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Appendix H: Dam Failure The probability of a dam failure impacting Eugene -Springfield is low; vulnerability to a dam failure event is high. HA Characteristics of Dams Dams are impervious structures that impound water by blocking the flow of a river or stream. Dams serve many purposes including water storage for potable water supply, flood control, hydroelectric power generation, agricultural irrigation, fire suppression, navigation, recreation, and others. Dams are typically multifunctional; however, the United States Army Corps of Engineers prioritizes flood control when operating Willamette Basin dams.1 Modern dams employ control mechanisms such as gated spillways or outlet pipes to manage the release of water, governing the natural variations in stream flow.' During periods of high flow, water is stored behind a dam then released to increase water levels during periods of low flow. Controlled releases results in lower peak flows and higher minimum flows than in uncontrolled streams. Water storage and release patterns vary from dam to dam, depending on the primary purpose(s) and a wide variety of economic, regulatory and environmental considerations. Large modern dams usually fall into two categories embankment or concrete. Both are typically constructed on a foundation, which may be concrete, natural rock or soils, or compacted soils. The surrounding natural valley walls become the abutments of the dam structure itself and dams are often sited along a constricted part of a river valley to minimize cost. Embankment dams are commonly termed earthfill or rockfill dams, depending on the primary material (soil or rock) used in their construction. Built as broad flat structures they are often twice as wide at the base to provide stability to the structure. Impervious layers may be added to the dam structure to reduce leakage. Subject to erosion by running water, embankment dams utilize erosion -resistant materials in the water release and control mechanisms of the dam. For example, concrete spillways with concrete or steel gates or outlet pipe systems with concrete or steel pipes as part of the water release control system help to reduce erosion.3 Modern concrete dams fall into three major classes: gravity, buttress and arch.4 Concrete gravity dams are designed on principles similar to embankment dams. They are broad structures with a flat base and a narrow top; the upstream side is flat while the 1 United States. Association of State Dam Safety Officials. Benefits ofDams. httos:Hdamsafety.Org/damsI01 Accessed August 2019. 2 Ibid 3 Paul Breeze. Power Generation Technologies (Second Edition) 2014. ° United States. Association of State Dam Safety Officials. Benefits ofDams. httos:Hdamsafety.Org/damsI01 Accessed August 2019. 6-129 January 2020