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<br />The maximum runoff used in the des i gn is <br />usually called the Spillway Design Flood <br />(SDF), representing the largest flood that <br />need be analyzed, regardless of whether or <br />not a spillway is provided. The magnitude <br />of the SDF (flood volume, peak flow, etc.) <br />as adopted in the United States for the <br />past 30 years is equal to that of the Pro- <br />bable Maximum Flood at the site of the dam. <br />Methodology to estimate the Probable Maxi- <br />mum Flood is available in Regulatory Guide <br />1.59. "Design Basis Floods for Nuclear <br />Power Plants", and other publications. <br />(References 10 and 11.) <br /> <br />For small retention dams built on isolated <br />streams in areas where failure would neither <br />jeopardize human life nor create damage to <br />property or the environment beyond the spon- <br />sor's legal liabilities and financial capa- <br />bilities, less conservative flood design <br />criteria may be used in the design. However, <br />the selection of the design flood needs to be <br />at least compatible with the guidelines set <br />forth by the Corps of Engineers. (Reference <br />12. ) <br /> <br />(Source: U.S. NRC Regulatory Guideline 3.11) <br /> <br />It is'Dravo's opinion that the PMF is not applicable in the flood- <br />way analysis of the San Miguel River at Uravan, Colorado. Regula- <br />tory Guideline 3.11 states that the tributary area associated with <br />a retention structure is based on "... the valley across which a <br />retention dam is constructed". In the Dames & Moore ER, the PMF <br />was determined for the 1510 square mile drainage area of the San <br />Miguel River upstream of the Uravan area. It appears that the <br />language of the 3.11 guidelines would indicate the design storm <br />is that which is intercepted by or stored by the retention struc- <br />tures, not the storm by which the flooding in a river channel is <br />determined. <br /> <br />4 <br />