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<br />- <br /> <br />-2- <br /> <br />j' <br /> <br />maximum probable flood that could have been determined through modem meteorological <br />teclmiques. <br /> <br />. <br /> <br />The current industry standard in the United States for determining maximum probable flood is to use <br />the PMP values from the appropriate Hydrometeorological Report (HMR) (HMR 49 for west of the <br />continental divide, HMR 52 for the eastern plains and HMR 55A for the Front Range). The 100- <br />year, 50-year, and 25-year events or normally determined from the National Weather Service NOAA <br />Atlas #2 "Precipitation-Frequency Atlas ofthe Western United States" Volume III-Colorado, U.S. <br />Department of Commerce, NOAA, National Weather Service, Silver Springs, Maryland, 1973. <br /> <br />II. Spillway Desil!n Requirements <br /> <br />The Colorado Dam Safety Program uses the inflow design flood (lOF), rather than the Probable <br />Maximum Flood (pMF), to determine the routing of the flood through the reservoir for determining <br />the necessary size of spillway to prevent the dam from overtopping during the flood event. The <br />current standards for determining the IDF from HMR PMP procedures are summarized in the table <br />below: <br /> <br />Standard Requirements for Determining the IDF and Spillway Size <br /> <br />New Dam Construction <br />INFLOW DESIGN FLOOD REQUIREMENTS . <br />Hazard Classification I II III IV <br />DAM SIZE <br />Large PMP .75 PMP 100 YR 50YR <br />Intermediate PMP .50 PMP 100 YR 50YR <br />Small PMP .50 PMP 100 YR 25YR <br />Minor .50 PMP 100 YR 50YR 25YR <br /> <br />Existing Dams <br /> <br />INFLOW DESIGN FLOOD REQUIREMENTS <br /> <br />Hazard Class <br />Dam Size <br /> <br />I <br /> <br />II <br /> <br />III <br /> <br />IV <br /> <br />Large <br />Intermediate <br />SmaIl <br />Minor <br /> <br />.75 PMP <br />.75 PMP <br />.75 PMP <br />.50 PMP <br /> <br />.50 PMP <br />.50PMP <br />.50 PMP <br />100 YR <br /> <br />100 YR <br />100 YR <br />100 YR <br />50YR <br /> <br />50YR <br />50YR <br />25YR <br />25YR <br /> <br />. <br />