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<br />The regression equations were used to develop discharges for specific frequencies on ungaged <br />tributaries. Spraddle Creek was an exception to this general rule because it is the only stream in <br />the study with a drainage area of less than 4 sq. mi. (2.2 sq. mi.). The discharges developed by <br />GAl during previous studies using rainfall-runoff relationships were adopted for this one small <br />area. <br /> <br />The discharges for the main stem of Gore Creek plotted significantly. above the average data <br />points from the stream gaging records. Discharges for ungaged reaches on the main stem were <br />determined using the three gages on that stream. One of those new stations, Gore Creek Lower <br />Station in the Town of Vail, with 77.1 sq. mi. of area, is in the Gore Creek study reach. It has <br />9 years of record that were extended by adjusting 6 years of data for an upstream gage that had <br />been in place from 1974 through 1979 for a total 15-year record. Annual peak flows recorded for <br />the 57.3-sq.-mi. upstream drainage area were adjusted to the downstream location based on the <br />ratio of their respective areas raised to the 0.7 power. While the 6-year adjustment had little <br />impact on the final results, the longer record is more reliable. The Gore Creek gage at the mouth <br />has 14 years of peak flow data, and the gage upstream of the confluence with Black Gore Creek <br />has 40 years of data. <br /> <br />The discharges for specific frequencies for the main stem Gore Creek stations, when plotted <br />versus their drainage areas, all plot higher than most of the other data points in the analysis. The <br />three stations are consistent with each other for each frequency. A line was drawn through those <br />three data points and parallel to the regression equations for each frequency. Consequently, the <br />slope of the relationship for the three main stem stations is the same as that for the regression <br />equations. This..curve was used to determine discharges for specific frequencies for ungaged <br />reaches along the main stem of Gore Creek. <br /> <br />The results of hydrologic analyses that are recommended for use in the Gore Creek basin <br />hydraulic analyses are presented in Table 1. The same computation points that were used in the <br />original PIS were adopted for this evaluation to facilitate comparison of results. In general, the <br />results of both studies are quite similar. Some of the specific discharges computed for tributaries <br />during current studies are slightly higher than those from previous studies. Discharges computed <br />for the main stem of Gore Creek are slightly lower than previously developed, particularly for <br />less frequent events. The 100-year floods computed for large areas are about 200 cfs less, with <br />smaller differences for small areas. <br /> <br />During review of previous studies, it was learned that the discharges that were adopted by GAl <br />for the main stem were from previous studies by Hydro-Triad. Hydro-Triad used a Log-Normal <br />distribution in its flood frequency computations for individual gaging stations. This procedure <br />overstates the magnitude of infrequent floods at stations such as those in this region whose peak <br />flow records exhibit negative skewness. They computed a mean curve through the data points <br />for individual stations and an upper envelope curve that was 1.5 standard deviations above the <br />mean curve. Hydro-Triad recommended using the upper envelope curve to be conservative and <br />because data on which to determine flood frequencies were limited at that time. That was a good <br />decision because main stem stations would have otherwise been understated when compared to <br />current results. <br /> <br />The 100-year discharges presented in a table in the FIS were compared to the upper envelope <br />curve presented by Hydro-Triad. This comparison revealed that the discharges came directly <br />from the Hydro-Triad report, even though this is not so stated. The additional years of data that <br />have become available for three main stem Gore Creek stations allows an analysis to be made <br />specifically for Gore Creek. These are the major reasons for differences between studies of <br />discharge for specific frequencies. <br /> <br />The slopes of the four regression equations computed for specific frequencies varied uniformly <br />from 0.73 at the 10-year event to 0.70 at the 500-year event. This is a reasonable range of slopes <br />since 0.7 is a rule of thumb value for a snowmelt-dominated relationship and provides comfort <br /> <br />20 <br />