Multi-Hazard Mitigation Plan: 11. DamFailures

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Chapter 11:Hazard Analysis—Dam Failures CHAPTER 11 - DAM FAILURES 11.1 Description of Hazard The severity of flooding caused by dam breaks has been analyzed by modeling by the Pacific Disaster Center and the US Army Corps of Engineers. Although no known dam break events have occurred on Hawaii Island, the potential damage is significant enough to merit prevention and disaster preparation. Dam failure can result from: prolonged periods of rainfall; inadequate spillway capacity; internal erosion resulting in structural failure; improper maintenance; improper design; negligent operation; failure of upstream dams on the same watercourse; landslides into reservoirs which may cause surges resulting in overtopping; high winds which can cause significant wave action resulting in substantial erosion; and earthquakes,which cause longitudinal cracks and weaken the entire structure. Hawaii County has 13 dams, all of which are earth dams.55 Most dams in Hawaii are old earthen berm reservoirs built during the plantation era originally for irrigation purposes. Of these dams, DLNR has rated two on this island to be "high" hazard based on the extent of potential downstream losses to residential/ commercial structures or agricultural crops (see Figure 11-1). A NASA-funded study applied a model to estimate flood losses.56 The report classified the inundated area into three risk zones: the zone of severe flooding corresponds to the region where flood depths are sufficient to cause severe structural damage or loss of life; the zone of probable damage corresponding to the region that is expected to be flooded to a depth of at least one foot; and the maximum possible flood extent. Because of the short duration of such flooding events from a dam break, coupled with the unknown reliability of the model, it would not seem practical to use the model for real time response. The value of the modeling is to assess priority for preventive monitoring and maintenance rather than to incorporate the results into the FIRM'S where it would be used for permitting and insurance purposes. 55 Goosby,S.,Chatman,A.,Michaud,J.,Kerper,D.,Dam Break Inundation Study for the State of Hawaii, Prepared for the State of Hawaii's Department of Land and Natural Resources,ASCE 2008 56 Johnson,C.and J.Michaud.Dam Failure Inundation Mapping Project,NASA Contract No.NASW-99044, January 15,2003. 11-1 Hawaii County Multi-Hazard Mitigation Plan