does not frovide for sufficicnt right-of-way to convey <br />this flow safely in a future flood channel. <br /> <br />A compromise fl"ou.iay "'as therefore <br /> <br />identified. <br /> <br />This' floodway <br /> <br />~~19g.9.':'~X f~.E.-t <br /> <br />would per~it encroachment <br /> <br />tc <br /> <br />. . <br />a po.l~_. <br /> <br />,;"hilr" <br /> <br />000 <br /> <br />A flaodway was calculated for the Dry Creek floo" plain using the <br />crit.€rion 0: encroachment to a I/2-foot ri"e in the laO-year <br />Wilt"" sllrfacE'. Thi,,,; i" the f100,1I""\I criterion ddol'ted by thto' <br /> <br />max~mum rise in the IOO-year water surface elevation would be nO <br />greater than one-foot. Thi,,; criterion provides for a floodway with an <br />average width of ,1pproximilt_f'ly 600-faet. This criterion is <br />consilitent. ,,'ith the guideline of the 1'ederal Insurance .'\rI",inistration. <br /> <br />F200dway data is l'resente'(l in '!:ab2e 'iI. <br /> <br />- 20- <br /> <br />- 21- <br />