Resolution No. 5303 - Laserfiche WebLink

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Eugene -Springfield Area Natural Hazards Mitigation Plan 2. Hazard Descriptions The failure rate for dams is less than 1%,51 with overtopping due to flooding accounting for 34% of the failures. Embankment dams cannot normally withstand a significant overtopping incident. For embankment dams, the most common failure mode is erosion during prolonged periods of rainfall and flooding. When reservoirs are full and water inflow rates exceed the capacity of the controlled release mechanisms (spillways and outlet pipes), overtopping may occur. Overtopping can scour and erode the dam itself and/or the abutments which may lead to partial or complete failure of the dam. Especially for embankment dams, internal erosion, piping, or seepage through the dam's foundation or abutments can also lead to failure. Willamette River Basin dams are designed for spillway opening only during infrequent severe events. Increased use can cause wear on spillway parts and lead to greater maintenance needs and an increased risk of failure. Vegetative growth and burrowing animals can cause erosion and weakening of smaller dams which can also be a common cause of failure. Levees are very similar to dams but are only used for flood control. They run parallel to the body of water. Both dams and levees fail in similar ways. Most levees in the United States (97%) are operated and maintained by local governments or private entities. 52 History of Impact in Eugene -Springfield There have been no reported flood -induced dam failures for the Eugene or Springfield area Although the likelihood of failure is very low, all dams upstream from the Eugene -Springfield area have the potential of causing widespread flooding, should they fail. Nine dams in the area could significantly impact the area if any one of them was to fail (Appendix H). All the major dams which could affect the area were built to flood standards and the probability of a failure is low according to the Army Corp of engineers. Additionally, the Hills Creek Dam is likely to withstand floods at least as large as a 1,000-year incident without damage expected. There are several non -certified levees in the Eugene -Springfield area. Though a failure of one of these structures would be limited in scope, compared to a dam failure, the likelihood of such an incident occurring is unknown. 51 "Dams' safety is at the very origin of the foundation of ICOLD." ICOLD. Accessed November 15, 2017. http://www.icold-eigb.net/GB/dams/dams_safety.asp 52 United States. Army Corps of Engineers. FENIA. National Levee Database. Accessed August 2019. httl)s://levees.see.usace.army.mil/#/public-dashboard 2-33 January 2020