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. HYDROLOGIC ANALYSIS . Existing and futur~ urbanization are shown on plate 3. two of these urbanization levels, existing and projected, are shown In the report entitled Larimer-Weld ReRion Land Use Alternatives prepared for the Larimer-Weld Regional Council of Governments in November 1977. The land uss patterns projected 1n the cited report Were slightly altered to tit hydrologic subareas in the hydrologic model. the existing urbanization reflects little urban development. Projected urbanization is based upon estimated year 2000 land use. This level of urbanization represents Sn inrermedlate level of deve10pmenr Int:,at theupstrulIIhalf of the Fossil Creek basin 1s urbanized except tor the rugged ares generally wast of Taft Hill Road. .otal urbanization assumes the entire basin is urbanized, except generally west of Taft Hill Road. A hydrologic analysis was carried out to establiSh the peak dis- charges for floods of various frequencies. Floods with 10-, 50-, 100-, snd 500-yesr recurrSnce intervals are presented in this report. A detailed description of the hydrologic analysis is contained in Volume II of this study. ComOlercial High Density Residential Medium 'Density Residential Low Density Rcsidential Agri.:."H"r...l 90 " " 90 5 Stream gaging records are not available for Fossil Creek. The Environmental Protection Agency's (EPA) Storm Water Management Model (S~) ~as used to model rainfall-runoff characteristics of the bssin. Basin characteristics needed for the model were taken from U.S. Geological Survey (USGS) 7.5 Minute Quadrangle mapping at a scale of 1:24,000 with a contour interval of 10 feet. Rainfall valuea for I-hour storm events of various frequency were obtained from the rainfall intensity-durstion curves developed by Resource Consultants, Inc. of Fort Collins, ColoradO. These curves were derived from the PrecIpitation-Frequency Atlss of the Western United States, Atlas 2, Volume III, Colorado, published by the National OceaniC and Atmospheric Administration (NOAA) in 1973. The SOO-year rainfall value wss e~trapolsted fro~ the 100-year and more frequent events. At the request of local interests, e~pected probability adjustments were not applied to the rainfall, to avoid conflict with flood frequency informs- tion developed in other studies. The time distribution of rainfall within the I-hour storm was obtained from a storm provided by Resource Consultanta, Inc., modified by using the Colorado Urban Hydrograph Procedure as a guide. Infiltration rates were obtained from a Larimer County soil map and the report Soil Resources of Colorado, Region 2- Larimer and Weld Counties, published by the Colorado State University Experiment Station and the Soil Conservation Service in 1976. Deten- tion storage values Were 0.3 and 0.2 inches for pervious and impervious aress, respectively. The percent of imperviousness for land areaS was estimated for e~isting, projected, and total urbanization conditions. Table 1 lists the percent of imperviousness for cach land use type considered in the hydrologic model. Urban land use hss a significant effect on runoff. To depict existing and future urban land patterns used in the hydro- logic model, areSs with approximately 40 percent or more impetviousness are indicsted on plate 3. Any changes in these land use projections and future urbani.ation boundarIes would change the hydtologic and hydraulic data presented in this study. Table 1 Percent of Imperviousness fot Land Uses in the Fosail Creek Basin Land Use Impervious Area (percent) . The effect of future urbanization, e~isting dams, road structures, andcaoals was considered. The reservoirs in the ba9inhllve some flood " . "