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I I I I I '. I I I I I I I I I I I I I HYDROLOGIC AND HYDRAULIC DETERMINATIONS Hydrologic Analysis A hydrologic WIalysis was performed to determine the peak discharge-frequency relationships for floods of the ID-, 50-, 100-, WId SOD-year reCUlTence intervals for the White :River, Sulphur Creek WId Sanderson Heights at Meeker. Peak flood flows on the White River were computed from a statistical analysis of 73 years of gaged data. A long-Pearson Type ill frequency distribution was used in the WIalysis WId three sets of discharges were computed from a station skew, a regional skew, WId a weighted skew coefficient. For further comparisons, a set of discharges was computed from a Gumbel extreme value distribution and Hazen plotting positions for the 66-year period of record from 1910 to 1976. Separation of the gaged data into snowmelt and rainfall events was /lOt considered. Historic, or pre-record, data could not be included in the analysis because the newspaper accounts contained insufficient details of the 1884 and 1897 flood events. Several forms of regional comparisons were made as an aid in evaluating and interpreting the statistical flood frequency analyses. These included flood discharges calculated from regional multiple regression equations (TM-l, reference 13), the index flood method (WSP 1683, reference 14), and comparisons with runoff (CSM) ratios for adopted 100-year frequency flood discharges in Northwest Colorado. The discharge frequency curve for the White River is shown in Figure 7. On Sulphur Creek and Sanderson Heights, no stream gaging records were available. Because of the size and altitude of the basins, it was assumed that floods flows would be caused primarily by rainfall from thunderstorms. For both Sulphur Creek and Sanderson Heights, synthetic flood hydrographs were developed according to the U.S.D.A. Soil Conservation Service procedure for determining peak flows in Colorado (Reference 4). - 10- This procedure requires the use of 24-hour rainfall values which were obtained from the National Weather Service (Reference 5). The 10-, 50-, WId 100-year peaks flows determined by the S.C.S. method for Sulphur Creek and Sanderson Heights Drainageway were plotted on probability paper, See Figure 7. To determine the SOD-year flood, the Probable Maximum Flood and the flood using "Assumption B" (Reference 3) were also plotted to establish the upper limit and to check the reasonableness of the magnitude of the 100-year flood. On Sulphur Creek, the 10- year, 50-year and 100-year floods were computed using 24-hour rainfall values for the May-October season to establish the trend for the lower frequency floods. After careful consideration of the results of the hydrologic analyses, the following peak flow discharges were adopted for use in this study. PEAK FLOOD DISCHARGE IN CUBIC FEET PER SECOND Study Reach Exceedance Interval in Years 10 50 100 500 White River 4600 5900 6500 7700 (D.A. = 872 sq.mi.) Sulphur Creek 820 1510 1850 3300 (D.A. = 27.2 sq.mi.) SWIderson Heights 46 107 145 300 (D.A. = 0.45 sq.mi.) Detailed descriptions, data and computations of the hydrologic techniques used for the White River, Sulphur Creek and Sanderson Heights Drainageway are in the Technical Addendum to this report.