Resolution No. 4814 - Laserfiche WebLink

Home Browse Search

history of these dams is given below in Table 12.5 (Corps of Engineers, Portland District Office, March, 2001). Table 12.5 Seismic Design, Evaluation and Inspection Data Corps of Engineers Dams Date of Last Seismic Design Basis Date of Last Dam Seismic Periodic Evaluation Original Current inspection Cottage Grove 1981 None 0 21 g 1997 Dexter 1981 0 10 g 0.21 g 1996 Fall Creek 1981 0.10 g 0 21 g 1999 Dorena 1981 none 0 21 g 1997 Lookout Point 1981 0.10 g 0 21 g 1999 Blue River 1994 0.t 0 g 0.24 g 1996 Hills Creek 2000 0.10 g 0.22 g 1999 Cougar 1994 0 10 g 0.24 g 1997 Fern Ridge 2001 none 0~35 g 2000 As shown in Table 12.6, the Corps has conducted at least preliminary seismic evaluations of all of these dams. However, some of lhese evaluations were conducted in the 1980s and thus do not reflect current understanding of the seismic hazard in Oregon or current state-of-the-art seismic evaluation engineering principles. The Corps has an ongoing regular inspection program and an ongoing seismic evaluation program. Presumably, updated seismic evaluations of these dams will be completed over the next few years. Seismic considerations were completely absent in the design of two of these dams: Dorena and Fern Ridge. The others were explicitly designed or probably designed to ground shaking levels of 0.10 g, which is the maximum seismic design level for any of the Corps dams in western Oregon. In contrast, the current Corps seismic design levels for dams at these sites (i.e., if new dams were to be built today) would be 0.21 g to 0.24g for the dams in eastern Lane County and 0.35 g for Fern Ridge. Thus, current seismic design requirements are for levels of ground shaking about two times higher than the probable design levels for most of these dams and about three times higher for Fern Ridge. Seismic evaluations of dam safety are a highly technical, highly specialized art. Separate evaluations must be done for each dam. The evaluation requires a detailed analysis of the design and construction of the dam, an analysis of the current condition of materials and components, geotechnical analysis of the foundation and site, and a site-specific seismic hazard analysis. For emergency planning purposes, a seismic evaluation should include the probabilities of failure for a scenario earthquake such as a large magnitude event on the Cascadia Subduction Zone. 12.5,3 Loss Estimates (Preliminary) Detailed loss estimates for possible failures of these dams are beyond the scope of this mitigation plan. However, we note that in 1987 the Oregon Emergency Management Division6 estimated that a completely catastrophic failure of the Hills Public Review Draft: August 6, 2004 12-8