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Table 1 Percent of Imperviouaness for Land Uaes io the Sheep Draw Basio fLOOD PROBLEMS Lsnd Use Imperv10usArea (percenr) Plood history is not readily available for Sheep Draw 8S the basin has been ajMrsely developed. Flood expedence in the area Indlcatea that snowmeit flooding Is uncommon and that Summer cloudbursta would be the most Ukely sourCe of flooding. Flood damages are aha possible in the event of large amounts of runoff from tributarieo Or omall droinage- ways flowing into Sheep Draw. There are no flood control works 1n the basin. Smallirrigatlondams,irrlgatloncanals,androadcroaalng sttuctures provide some incIdental flood dischatge teduction, as discussed in the hydrologic analysis. CCII.o'.ercts.1 Medium Density Residential Agricultural " " , The land use patterns were alightly altered to fit hydrologic sub- aress in the hydrologic model. To depict urban land use ss actually used in the hydrologic model, areas with approximately 40 psrcent or more Imperviousuess are indicated on plate 3. Any changea in these lsnd use projections and future urbanisation boundaries would Change the hydrologic and hydraulic data presented in this study. ALTERNATIVE LAND USE CONDITIONS The Sheep Drsw flood hazard was evaluated under future 8S well as existing conditions since the Sheep Draw basin is subject to increasing urban develop~nt. Changes in Imperviousness that would result from land use chs.nges will affect the runoff potential. To reflect the influence of changing development, three levela of urbanization were considered. Existing and future urbanization are shown on plate 3. Two of theae urbanization levels, existing and projected, are ahown 1n the report entitled Larimer-~eld Region Land Uae Alternatives prepared for the Larimer-Weld Regional Council of Govern~Pnt8 in November 1977. The existing urbeniration reflects no significant urban development. Projected urbanization Is based upon year 2000 land use. Thl8 level of urbanization representa an intermediate level of development in that 11,,, <l""~Llbtr"."s I,..H or the Sh""p Dr..... n..~ln b u,,,,,,,h,,d. Tural HYDROLOGIC ANALYSIS A hydrologic analysIs waS carried out to establish the peak d1s- c.hargesfor flood. of varIous frequencieo. PloodswtthlO-, 50-, 100-, and SOO-year recurrence intervala are preaented in this report. A detatled descriptIon of the hydrologic analYOi8 ia contained in Volume 11 of this study. Stream gaging recorda are not availsble for Sheep Draw. EPA'a SWMM wss uaed to model rainfall - runoff characteristics of the basin. Basin charocteriatics needed for the model were taken froa U.S. Geolog- ical Survey (USGS) 7.S-Minute Quadrangle ~pping having a scsle of 1:24 000 wIth 8 contour interval of 10 feet. Rainfall values for , urbanhation SS8Ulll88 the entire badn 18 urbanized. I-hour 8to~eventa of vadoua frequency wer..obl..tn,,Precipitation-Frequency Atlaa of the Weatern United States, Atlas 2, Volume III, Colorado, published by the National Oceanic snd AtmospherIc Administration (NOAA) in 1973. The 500-year rainfall value was extrapolated from the 100-year and more frequent eventa. Eepected The percent of imperviousness for the land areas WaB estimated for "eiatlng, projected urbanization. and total urbanization conditions. Table 1 lists the perc~nt of impervlousnes8 for esch land uSe that was consid~red In the hydrologIc ~del. " "