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CHAPTER 8. DAM FAILURE <br />8-9 <br />The Corps of Engineers developed the hazard classification system for dam failures shown in Table 8-2. <br />The Washington and Corps of Engineers hazard rating systems are both based only on the potential <br />consequences of a dam failure; neither system takes into account the probability of such failures. <br />8.2.5 Warning Time <br />Warning time for dam failure varies depending on the cause of the failure. In events of extreme precipitation <br />or massive snowmelt, evacuations can be planned with sufficient time. In the event of a structural failure <br />due to earthquake, there may be no warning time. A dam’s structural type also affects warning time. Earthen <br />dams do not tend to fail completely or instantaneously. Once a breach is initiated, discharging water erodes <br />the breach until either the reservoir water is depleted or the breach resists further erosion. Concrete gravity <br />dams also tend to have a partial breach as one or more monolith sections are forced apart by escaping water. <br />The time of breach formation ranges from a few minutes to a few hours (U.S. Army Corps of Engineers, <br />1997). <br />Kittitas County and its planning partners have established protocols for flood warning and response to <br />imminent dam failure in the flood warning portion of adopted emergency operations plans. These protocols <br />are tied to emergency action plans (EAPs) created by the dam owners. Not all dams have EAPs; only those <br />rated as high hazard are mandated to do so by state and federal regulations. <br />8.3. SECONDARY HAZARDS <br />Dam failure can cause severe downstream flooding, depending on the magnitude of the failure. Other <br />potential secondary hazards of dam failure are landslides around the reservoir perimeter, bank erosion on <br />the rivers, and destruction of downstream habitat. <br />Table 8-2. US Army Corps of Engineers Hazard Potential Classification (1995) <br />Hazard Categorya Direct Loss of <br />Lifeb Lifeline Lossesc Property Lossesd Environmental <br />Lossese <br />Low None (rural location, <br />no permanent <br />structures for human <br />habitation) <br />No disruption of <br />services (cosmetic or <br />rapidly repairable <br />damage) <br />Private agricultural <br />lands, equipment, and <br />isolated buildings <br />Minimal incremental <br />damage <br />Significant Rural location, only <br />transient or day-use <br />facilities <br />Disruption of <br />essential facilities and <br />access <br />Major public and <br />private facilities <br />Major mitigation <br />required <br />High Certain (one or more) <br />extensive residential, <br />commercial, or <br />industrial <br />development <br />Disruption of <br />essential facilities and <br />access <br />Extensive public and <br />private facilities <br />Extensive mitigation <br />cost or impossible to <br />mitigate <br />a. Categories are assigned to overall projects, not individual structures at a project. <br />b. Loss of life potential based on inundation mapping of area downstream of the project. Analyses of loss of life potential should <br />take into account the population at risk, time of flood wave travel, and warning time. <br />c. Indirect threats to life caused by the interruption of lifeline services due to project failure or operational disruption; for example, <br />loss of critical medical facilities or access to them. <br />d. Damage to project facilities and downstream property and indirect impact due to loss of project services, such as impact due to <br />loss of a dam and navigation pool, or impact due to loss of water or power supply. <br />e. Environmental impact downstream caused by the incremental flood wave produced by the project failure, beyond what would <br />normally be expected for the magnitude flood event under which the failure occurs.