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<br />-18- <br /> <br />-19- <br /> <br />sheds. The Gore Creek catchment area below Black Gore Creek ranges <br />from 34 to 100 square miles. In these larger areaS. the snowmelt <br />discharges are significantly higher than are the rainfall runoff <br />discharges. <br />In contrast, the general storm will cover the entire basin and <br />shouldprodllce its greatest effect on Gore Creek near the Eagle R iver. <br />However, the general rainstorms precipitate at such a low rate that <br />the infiltration absorbs most of t~ water. 8reaking the 100 year <br />general storm into time increments in accordance with the recommendations <br />of the "Design of Small Dams" results on 0.44 inches of precipitation <br />in the most intense hour.I9 The peak intensity during this hour is <br />unlikely to be greater than the 0.8 inch per hour infiltration rate.1 <br />Additionally, portions of this precipitation will also fall as snow <br />or hail pellets whiCh will further delay runoff. <br />GoreCreekF1oodP..ak <br /> <br />comparative purpos<,s. The spring snowmelt phenomena prOduces the peak <br />streamflow discharges for the main stem Gore Creek. Thevaluesder1ved <br />from the fourteen Log-Normal snowmelt discharge probability analysis <br />were utilized in developing the Snowmelt Discharge curves, as pre- <br />sented on Figures III-7 to III-12. The curve fitting process included <br />a least squares computation for the best fit or mean discharge curve <br />and fitting an upp<,r envelope curve based upon a 1.5 standard deviation. <br />The upper envelope curve provides a confidence level of 93+% that <br />resultant peak discharges w111 fall below the curve. Due to the relative <br />shortness of many of the streamflow records, the upper envelope curve <br />was utilized in defining peak discharges for the floodplain compu. <br />tations in this study. <br />The 500 year snowmelt discharge curves were established by <br />extrapolating the curves for other recurrence intervals based upon a <br />criteria defined by the Colorado Water Conservation Board and in <br />accordance with F.r.A. practices. A more detailed discussion of the <br />procedures and the data are contained in the technical addendum to <br />this report. <br />Effects of Development on DischarQes <br />Increased imperviousness within the Gore Creek Valley is not <br />expectedtoincreasesno~1tf1ooding. This is because snow in the <br />lower valley will be melted early in the thaw before the creek is <br />near flood stage. Consequently, any additional runoff caused by <br />development in the valley will have already been discharged days <br />before the peak. Increased imperviousness will, however, increase <br />discharges within the very small basins where the development occurs. <br /> <br />The U.S.G.S. streamflow gages provide a high-quality record of the <br />runoff rates and wateryields.20 Within the Gore Yalley. there are ten <br />streamf1owgages. Two of these have just been installed. Theothers <br />have periods of record from eight to twenty years, see Figure 111-6. <br />1n addition to these eight stations. streamflow recordS at six other <br />locations with Similar orientation and orographic character were <br />analyzed. The six additional stations have periOdS of record ranging <br />from fourteen to forty-five years. The catchment areas for these <br />streamflow gages range in size from 4.37 to 650 squilre miles. <br />The streamflow records from the eight Gore basin stations plus <br />the six similar basin stations were statistically analyzed using a <br />Log.Norma1 relationship as well as a Log-Pearson III relationship for <br />