Multi-Hazard Mitigation Plan: 10. Floods

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Chapter 10:Hazard Analysis—Floods March. They originate over the Pacific Ocean as a result of the intersection between polar and tropical Pacific air masses and move eastward over the islands. These storms reflect orographic influences and are accompanied by widespread precipitation(see Figure 10-1). Subtropical Cyclonic Storm Mid-latitude Cyclonic Storm 1075 L lots f\ 707o L roz5 99',0 1020 107 995 .'.5 1011.5 1015 X7005 H rOO5 1010 1000 ` H 1075 KONA STORM Hawafi� c 7011.5 1011.5 Heweri 7015 1010 7070 COLD FRONT 1071.5 780° 150` _ 170' `�•1�' 180° 150' 120' Kona storms are low-pressure areas(cyclones)of subrrop- between CMtober and April,mark the leading edges of ical origin that usually develop northwest of Hawaii in cold air masses associated with low-pressure systems winter and move slowly eastward,accompanied by south- moving north of the Islands in the prevailing westerlies. erly winds,from whose direction the storm derives its They bring widespread clouds,heavy rain,and occasional name,and by the clouds and rain that have made these thunderstorms.Vigorous fronts may be preceded by strong storms synonymous with bad weather in Hawaii.Kona southwest winds and followed by gusty northerly winds. storms vary in number from year to year.Some winters Kauai experiences as many as 20 cold fronts per year,but have had none,others five or more.Cold fronts,common generally only about half of them reach Hawaii Island. Figure 10-1. Kona Storms and Cold Fronts • Upper troughs. Upper troughs are low pressure storms over wide areas that cause heavy rain and strong winds. • Convective-type storms. Convective-type thunderstorms can occur at any time of the year. They are most common during periods of relatively high humidity and unstable air conditions. These storms cover comparatively small areas and result in high-intensity rainfall of short duration. • Hurricanes or tropical storms. Hurricanes and tropical storms cause heavy rains, strong winds, and high surf. Heavy rainfall creates three types of flooding: 1) channel overflow, 2) overland sheet flow, and 3)ponding of standing water in poorly drained low-lying areas. Channel overflow occurs when the carrying capacity of the channel is exceeded,which can be exacerbated by develop- ment changes within the drainage basin or clogging by debris or overgrown streambed vegetation. Overland sheet flow occurs primarily in areas with undefined drainage ways, such as Puna and the leeward side of the island (e.g., Kona, Waikoloa, and Kawaihae). Poorly drained low-lying areas are a problem when flooding occurs even when rainfall is not heavy. Intense rainfall may trigger "flash-floods"which provide little warning (less than six hours)before the affected area experiences flood conditions. Prolonged rainfall may result in an accumulation of water creating flooding conditions that last several days, or even weeks. Factors influencing flooding conditions include rainfall intensity and duration, topography, soil type, antecedent soil moisture, and ground cover. 10-2 Hawaii County Multi-Hazard Mitigation Plan