Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods Flash floods are characterized by rapid rise in water level,high velocity, large amounts of de- bris, and concentration in stream beds that are often normally small or even dry. The major factors are the intensity and duration of rainfall, rain shed area, and steepness of the streambed. They can also be caused by a dam break. Flash floods typically carry large amounts of debris which increase the damage they do, and are very capable of undermining bridges and carrying away vehicles. There is a major public safety hazard from these events, as the National Weather Service (NWS) estimates that over 50% of the flash flood deaths in Hawaii involve vehicles. The records provided by Mr. Tom Heffner of NWS show that since 1959, when their records began, 30 deaths from flashfloods have occurred. They do not have the data by county, but assuming that this County had one fourth of the deaths, that averages one every six years. 10.1.2 Dams A dam is a barrier constructed across a watercourse for the purpose of detention (control), storage, or diversion of water. Detention dams retard and minimize the effects of flood runoff by storing the flood flow and releasing it at a rate that does not exceed the carrying capacity of the downstream channel. Storage dams impound water during periods of surplus supply for use during drought periods. Diversion dams provide hydraulic head for diverting water into ditches, canals, or other means of conveyance. These various types of dams are constructed for one or a combination of the following reasons: flood control;water source for domestic or irrigation use; recreation; and/or hydroelectric energy production. The volume of water impounded is measured in acre-feet referring to the number of acres of land that would be covered to a depth of one foot. A list of all dams on Hawaii County is provided below and mapped in Figure 10-2: 1. HA00026 Lalakea Reservoir 2. HA00051 Hawi No. 5 Reservoir 3. HA00027 E-13 Reservoir 4. HA00052 Kehena Reservoir 5. HA00040 Waikoloa 50 MG Reservoir 6. HAO0122 Waikoloa 50 MG Reservoir 2 7. HA00042 Puukapu Reservoir S. HA00123 Pint Pulehu Reset voir 9. HA00043 Puukapu Watershed Retarding Dam R-1 10. HA00131 Paauilo Reservoir 11. HA00049 Keaiwa Reservoir 12. HA00136 Waikoloa 50 MG Reservoir 3 13. HA00050 Hawi No. 3 Reservoir 10-3 Hawaii County Multi-Hazard Mitigation Plan