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<br />Table I <br />Percent of Imperviousness for Lsnd Uses <br />in the Sheep Drs'" Basin <br /> <br />FLOOO PROBLEMS <br /> <br />Lsnd Use <br /> <br />Impervious Area <br />(percent) <br /> <br />Floodh1storyisnot readflysvallable for Sheep Dta"'u the basin <br />has been sparsely developed. Flood e~perience in the area indicstea <br />that snowaelt flooding is uncommon and that su~er cloudburst. would be <br />the..oatl1kelysourceofflooding. Flood damages are also p09sible 1n <br />the event of large amount. of runoff from tributaries or s<Da11 drai~ge- <br />"'ays flo..ing into Sheep Draw. There are no flood control works in the <br />basin. S<Dallirrigation da...., irr1gation cand., and road cross1ng <br />structures provide ao.e incidental flnnd discharge reduction, as <br />discussed in the hydrnlogic analy.is. <br /> <br />COlll1llerciai <br />Medium Denaity Residential <br />Agricultural <br /> <br />" <br />" <br />5 <br /> <br />The land uae patterns were slightly altered to fit hydrologic sub- <br />areas in the hydrologic model. To depict urbsn land use as actually <br />used in the hydrologic model, areaS with approxi~tely 40 percent or <br />more imperviousness are indicated all plate 3. Any changes in these <br />land use projections and future urbani.stion boundaries ..auld change <br />the hydrologic and hydraulic data preaented in thia study. <br /> <br />ALTERNATIVE LAND USE CONDITIONS <br /> <br />The Sheep Draw flood ha.ard was evaluated under Cuture as ..ell a. <br />existing condItions sInce the Sheep Draw basin i" subject to increasIng <br />ur~n developmP.nt. Changes in impervlousnesa that would result Crom <br />landuae changes ",111 affect the runoff potential. To reflect the <br />1nfiuence of changing development, three levels of urbani.atIon were <br />con.1d..r,od. ~,,1.t1ng >lnd fut\1r.. urbolnhation ar.. aho..n on plate 3. <br />T",o of theae urbanl~Ation levels, existing and projected, are shown in <br />the report entitled Larl~er-~eld Region Land Use Alternatives prepared <br />Cor the Larimer-Weld Regional Council of Governments 1n November 1977. <br />The existIng urbani~ation reflect. no algntficsnt urban develop~nt. <br />Projected urbani~ation is baaed upon year 2000 land use. This level of <br />urbani~atlon represents an interMediste level of develop..ent in that <br />the dO"'natream hslf at the Sheep Oraw baa1n Is urbanized. Total <br />urbanization asaumea the enttre ~s1n 1s urbanl.ed. <br /> <br />HYDROLOGIC ANALYSIS <br /> <br />A hydrologIc analyais WS8 carried out to establlsh the peak di.- <br />charges for floods of various frequen<:lea. Floods "'ith 10-, 50-, 100-, <br />and SOO-year recurrence intervals are presented in this report. A <br />detailed description of the hydrologic analys1s is conta1ned In Volu~ <br />II of this st\1dy. <br /> <br />Strea~ gsging records are not available for Sheep Oraw. EPA's <br />~~ was used to model ratnfall - runoff characteristics of the basin. <br />Baainchsracter1aticsneededfortheIDodelweretakenfrOlllU.S.Ceolog- <br />leal Survey (USCS) 7.S-Minute Quadrangle mapping having a acale of <br />1:24,000 wIth S contout lnterval oC 10 Ceet. Rainfall values for <br /> <br />The percent of imperviousneu for the land areas "'S8 eselmated for <br />e~istIng, projected urbanization, and total urbanization conditions. <br />Table IUats the percent oC 1l1lpervlousnes. for each land uS" that wu <br />considered In the hydrologic ~del. <br /> <br />I-hour storm event a of varioua frequency were obtained from the <br />Precip1tation-Frequency Atlas of the Western United State6, Atlas 2, <br />Volume III, Colorado, published by the National Oceanic and Atmospheric <br />Adminiatration (NOAA) 1n 1973. The SOO-year rainfall value was <br />extrapolated from the laO-year and mofe frequent event.. Expected <br /> <br />" <br /> <br />13 <br />