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<br />8-1 <br />CHAPTER 8. <br />DAM FAILURE <br /> <br />8.1. GENERAL BACKGROUND <br />8.1.1 Causes of Dam Failure <br />Dam failures in the United States typically occur in one of <br />four ways (see Figure 8-1): <br />• Overtopping of the primary dam structure, which <br />accounts for 34 percent of all dam failures, can <br />occur due to inadequate spillway design, <br />settlement of the dam crest, blockage of spillways, <br />and other factors. <br />• Foundation defects due to differential settlement, <br />slides, slope instability, uplift pressures, and <br />foundation seepage can also cause dam failure. <br />These account for 30 percent of all dam failures. <br />• Failure due to piping and seepage accounts for <br />20 percent of all failures. These are caused by <br />internal erosion due to piping and seepage, erosion <br />along hydraulic structures such as spillways, <br />erosion due to animal burrows, and cracks in the <br />dam structure. <br />• Failure due to problems with conduits and valves, <br />typically caused by the piping of embankment <br />material into conduits through joints or cracks, <br />constitutes 10 percent of all failures. <br />The remaining 6 percent of U.S. dam failures are due to <br />miscellaneous causes. Many dam failures in the United <br />States have been secondary results of other disasters. The <br />prominent causes are earthquakes, landslides, extreme <br />storms, massive snowmelt, equipment malfunction, <br />structural damage, foundation failures, and sabotage. The <br />most likely disaster-related causes of dam failure in Kittitas <br />County are earthquakes. <br />Poor construction, lack of maintenance and repair, and <br />deficient operational procedures are preventable or <br />correctable by a program of regular inspections. Terrorism <br />and vandalism are serious concerns that all operators of public facilities must plan for; these threats are <br />under continuous review by public safety agencies. <br />•DEFINITIONS <br />Dam—Any artificial barrier and/or any <br />controlling works, together with <br />appurtenant works, that can or does <br />impound or divert water. (Washington <br />Administrative Code, Title 173, Chapter <br />175.) <br />•Dam Failure—An uncontrolled release of <br />impounded water due to structural <br />deficiencies in dam. <br />Emergency Action Plan—A document <br />that identifies potential emergency <br />conditions at a dam and specifies actions to <br />be followed to minimize property damage <br />and loss of life. The plan specifies actions <br />the dam owner should take to alleviate <br />problems at a dam. It contains procedures <br />and information to assist the dam owner in <br />issuing early warning and notification <br />messages to responsible downstream <br />emergency management authorities of the <br />emergency situation. It also contains <br />inundation maps to show emergency <br />management authorities the critical areas <br />for action in case of an emergency. (FEMA <br />64) <br />•High Hazard Dam—Dams where failure or <br />operational error will probably cause loss of <br />human life. (FEMA 333) <br />•Significant Hazard Dam—Dams where <br />failure or operational error will result in no <br />probable loss of human life but can cause <br />economic loss, environmental damage or <br />disruption of lifeline facilities, or can impact <br />other concerns. Significant hazard dams <br />are often located in rural or agricultural <br />areas but could be located in areas with <br />population and significant infrastructure. <br />(FEMA 333)