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CHAPTER 8. DAM FAILURE <br />8-8 <br /> <br />Figure 8-4. Keechelus and Kachess Dam Inundation Area <br />8.2.3 Frequency <br />Dam failures are infrequent and usually coincide with events that cause them, such as earthquakes or <br />excessive rainfall. The probability of any type of dam failure is low in today’s regulatory environment. <br />There is a “residual risk” associated with dams that remains after safeguards have been implemented. The <br />residual risk is associated with events beyond those that the facility was designed to withstand. <br />8.2.4 Severity <br />The DSO classifies dams and reservoirs in a hazard rating system based solely on the potential <br />consequences to downstream life and property that would result from a failure of the dam and sudden <br />release of water. The following codes are used as an index of the potential consequences in the downstream <br />valley if the dam were to fail and release the reservoir water: <br />– 1A = Greater than 300 lives at risk (High hazard) <br />– 1B = From 31 to 300 lives at risk (High hazard) <br />– 1C = From 7 to 30 lives at risk (High hazard) <br />– 2 = From 1 to 6 lives at risk (Significant hazard) <br />– 3 = No lives at risk (Low hazard)