FLOOD00191 - Laserfiche WebLink

Home Browse Search

3.) Hydrologic Analyses Hydrologic analyses were carried out to establish the peak discharge-frequency relationships for floods of the selected recurrence intervals for each stream studied in detail in the community. Discharges for Spring Creek and Dry Creek were computed using the U.S. Environmental Protection Agency's Storm-Water Management Model (SWMM) (Reference 6). The drainage basins were broken down into 14 and 190 subcatchments for Spring and Dry Creeks respectively which were analyzed individually to determine flood hydrology character- istics and flood peaks. The individual flood hydrographs were then combined at various design points along each stream to determine peak discharges at these design points. The level of urban development within the basins was determined by field inspection. The level of urbanization led to estimates of percent imperviousness for each of the 14 and 190 individual sub- catchments. Values recommended by the Denver Area Urban Drainage and Flood Control District and the Environmental Protection Agency were used for infiltration, overland flow roughness coefficients, and surface storage depths for pervious and impervious areas. Rainfall information was taken from the 1973 National Oceanic and Atmospheric Administration Atlas, and a typical 2-hour thunderstorm configuration was estimated for purposes of computi~g the runoff peak discharges. These procedures were followed in the computation of the 10- and 100-year discharges. The SO- and SOD-year discharges were estimated by assuming E straight-line relationship on log- probability paper. Dry Creek drains a total of approximately 80 square miles of agricul- tural rangeland located north of the City of Fort Collins. Approxi- mately 13 percent of the overall drainage area does not contribute to the floodflow of Dry Creek. This 13 percent reduction in drainage area is due to several lakes which intercept the flow before it enters Dry Creek. These lakes include Lindenmeir Lake, Long Pond, Richard Lake, Terry Lake, Black Lake, Dixon Reservoir, Reservoir No. 4, Kulver Reservoir, Reservoir No.3, Rocky Ridge Lake, Curtis Lake, Gilmore Lake, Cavalry Lake, Spitzer Lake, and Miner's Lake. Precipi- tation information used in the study of Dry Creek was obtained from the National Oceanic and Atmospheric Administration Atlas, Volume III, 1973, for Colorado. Runoff hydro graphs were computed for the areas above and below Douglas Lake at the Dry Creek crossing of U.S. Highway 287 and State Highway 14. The upper portion of the drainage basin, 46 square miles, drains directly into Douglas Lake, a major irrigation reservoir. Runoff hydrographs for the area above Douglas Lake were, therefore, routed through Douglas Lake, resulting in considerable attenuation due to available store. Peak discharges throughout the study reach were found to be generated totally below Douglas Lake. 6