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The 1987 Simons, Li & Associates, Inc., SWMM model was updated by creating inflow hydrographs to include the discharge contributions from Boxelder Creek. Discharge contributions were added along 1-25 at three locations: at Vine Drive (includes the peak discharges for the Larimer-Weld Canal as well as the Vine Drive discharge), at the C&SRR (includes the peak discharge for the overtopping of 1-25 south of Vine Drive and the C&SRR), and at SH-14. Peak discharges for each of the hydrographs were set from values listed in the tables above and the 1987 Simons, Li & Associates, Inc., model was converted from a 3-hour rainfall event to a I-hour rainfall event. Additional information on the hydrologic analyses can be found in the technical addendum prepared by Love & Associates, Inc. (Reference 25). ~) Discharges for Dry Creek were computed using the EPA's SWMM (Reference 20). The drainage basin was broken down into 190 subcatchments that were analyzed individually to determine flood hydrology characteristics 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 basin was determined by field inspection. The level of urbanization led to estimates of percent imperviousness for each of the 190 individual subcatchments. Values recommended by the Denver Area UDFCD and the EP A were used for infiltration, overland-flow roughness coefficients, and surface-storage depths for pervious and impervious areas. Rainfall information was taken from the 1973 NOAA Atlas, and a typical2-hour thunderstorm configuration was estimated for purposes of computing the runoff peak discharges (Reference 26). These procedures were followed in the computation of the 10- and 100-year discharges. The 50- and SOD-year discharges were estimated by assuming a straight-line relationship on log-probability paper. Dry Creek drains a total of approximately 80 square miles of agricultural rangeland located north of the City of Fort Collins. Approximately 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 that incercept the flow before it enters Dry Creek. These lakes include: Lindenmeir Lake, Long Pond, Richard Lake, Terry Lake, Black Lake, Douglas Lake, Reservoir No.4, Kulver Reservoir, Reservoir No.3, Rock Ridge Lake, Curtis Lake, Gilmore Lake, Cavalry Lake, Spitzer Lake, and Miner's Lake. Runoff hydrographs were computed for the areas above and below the Douglas Reservoir at the Dry Creek crossing of U.S. Highway 287 and SH-14. The upper portion of the drainage basin, 46 square miles, drains directly inco the Douglas Reservoir, a major irrigation reservoir. Runoff hydrographs for the area above the Douglas Reservoir were, therefore, routed through the reservoir, resulting in considerable attenuation due to available storage. The Douglas Reservoir reduces the peak flows by an estimated 15 to 20 percent (Reference 19). Peak discharges throughout the study reach were found to be generated totally below the Douglas Reservoir. t-, . The peak discharges for the lOD-year flood for Boxelder Creek were developed by Simons, Li & Associates, Inc., as outlined in the 1981 Master Drainageway Planning Study (Reference 2). The SWMM model, modified by the USACE, Missouri River Division, was used by Simons, Li & Associates, Inc., to estimate the peak flows. A more detailed explanation of the hydrologic analysis can be found in the Simons, Li & Associates, Inc., study. 10