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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />elevation 10,470 on the ridge between Trap Creek and the Cache la Poudre <br />River. The drainage area is 3.01 square miles and the basin has an average <br />slope of 528 feet per mile. The distance from the basin divide to the <br />reservoir is approximately 2.6 miles. From these values, a time of <br />concentration for basin runoff to reach the reservoir of 0.87 hours was <br />computed. <br /> <br />The probable maximum precipitation (PMP) for Barnes Meadow Reservoir was <br />obtained from the revised version of . the NOAA Hydrometeorological Report <br />Number 55 (Hydrometeoro 1 ogi ca 1 Report Number 55A). The Barnes Meadow <br />Reservoir drainage basin lies just outside the area shown as potentially <br />controll ed by the local (thunderstorm) PMP. Consequently, both the general <br />and local storms were included in the analysis. The local storm one-hour and <br />six-hour rainfall values of 7.1 inches and 9.6 inches exceeded the general <br />storm one-hour and six-hour rainfall values of 4.75 inches and 9.75 inches, <br />respectively. <br /> <br />A desi gn flood was determi ned for both the local and general storm for the <br />Barnes r4eadow drai nage. For the development of rehabil i tati on alternatives <br />the probable maximum flood (PMF) was calculated in accordance with SEO <br />guidelines for high hazard dams in force at that time. Infiltration losses <br />were determi ned util i zi ng the Green Ampt i nfi ltrati on model, whi ch <br />incorporates several soil characteristics as well as initial and final <br />saturation rates. The model resulted in an average infiltration rate of 0.6 <br />inches per hour. This is conservative when compared to the U. S. ArTl\Y Corps <br />of Engineers Phase I Inspection Report, which used an infiltration rate of one <br />inch per hour for mountainous and forested areas of Colorado. The runoff <br />hydrograph for the local storm had a peak di scharge of 11,811 cfs with a <br />vol ume of 1,031 acre-feet and the general storm a peak of 5,072 cfs with a <br />volume of 783 acre-feet. The local storm was considered to be the more <br />critical event because of its high peak and was adopted for subsequent designs. <br /> <br />During the course of the study the SEO amended the guidelines for the design <br />flood for existing high hazard dams to be based on 75 percent of the PMP. The <br />basin hydrology was re-examined to reflect this significant modification. For <br /> <br />1231 H <br /> <br />-62- <br />