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<br />The 1987 Simons, Li & Associates, Inc., SWMM model was updated by creating inflow
<br />hydrographs to include the discharge contributions from Boxelder Creek. Discharge
<br />contributions were added along 1-25 at three locations: at Vine Drive (includes the peak
<br />discharges for the Larimer-Weld Canal as well as the Vine Drive discharge), at the
<br />C&SRR (includes the peak discharge for the overtopping of 1-25 south of Vine Drive and
<br />the C&SRR), and at SH-14. Peak discharges for each of the hydrographs were set from
<br />values listed in the tables above and the 1987 Simons, Li & Associates. Inc., model was
<br />converted from a 3-hour rainfall event to a I-hour rainfall event. Additional information
<br />on the hydrologic analyses can be found in the technical addendum prepared by Love &
<br />Associates, Inc. (Reference 25).
<br />
<br />Discharges for Dry Creek were computed using the EPA's SWMM (Reference 20), The
<br />drainage basin was broken down into 190 subcatchments that were analyzed individually
<br />to determine flood hydrology characteristics and flood peaks. The individual flood
<br />hydro graphs were then combined at various design points along each stream to determine
<br />peak discharges at these design points,
<br />
<br />The level of urban development within the basin was determined by field inspection. The
<br />level of urbanization led to estimates of percent imperviousness for each of the 190
<br />individual subcatchmems, Values recommended by the Denver Area UDFCD and the
<br />EPA were used for infiltration, overland-flow roughness coefficients, and surface-storage
<br />depths for pervious and impervious areas, Rainfall information was taken from the 1973
<br />NOAA Atlas, and a typical2-hour thunderstorm configuration was estimated for purposes
<br />of computing the runoff peak discharges (Reference 26). These procedures were followed
<br />in the computation of the 10- and lOO-year discharges, The 50- and 500-year discharges
<br />were estimated by assuming a straight-line relationship on log-probability paper.
<br />
<br />Dry Creek drains a total of approximately 80 square miles of agricultural rangeland
<br />located north of the City of Fort Collins. Approximately 13 percent of the overall
<br />drainage area does not contribute to the floodflow of Dry Creek, This 13-percent
<br />reduction in drainage area is due to several lakes that intercept the flow before it enters
<br />Dry Creek These lakes include: Lindenmeir Lake, Long Pond, Richard Lake, Terry
<br />Lake, Black Lake, Douglas Lake, Reservoir No, 4, Kulver Reservoir, Reservoir No.3,
<br />Rock Ridge Lake, Curtis Lake, Gilmore Lake. Cavalry Lake, Spitzer Lake, and Miner's
<br />Lake. Runoff hydrographs were computed for the areas above and below the Douglas
<br />Reservoir at the Dry Creek crossing of V,S, Highway 287 and SH-14. The upper portion
<br />of the drainage basin, 46 square miles, drains directly into the Douglas Reservoir, a
<br />major irrigation reservoir. Runoff hydrographs for the area above the Douglas Reservoir
<br />were, therefore, routed through the reservoir, resulting in considerable attenuation due
<br />to available storage. The Douglas Reservoir reduces the peak flows by an estimated 15
<br />to 20 percent (Reference 19). Peak discharges throughout the study reach were found to
<br />be generated totally below the Douglas Reservoir.
<br />
<br />The peak discharges for the lOO-year flood for Boxelder Creek were developed by
<br />Simons, Li & Associates, Inc" as outlined in the 1981 Master Drainageway Planning
<br />Study (Reference 2). The SWMM model, modified by the VSACE, Missouri River
<br />Division, was used by Simons, Li & Associates, Inc., to estimate the peak flows. A
<br />more detailed explanation of the hydrologic analysis can be found in the Simons, Li &
<br />Associates, Inc., study.
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