Res 2006-163 - Laserfiche WebLink

Home Browse Search

':, '-.. ;U:1. \, 2.5 High Wind I Hazard Identification: Wind is defined as the motion of air relative to the earth's surface. The horizontal component of the three-dimensional flow and the near-surface wind phenomenon are the most significant aspects of the hazard. Extreme windstonn events are associated with extra-tropical and tropical cyclones, winter cyclones, and severe thunderstonns and accompanying mesoscale offspring such as tornadoes and straight-line winds. Straight-line winds are often responsible for most of the wind damage associated with a thunderstonn. These winds are often confused with tornadoes because of similar damage and wind speeds. However, the strong and gusty winds associated with straight-line winds blow roughly in a straight line unlike the rotating winds of a tornado. Downbursts or microbursts are examples of damaging straight-line winds. A downburst is a small area of rapidly descending rain and rain-cooled air beneath a thunderstorm that produces a violent, localized downdraft covering 2.5 miles or less. Wind speeds in some of the stronger down bursts can reach 100 to 150 miles per hour, which is similar to that of a strong tornado. The winds produced from a downburst often occur in one direction, and the worst damage is usually on the forward side of the downburst. In the mainland United States, the mean annual wind speed is reported to be 8 to 12 mph, with I frequent speeds of 50 mph and occasional wind speeds of gr.eater than 70 mph. For coastal areas from Texas to Maine, tropical cyclone winds may exceed 100 mph. Property damage and loss of life from windstorms are increasing due to a variety of factors. Use of manufactured housing is on an upward trend, and this type of structure provides less resistance to wind than conventional construction. Uniform building codes for wind resistant construction are not adopted by all states, and population trends show rapid growth in the highly exposed areas. Areas experiencing the highest wind speeds are coastal regions from Texas to Maine, under the influence of North Atlantic Ocean and Gulf of Mexico windstorms associated with tropical cyclones, and the Alaskan coast, under the influence of winter low-pressure-system windstorms in Gulf of Alaska and North Pacific Ocean. The Hays County Mitigation Planning Committee classified high wind as "major" in severity and "likely" in frequency. The high wind hazard is also associated with tornados and severe weather or winter stonns. Hays County is located in Zone III with a design wind speed of 200 mph based on the "Design Wind Speed Map for Community Shelters" from FEMA's "Taking Shelter from the Storm: I Building a Saferoom in Your House", publication 320. This information is based on ASCE 7- 98 based on 3 second gust, 33 feet above grade and Exposure C. Ha~'s counl~', Texas Miligalion Plan 63