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2025 Hazard Mitigation Plan Kittitas County, Washington ,@tffisdi 4.6.2. Dam and Levee Failure 4.6.2.1. Description Dams are a critical and vital part of the nation's infrastructure as they provide a range of benefits, such as irrigation, electrical generation, flood control, renewable/clean energy, water storage, sediment/hazardous materials control, navigation, fisheries, and recreation..Ts Dams are artificial/human- made structures built across a watercourse for the purpose of retaining water.-7e A levee is a human- made structure, usually an earthen embankment or concrete floodwall, designed and constructed to contain, control, or divert the flow of water to provide reasonable assurance of excluding temporary flooding from the levee area..8o Dam Failure The sudden rapid and uncontrolled release of impounded water or liquid-borne solids characterizes a failure. The term "dam failure" encompasses a wide variety of circumstances. Situations that would constitute a dam failure vary widely, from developing problems to a partial or catastrophic collapse of the entire dam. There are numerous potential causes of a dam failure and can be attributed to deficiencies in the original design of the dam, the quality of construction, the maintenance of the dam, operation of the appurtenances and acts of nature including precipitation in excess of the design, flood, and damage from earthquakes. Dam failures are most likelyto happen forone (1) of five (5) reasons listed in Table 4-30. Table 4-30. Dam Failure Typessl Dam failure is potentially extremely dangerous if it occurs catastrophically with little or no warning. Dam inundation areas may contain large populations and significant critical infrastructure. During a catastrophic failure, the water flow may be dozens of feet high and move at high velocity. For large dams, evacuation procedures may not be adequate to prevent large loss of life and property damage. 78 FEMA. (2020). The National Dam Safety Program: 25 Years of Excellence. Retrieved from https://www.fema.qov/sites/defaulUfiles/2020-08/national-dam-safetv-25-years.pdf. 7e Merriam-Webster. (n.d.). Dam. Retrieved from https://www.merriam-webster.com/dictionarv/dam. 80 United States Army Corps of Engineers. (n.d.). Levees and Flood Risk Management. Retrieved from https://www.nwp.usace.armv.mil/Missions/Flood-Risk-Manaqement/Levees/. 81 Association of State Dam Safety Officials. (n.d.). Understanding Dam Failures. Retrieved from https ://damsafetv.orq/dam -fai lu res. Overtopping Caused by water spilling over the top of a dam. Overtopping of a dam is often a precursor of dam failure. National statistics show that overtopping due to inadequate spillway design, debris blockage of spillways, or settlement of the dam crest account for approximatelv 34% of all dam failures in the United States. Foundation Defects lnclude settlement and slope instability, cause about 30% of all dam failures Cracking Caused by movements like the natural settling of a dam. lnadequate Maintenance and Upkeeplnadequate Maintenance and Upkeep Piping When seepage through a dam is not properly filtered and soil particles continue to progress, and form sink holes in the dam. Another 20o/o of United States dam failures have been caused by piping (internal erosion caused by seepage). Seepage often occurs around hydraulic structures (e.9., pipes and spillways), through animal burrows, around roots of woody vegetation, and through cracks in dams, dam appurtenances, and dam foundations. DescriptionFailure Type Chapter 4: Hazard ldentification and Risk Assessment 80