Resolution_2025 Kittitas County Hazard Mitigation Plan

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2025 Hazard Mitigation Plan Kittitas County, Washington Chapter 4: Hazard Identification and Risk Assessment 81 The State of Washington Department of Ecology Dam Safety Office (DSO) is responsible for overall dam safety within the State. As of December 2023, DSO’s inventory of dams includes a total of 1,242 regulated dams in the State of Washington. These dams are defined as structures that can impound 10 acre-feet or more of watery material at the dam crest elevation. Approximately, 51% of the dams in the State are privately-owned and 21% are owned by local government.81F 82 Levee Failure A levee failure can occur when flood waters exceed the height of a levee and flow over its crown (i.e., overtopping); or when breaching occurs. A levee breach can occur gradually or suddenly when part of the levee gives way, resulting in an opening through which floodwater may pass. The most dangerous breaches happen quickly during periods of high water. The resulting stream of water can quickly flood a large area behind the failed levee with little to no warning.82F 83 Earthen levees can be damaged in several ways and if severe enough these scenarios can lead to a zone of weakness that could cause a levee breach. • Strong river currents and waves can erode the surface. • Trees growing on a levee can blow over, leaving a hole where the root ball and soil used to be. • Burrowing animals can create holes that enable water to pass through a levee. • In seismically active areas like Kittitas County, earthquakes and ground shaking can cause a loss of soil strength, weakening the levee potentially resulting in a levee failure. Seismic activity can also cause levees to slide or slump, both of which can lead to failure.83F 84 The United States Army Corps of Engineers (USACE) operates, maintains, and evaluates flood protection levees to determine if they meet accreditation requirements. Most levees are owned by local communities and flood control districts that must ensure proper operation and maintenance of the levee system as well. 4.6.2.2. Hazard Location The National Inventory of Dams (NID) consists of dams meeting at least one (1) of the following criteria: • High hazard potential classification – loss of human life is likely if the dam fails. • Significant hazard potential classification – no probable loss of human life but can cause economic loss, environmental damage, disruption of lifeline facilities, or impact other concerns. • Equal or exceed 25 feet in height and exceed 15 acre-feet in storage. • Equal or exceed 50 acre-feet storage and exceed six (6) feet in height. Although the goal of the NID is to include all dams in the United States that meet these criteria, in reality, this inventory is limited to information that can be gathered and properly interpreted. In most cases, dams within the NID criteria are regulated (e.g., construction permit, inspection, and/or enforcement) by federal or state agencies, who have basic information on the dams within their jurisdiction. All high hazard dams have Emergency Action Plans (EAP) in place, however due to data privacy and pr otected information, 82 State of Washington Department of Ecology. (2023). Inventory of Dams Report. Retrieved from https://apps.ecology.wa.gov/publications/documents/94016.pdf . 83 American Society of Civil Engineers. (2010). So, You Live Behind a Levee! Retrieved from https://www.spl.usace.army.mil/Portals/17/SoYouLiveBehindLevee.pdf. 84 Ibid.