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<br />, (J liVJI <br />~ flY <br />J('" <br />tf( <br />f /J <br />,!,10 <br />3.1.4 Spillway Design Criteria ~ <br /> <br />Present hydrologic criteria for evaluating the safety of a <br />dam or the design of a dam spillway are based on the dams hazard <br />classification and size. <br /> <br />The discharge capacity of the spillway and/or the storage <br />capacity of the reservoir should be capable of safely handling <br />the recommended spillway design flood for the size and hazard <br />potential classification of the dam as indicated below. The <br />magnitude of the spillway design flood (SDF) is intended to <br />represent the largest flood that need be considered in the <br />evaluation of a given project, regardless of whether a spillway <br />is provided. <br /> <br />SPILLWAY DESIGN FLOODS <br /> <br />Hazard <br /> <br />Size <br /> <br />Spillway Design Flood (SDF) <br /> <br />50 to 100-yr frequency <br />100-yr to 1/2 PMF <br />1/2 PMF to PMF <br /> <br />100-yr to 1/2 PMF <br />1/2 PMF to PMF <br />PMF <br /> <br />Low <br /> <br />Small <br />Intermediate <br />Large <br /> <br />Moderate <br /> <br />Small <br />Intemediate <br />Large <br /> <br />High <br /> <br />Small <br />Intermediate <br />Large <br /> <br />1/2 PMF to PMF <br />PMF <br />PMF <br /> <br />100-yr = 100-Year Exceedence Interval. The flood magnitude <br />expected to be exceeded, on the average, once in <br />every 100 years. It may also be expressed as an <br />exceedence frequency with a one-percent chance of <br />being exceeded in any given year, <br /> <br />PMF = probable Maximum Flood. The flood that may be <br />expected from the most severe combination of <br />critical meteorologic and hydrologic conditions <br />that are reasonably possible in the region. <br /> <br />The classification for size is determined by the height of <br />the dam and/or the storage capacity, whichever gives the larger <br />size category, as indicated below. <br /> <br /> SIZE CLASSIFICATION <br /> Impoundment <br />Category Storage (Ac-Ft) Height (Ft) <br />Small < 1000 and > 50 < 40 and > 25 <br />Intermediate > 1000 and < 50,000 > 40 and < 100 <br />Large > 50,000 > 100 <br /> <br />54 <br />