Multi-Hazard Mitigation Plan: 04. High Wind Storms

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Chapter 4:Hazard Analysis—High Wind Storms CHAPTER 4 - HIGH WIND STORMS 4.1 Description of Hazard Wind is one of the most costly property perils, causing more damage nationwide than earthquakes, freezing, or other natural perils. (IIPLR, 1994) In wind storms and especially hurricanes, windborne debris can be a major factor in causing damage. Flying objects such as tree limbs, outdoor furniture, signs, roofs, gravel, and loose building components from progressively failing adjacent buildings can impact the building envelope, creating openings that allow internal pressure to build within. The internal pressures add to the external pressures producing more severe pressures on the building components of the structure. (IIPLR, 1994) The roof then is subjected to tremendous internal pressure building from inside, together with the negative wind pressures lifting the roof from outside. The resulting combined forces may cause roof system failure if the roof has not been adequately designed and constructed. If the roof is breached, high winds and rain destroy the inside of the building. Winds in Hawaii originate from three main sources: trade winds, Kona winds, and hurricanes or tropical storms. Northeast trade winds are dominant throughout most of the year and generally range in velocity between 10 and 20 mph. However, trade winds of 40-60 mph occasionally occur for several days at a time when the sub-tropical high-pressure cell located in the central North Pacific Ocean intensifies. During the 1993-1994 and 1994-1995 winter seasons, for example, strong and gusty trade winds of 40 to 50 mph lasted several days and inflicted damage to roof tops, tree limbs, and telephone equipment. The east-facing coastlines, as a result, are the windward coasts and most impacted by trade wind energy. Kona winds are southerly winds and occur as light and variable winds during summer months when trade wind circulation breaks down,but in winter they can be very strong when storm systems moving across the central North Pacific draw air from the south toward their low pressure troughs. Damaging Kona winds from storms generally occur during the winter and spring seasons and have reached velocities of 50 mph for several days on end. 4.1.1 Utilities Power distribution lines are susceptible to strong winds due to the relatively low design standards for older portions of the grid which may also have preexisting damage in the wood poles due to decay and termite attack. One of the most common impacts of a wind storm is the loss of electrical service to some communities. [A comprehensive discussion of the historic design criteria for electrical transmission and distribution systems in Hawaii County, and recent updates, is provided in Chapter 5.] 4.1.2 Trade Winds Trade winds are by far the most common winds over Hawaiian waters and play a major role in defining the climatology of the region. (Kodama 1998) These persistent winds, which blow from a NE to ENE direction, became known as trade winds long ago when clipper ships 4-1 Hawaii County Multi-Hazard Mitigation Plan