Multi-Hazard Mitigation Plan: 11. DamFailures

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Chapter 11:Hazard Analysis-Dam Failures 11.3 Probability of Occurrence Given the increased monitoring procedures enacted following the breach of the Ka Loko Dam on Kauai, the probability of a dam failure anywhere in the state of Hawaii has been significantly reduced. A major dam failure is a very rare event for which there is no defined recurrence interval. However, the potential does exist during an extreme rainfall event, or major earthquake at any unmaintaincd or under maintained location. 11.4 Risk Assessment After the failure of the Kaloko Dam, the DNLR Dam Safety Division initiated a project to study the potential inundation areas downstream of all 135 registered dams in Hawaii. The failure scenarios considered were: • Sunny day failure with dry downstream conditions • Failure occurs when dam is at maximum capacity • Failure occurs by piping failure halfway up the dam face • Spillways and dam outlet works are inoperable at the time of the breach The water flow from the dam failure scenarios was modeled to determine maps of the inundation area, maximum water depth, maximum flow velocity and time of inundation at each dam. The various impacts of the dam break were considered for each dam, including: • Impact to the population by the number of people potentially impacted downstream • Impact to transportation including water depth and speed at bridges and road crossings • Impact to buildings based on replacement cost value • Impact to critical facilities including schools, hospitals, fire and police stations, government buildings, airports/seaports, shelters The results of this study could be used to infer some type of expected annual loss based on a probability of a particular dam's failure, however, these probabilities very small and ideally zero if proper monitoring and maintenance is provided. The intention of this study was prioritize critical areas, and to provide a reasonable assessment of risk, not to determine annual expected losses for comparison with other hazards. 11.5 Mitigation Strategies 11.5.1 General 11.5.1.1 National Dam Inspection Act The National Dam inspection Act (NDIA) of 1972 as expressed in Public Law 92-367 authorized for the first time the federal regulation of privately own dams. Per the NDIA, the United Sates Army Corps of Engineers (Corps) were authorized to inventory and inspect privately owned dams in the United States. The NDIA provided funding for the Honolulu 62 Image courtesy of Casey and Cyndi Riemer from Jack Harter Helicopters,Retrieved from Star Bulletin-, Website on October 26,2009 from http://archives.starbulletin.com/2006/03/15/news/stoKyOl.html 11-7 Hawaii County Multi-Hazard Mitigation Plan