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<br />m. HYDROLOGIC EVALUATION <br /> <br />A. Basin Hvdrologv <br />The hydrologic model used to determine peak flows ,md volumes throughout the Orchard Avenue <br />Drainage Basin was the TR-20 Computer Program for Project Formulation Hydrology developed <br />by the Soil Conservation Service. <br /> <br />The overall basin was divided into tributary basins and then into smaller subbasins. The subbasins <br />were then numbered and design points designated with leners (see Ihe Basin Dh:charge Map in <br />the back pocket of this report). TIle subbasins w(:re chosen with respect to the natural topography, <br />roadway crossings and development considerations. <br /> <br />Peak flows for the lOO-year, 50-year and 25-year, 24-hour storms, were calculated and evaluated. <br /> <br />B. Time of Concentration <br />The time of concentration (Tc) used in the TR-20 calculations was determined by first calculating <br />an initial ovcrland flow time from the subbasin boundary to the naturally occurring swales and <br />charmels. Then a travel time was calculated in these natural swales to the bottom of the subbasins <br />and added to the initial overland flow time to d,:tcrmine the overall time of com~mtration for <br />existing conditions. For future dcveloped conditions, the charmel travel times were adjusted to <br />reflect improved conditions and therefore a shOlter time of concentration. <br /> <br />C. Rainfall <br />Rainfall amounts for the Orchard Avenue Basin were: detcnnined from the National Oceanic and <br />Atmospheric Administration Atlas 2, Precipitation-Frequency Atlas of the Western United States, <br />Volume III- Colorado, 1973. <br /> <br />Precipitation for the lOO-year 50-year and 25-year, 24-hour stonns were 3.4, 3.05 and 2..75 inches, <br />respectively. <br /> <br />D. Land use <br />Existing land uses in the Orchard Avenue drainag'; basin were detennined by examining current <br />development plans supplemented with field reconnaissance. Currently most of the development is <br />occuning in the western and southern portion of the basin with the eastern and northern areas <br />remaining in their natural state. <br /> <br />Proposed land use for the area was detennined through examination of current development plans <br />and through discussions with Fremont County Planning Department officials and C,mon City <br />officials. For design purposes undeveloped areas were assumed to be fully developed using <br />projected densities. The land use map is a composite of this land use information. There is not a <br />time frame or date associated with this ultimate projected land use. <br /> <br />E. Soil Characteristics <br />The soils information contained in this report is derived from the "Soil SUlVey of Fremont County <br />Area, Colorado", currently being completed by the USDA Soil ConselVation Servic,:. Of the eight <br />soils classifications found within the Orchard A venue drainage basin, three belong to Hydrologic <br />Soil Group C, and five belong to the Hydrologic Soil Group D (see the Soils Map for location). <br />The following is a table of the soils located within the basin: <br />