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Eugene -Springfield Area Natural Hazards Mitigation Plan <br />2. Hazard Descriptions <br />Dam or Levee Failures <br />Dam failures can occur at any time in a dam's life. Failures are most common when <br />water storage is near or exceeding design capacity. At high water levels, the water <br />force on a dam is higher and several of the most common failure modes are more <br />likely to occur. Correspondingly, for any dam, the probability of failure is much <br />lower when water levels are substantially below the design capacity for the <br />reservoir. <br />There are several ways an earthquake can cause an earthen fill dam, embankment <br />dam, or levee to fail. <br />Compaction failure <br />The most common mode of dam failure, due to an earthquake, occurs when the <br />embankment is not properly compacted. Dams can settle or spread laterally. By <br />itself, such settlement does not generally lead to immediate failure. However, if the <br />reservoir is full, relatively minor amounts of settling may cause overtopping to <br />occur, resulting in scour and erosion which could progress to failure. <br />Structural failure <br />Ground shaking can also cause structural failures or overtopping of dams. For any <br />dam, improper design or construction, or inadequate preparation of foundations and <br />abutments can also cause failures. <br />Landslide tsunamis <br />Landslides into the reservoir, which may occur on their own or triggered by <br />earthquakes, may lead to surge waves which overtop dams, or hydrodynamic forces <br />which cause dams to fail under the unexpected load. <br />Seismic Seiches <br />Overtopping or overloading of a dam structure can also occur when an earthquake <br />causes seismic seiches (waves) in the reservoirs. A seismic seiche is a standing <br />wave in which the largest vertical oscillations are at each end of a body of water <br />with very small oscillations at the center. <br />Equipment Failure <br />An earthquake can damage spill ways, gates, turbines, and electrical equipment <br />used to operate the dam. When such failures occur water can quickly rise behind a <br />dam causing it to be overtopped. <br />More information on local dams is in Appendix H. <br />2-12 January 2020 <br />