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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 /> <br />The procedures used and findings of the PMF study are described <br /> <br />below. <br /> <br />2. Una Reservoir Hydrologic Conditions <br />Applicable to PMF Estimate <br /> <br />The drainage area at the dam site is 7,370 square miles with ap- <br />proximately 2,800 square miles above El 9000, and the Colorado Ri vel" at this <br />location has an average annual flow of approximately 3,500 cfs. The 500-year <br />flood peak discharge is estimated as approximately 44,000 cfs. The mean annu- <br />al precipitation in the basin is approximately 16.9 inches and the 100-year, <br />6-hour precipitation is estimated to vary frc:m approximately 1.8 inches at <br />Grand Junction to over 2.0 inches in the upper basin. The average 72-hour PMP <br />over the drainage basin is 10.7 inches, based upon infonnation provided in the <br />September 1977 publication, Hydrometeorological Report No. 49, Probable Maxi- <br />(23) <br />mum Precipitation Estimates, Colorado River and Great Basin Drainages <br />(HMR49). The typical May 1 snow pack water equivalent above El 9000 is ap- <br />proximately 16 inches, based on information provided in Colorado and New <br />(21 ) <br />Mexico Water Supply Outlook. <br /> <br />I <br />I <br />I <br />I <br /> <br />3. Tri-County Project PMF Analysis <br /> <br />I <br />I <br />I <br />I <br />I <br /> <br />As noted above, the PMF estimate for the Una Reservoir Project is <br />based on the analysis perfonned for the Tri-County Project on the Gunnison <br />River, since the two basins are hydrologically similar and that study is the <br />most recent available PMF study. It included detailed analyses based upon <br />watershed calibration of historic floods utilizing the Corps of Engineers <br />(12) <br />Hydrologic Engineering Center's oomputer program HEC-1 and the companion <br />Clark Unit Hydrograph Method, and the energy budget snownelt equations. Esti- <br />mates were made of the PMP utilizing the data and procedures of HMR49. Mete- <br />orological infonnation including probable maximum snow packs , historic snow- <br />packs, solar radiation, and temperatures was also utilized for the analyses. <br /> <br />The Tri-County PMF estimate resulted in a peak flow of 72,000 cfs <br />and a volume of 3,670,000 ac-ft for the May-August snowmelt flood. The total <br />drainage area of that basin is 5,241 square miles of which 3,320 square miles <br /> <br />II-13 <br />