Multi-Hazard Mitigation Plan: 05. Tropical Cyclones

Home

Chapter 5:Hazard Analysis—Tropical Cyclones CHAPTER 5 - TROPICAL CYCLONES 5.1 Description of Hazard Tropical cyclones are storms that originate over a warm tropical ocean. When such a storm develops sustained winds (one minute average) over 74 miles per hour, it is classified as a hurricane.15 They are characterized by a large counterclockwise circulation of air and lower barometric pressure near the center. These actions produce the major hazards of hurricanes: high winds, high waves, storm surge, and heavy rain. These hazards cause flooding, salinization of water supplies, agricultural crop losses, erosion, loss of life, and damage to structures and infrastructure. The winds can affect all parts of an island and can be intensified by mountain ranges (orographic or topographic amplification). A good example was Hurricane Iwa in 1982; it was barely a Category 1 hurricane but winds in the Wahiawa area of Oahu equaled those in areas on Kaua'i, which was closest to the path of the storm. Meteorologists consider the notion that Mauna Loa and Mauna Kea "protect" Hawai'i Island from hurricanes to be unfounded. Waves and storm surges normally hit coasts ahead of high winds, as waves move faster than a hurricane advances. Locally intense rainfall may occur as the hurricane makes landfall.16 Thus hurricanes,though rare in Hawai'i, are complex hazards. 5.1.1 Coastal Wave/Surge Storm surge is the rise of water above sea level at the time of storm onset. The height of storm surge along the open coast depends on a number of factors, which include: (1) wind speed and associated barometric pressure, (2) depth of water or shoaling factor, (3) storm trajectory, and (4) speed of the storm. Coastal configuration in the form of estuaries or bays can cause a funneling or amplification effect. Coincidence with high tide will also increase surge height. Although the maximum surge usually affects only a relatively short length of coastline, combined storm surge and wave action may have damaging effects over the entire coastline facing a major storm center. Wind-driven waves on top of the storm surge pose a number of added problems. In Hawaii the wave run-up typically floods areas not reached by the surge itself. The scouring power of waves is considerable. The storm surge is the most dangerous part of a hurricane as pounding waves create very hazardous flood currents. About 90% of the deaths experienced in the past near the coast resulting from hurricanes are caused not by wind,but by storm surge. Stream flooding is also much worse inland during the storm surge because of backwater effects. 15 In the western Pacific,hurricanes are called"typhoons"; similar storms in the Indian Ocean are called"cy- clones". An organized system of clouds and thunderstorms with a defined circulation and maximum sus- tained winds of 38 mph or less is called a "tropical depression". An organized system of strong thunderstorms with a defined circulation and maximum sustained winds of 39 to 73 mph is called a"tropical storm". 16 Schroeder, Tom, 1993,"Hawaii Hurricanes: Their History, Causes, and Future," Office of State Planning, December 1993. 5-1 Hawaii County Multi-Hazard Mitigation Plan