Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods streams in East Hawaii had recurrence intervals between 5 and 30 years.49 Governor Cayetano declared the island of Hawaii a disaster area and President Clinton declared Hawaii County a federal disaster area, which authorized federal assistance (FEMA DR-1398-HI). More than 1,131 Big Island flood victims registered for assistance through FEMA's toll-free tele-registration number.50 FEMA DR-1743-HI December 2007 A complex storm system developed in the northwest Pacific Ocean and moved southeast toward Hawaii on December 3, 2007. As the system moved southeast, the associated cold front intensified and approached the island chain from the west. Strong southwest winds ahead of the cold front were reported across Kauai and Oahu on the fourth and fifth. Lingering atmospheric instability behind a previous frontal system combined with warm, moist conditions ahead of the cold front lead to extremely heavy rains across the state with Maui and the Big Island receiving the heaviest rains between the sixth and the eighth of December. The storm weakened and drifted northeast toward the end of the period, but a surface trough remained across the state keeping conditions unsettled until roughly December 11. Widespread property damage was reported all across the state during this weather event. The approach of the front and the strong southwest flow prompted forecasters to issue high wind warnings for parts of Kauai and Oahu. As the front moved down the island chain, additional warnings were issued. Although winds were generally strong across the state, downslope zones of northeast Kauai, central Oahu, windward Oahu, and Molokai saw the highest gusts, in the 60-70 mph range. Very strong winds were also reported at the summits of Haleakala on Maui and the summits of Mauna Kea and Mauna Loa on the Big Island of approximately 70-80 mph. Maui and the Big Island experienced the heaviest rainfall during the event. Widespread flooding was common across portions of central and upcountry Maui. Flash flooding in the Waiohuli area of Maui swept a house from its foundation. Two day totals for the period ending at 8:00 am HST Thursday, December 6, 2007 were between ten and twelve inches at the Kapapala Ranch and Hawaii Volcanoes National Park Headquarters gauges. Up to two feet of water covered portions of Highway 1 1 in the Kau district of the Big Island. State wide totals for the same period ranged from one to six inches. Conditions on the summits of Mauna Kea and Mauna Loa deteriorated rapidly during the approach of the front on the fifth of December. High winds and snow showers created white- out conditions on the summits. Snow levels dropped down to around eleven thousand feet. Seven foot snow drifts and icing forced park rangers to shut down the Mauna Kea access road on the fifth. Conditions did not allow for the road to be reopened until the end of the storm period. 49 Data on the November 2000 flood cited in this paragraph are from Fontaine,R.and B.Hill,Streamflow and Erosion Response to Prolonged Intense Rainfall of November 1-2,2000,Island of Hawaii,Hawaii,U.S. Geological Survey,Water-Resources Investigations Report 02-4117,2002. 50 Hawaii Hazard Mitigation Forum website,http://www.mothemature-hawaii.com. 10-6 Hawaii County Multi-Hazard Mitigation Plan