Resolution No. 4814 - Laserfiche WebLink

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and the risk of property damage is minimal or Iow. Additional data on these NID high potential hazard dams are given below in Table 12.4. Table 12.4 Additional Data on NID High Hazard Potential Dams Storage Date Dam County Dam Name River (acre EAP Owner feet) Built Type Lane Cottage Grove Coast Fork Willamette 50 000 1942 RE Y Corps Lane Dexter Middle Fork Willamette 29 900 1955 RE Y Corps Lane Fall Creek Fall Creek 125.000 1965 ER Y Corps Lane Dorena Row River 131.000 t949 RE Y Corps Lane Lookout Point Middle Fork W~llamette 477.700 1953 RE Y Corps Lane Blue River Dam Blue River 89 000 1968 RE Y Corps Lane. Hills Creek Middle Fork Willamette 356000 1962 RE Y Corps Lane Cougar South Fork McKenzie 219,000 1964 ER Y Corps Lane Fern Ridge Long Tom 121,000 1941 RE Y Corps In the table above, the Owner Type Classification is: Federal (F), Local (L), Private (P) and Utility (U). The NID dam type classification includes the following types of dams: RE rockfill/earthfill embankment dams, primarily rockfill (fill >3" size) ER rockfill/earthfill embankment dams, primarily earthfill (fill <3" size) PG concrete gravity dams REPG combination dams incorporating rockfill/earthfill and concrete gravity. These dams were completed between 1941 and 1968. All dams are rockfill/earthfill embankment dams, except Cougar which is an earthfill/rockfill embankment dam. All dams are operated by the US Army Corps of Engineers and all have emergency operations plans in place. All Corps dams are maintained on a regular schedule and undergo regular inspections, with major re-inspections every five years. Furthermore, the Corps is highly experienced in the construction, operation, and maintenance of dams. As noted previously, the NID classification as High Potential Hazard means only that there is probable loss of life i_f one of these dams fails. The NID classification contains no information whatsoever about the safety or lack of safety of a given dam and no information about the probability of failure. For embankment dams, as discussed above, the most common failure modes are overtopping, foundation failures, and seepage through the dam. For concrete dams, the most common failure modes are overtopping and foundation failures. Under normal or flood conditions, failure of the Corps operated dams appears highly unlikely. Failure is perhaps possible, however, in extreme flood events well above the design basis, especially if the reservoirs were close to full at the onset of flooding. The spillway capacities could be exceeded with a potential for overtopping failures. There are, however, two other circumstances that may pose significant threats to any of these dams: landslides and earthquakes. Public Review Draft: August 6, 2004 12-6