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<br />Straight-line extrapolations were made from the regional flood-frequency <br />curve (or weighted curve) to the PMF value. The results listed in table 7 <br />indicate that estimates of PMF have recurrence intervals that extend <br />throughout several orders of magnitude. In the study area, these data <br />indicate projects designed for PMF floods do not have the same margins of <br />safety. Dams on the plains and in the foothills are designed for floods <br />that have recurrence intervals generally in the range of 2,000 to 3,000 <br />years, whereas dams above 7,500 feet are designed for floods that have <br />recurrence intervals far in excess of 10,000 years. The present 01ympus <br />dam spillway design has a capacity of 22,500 cubic feet per second and has <br />a recurrence interval well in excess of 10,000 years. <br /> <br />Table 7.--Recurrence iftterva1s from regression analysis for selected <br />probable maximum floods <br />[--, not applicable] <br /> <br />Site name <br /> <br />Streamf1ow- <br />gaging <br />station <br />number <br /> <br />Total <br />drainage <br />area <br />(square <br />miles) <br /> <br />Probable <br />maximum <br />flood <br />(cubic feet <br />per second) <br /> <br />Recurrence <br />interval <br />(years) <br /> <br />Big Thompson River <br />at Estes Park-------- 06733000 <br /> <br />137 <br /> <br />84,000 <br /> <br />>>10,000 <br /> <br />Big Thompson River <br />above Drake---------- <br /> <br />189 <br /> <br />1116,000 <br /> <br />>10,000 <br /> <br />Big Thompson River at <br />mouth of canyon, <br />near Drake----------- 06738000 <br /> <br />305 <br /> <br />1180,000 <br /> <br />2,200 <br /> <br />Plum Creek near <br />Louviers------------- 06709500 <br /> <br />302 <br /> <br />550,000 <br /> <br />2,700 <br /> <br />Cherry Creek near <br />Franktown------------ 06712000 <br /> <br />169 <br /> <br />265,000 <br /> <br />3,000 <br /> <br />1Prorated by drainage area from Big Thompson River at Estes Park. <br /> <br />This study has indicated the lack of large floods in areas above 7,500 <br />feet in the mountains of Colorado. In Colorado, there are more than 27,000 <br />dams of which probably several thousand are above 7,500 feet. Since 1890, <br />more than 130 dams have failed (Colorado Water Conservation Board, 1983), <br />but none have failed above 7,500 feet because of overtopping from rainfall <br />runoff. The dams above 7,500 feet have failed as a result of embankment or <br />piping failures, such as the 1982 Lawn Lake Dam failure at an elevation of <br />11,000 feet (Jarrett and Costa, 1986). Evaluation of streamflow data <br />and paleoflood investigations provide an alternative method for evaluating <br />flood hydrology and the safety of dams. <br /> <br />32 <br /> <br />3& <br />