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<br />The SCS-computed discharges an Boxelder Creek were based upon none <br />of the proposed reservoirs being completed and all three proposed <br />reservoirs above Wellington being complete. At the time of this <br />study, only two of the reservoirs had been constructed, so the <br />assumed storage for the third structure (B-2) was eliminated based <br />upon the information suppli~d by the SCS. <br /> <br />Discharges for Dry Creek were computed using the SWHM <br />(Reference 12). The drainage basin was divided into 190 subbasins <br />that drained into 139 channel sections. Basins were selected so <br />that hydrographs would be available at critical design points, <br />particularly at irrigation canal crossings and at onstream <br />reservoirs (Reference 1). Precipitation information was obtained <br />from the National Oceanic and Atmospheric Administration Atlas <br />(Reference 18). The SOO-year rainfall, not available from the <br />atlas, was extrapolated assuming a Gumbel probability distribution <br />(Reference 1). <br /> <br />Discharges for Fish Creek and the Fall River were computed based <br />upon records of stream gages located on the two streams. The Fish <br />Creek gage has 30 years of record, and the Fall River gage has 9 <br />years of record. These records were analyzed using a log-Pearson <br />Type III distribution as recommended in U.S. Water Resources <br />Council Bulletin 17 (Reference 11). These discharges were weighted <br />with those obtained using regression equations from the Colorado <br />Water Conservation Board's Technical Manual No.1 (Reference 10). <br /> <br />Peak discharges for the 10-, SO-, and 100-year floods along the <br />Cache La Poudre River were provided by the COE, Omaha District <br />(Reference 6). The SOO-year discharges were estimated by straight <br />line extrapolation of the 10-, SO-, and 100-year floods provided by <br />the COE. <br /> <br />The SWHH (Reference 12) was used to compute discharges for approxi- <br />mate flooding on Boxelder Creek and Cooper Slough in the vicinity <br />east of the City of Fort Collins (Reference 9). <br /> <br />Peak discharge-drainage area relationships for streams studied in <br />detail are shown in Table 1. <br /> <br />3.2 Hydraulic Analyses <br /> <br />Analyses of the hydraulic characteristics of flooding from the <br />sources studied were carried out to provide estimates of the eleva- <br />tions of floods of the selected recurrence intervals. <br /> <br />Cross sections for the Big Thompson River and its tributaries were <br />taken from topographic maps at a scale of 1:2,400, with a contour <br />interval of 2 feet (References 19, 20, and 21) and field surveys. <br />These tributaries include the North Fork Big Thompson River, Big <br />Thompson River Tributaries 1, 2, 3, and 4, Long Gulch, Rabbit <br />Gulch, Quillan Gulch, West Creek, Fox Creek, Dark Gulch, Cedar <br /> <br />13 <br />