Hurricane Irene Targeting the East Coast: building and infrastructure failures likely.

There are no shortages of wind related disasters in the US this year.  On the heels of a record setting tornado season, the east coast is bracing for Hurricane Irene.  Irene is currently moving slowly north at a speed of about 14 mph and is expected to make landfall along the outer banks area of North Carolina on Saturday.  The large slow moving storm which is estimated to reach 450 miles in width is generating a lot of warnings to the public in multiple impact areas including; wind speed, flooding dangers from the large amounts of rain expected, storm surge and possilby the generation of tornadoes.  To make matters worse, by the time Irene reaches the New York City area, it will coincide with high tide prompting officials in NYC to issue the first  mandatory evacuation in history for low lying districts and a phased shutdown of mass transit infrastructure including subways and bridges (if winds of  60 mph are reached).

Irene is generating wind speeds that put it at the high end of a Category 2 storm (see the Saffir–Simpson Hurricane Wind Scale below) with some models predicting that it could rise to a low end Category 3.   Storm surge in conjunction with wave height is expected to be a major danger for the public and could generate extreme physical damage in many locations ranging from beach erosion to flooding to structural damage and possible collapse. 

Currituck Beach Lighthouse near Corolla, NC (Outer Banks) in August 2011. Photo Credit: m-kev

Carolla Island Bridge - Outer Banks: National Historic Landmark (Photo credit: m-kev)

It is important to note that the Saffir-Simpson scale is based on wind speed, not storm surge.  One of the lessons learned from Hurricane Katrina was that the magnitude of storm surge is not necessarily related to storm category.  Storm surge in the Outer Banks area of North Carolina is being predicted at 6 – 11 feet.  In conjunction with high waves and wind a surge of this magnitude has the potential to cause major destruction to the point of changing the landscape of the islands.  The narrow islands are packed with vacation homes and various forms of residential development.  Many of the newer structures are elevated one floor above grade, either for flood considerations or in an effort to obtain a better ocean view.  Regardless, those homeowners at least have a chance at literally riding over the surge.  Grade level structures, roads, commercial buildings and the fragile dune landscape will be the hardest hit.   The area is not without a number of historical structures which have lived through a number of severe storms.  The Currituck Beach Lighthouse on the Outer Banks near Corolla,  NC is one such structure that has survived well over 100 years of storms.  It is shown in the adjacent photograph in calm weather in August of 2011 waiting to ride out another storm.

To many, the realization of an Irene sized storm this year comes as no surprise.  Early information releases by agencies such as NOAA predicted this hurricane season to be above average in number and severity.  See this BFF Hurricane Prediction Post for details.

Relationship Between Categories and Wind Speed

Hurricanes are rated in categores based on wind speed ranging from One to Five with a Category Five being the most severe resulting in sustained winds of 156  miles per hour (mph) or greater. 

The summary below shows the wind speeds associated with each category based on the current Saffir–Simpson Hurricane Wind Scale:

  • Category One — Winds 74-95 mph                                                   
  • Category Two — Winds 96-110 mph
  • Category Three — Winds 111-130 mph
  • Category Four — Winds 131-155 mph
  • Category Five — Winds greater than or equal to 156 mph

The definition of sustained winds used by most weather agencies is that of a 10-minute average at a height of 33 ft.  Note however that some agencies and organizations use a 1-minute average speed (also at 33 ft) which can often result in confusion when storm statistics are reported in the media. 

It is also important to note that the definitions of wind speed noted above are not used for design of structures which are often based on 3-second wind gusts.  A 3-second gust as used by engineers for lateral load design of buildings and related structures is defined in ASCE Manual 7 as the highest sustained gust over a 3 second period having a probability of being exceeded per year of 1 in 50.

Relationship Between Damage and Hurricane Categories

Galveston disaster, interior St. Patrick's church - Photo Credit: Zahner/Library of Congress via pingnews (Public Domain)

Damage associated with a hurricane is the result of many factors, not just sustained winds.  Duration, landfall location, amount of rainfall, surrounding terrain, type and quality of construction, height of storm surge, and other items can all impact the amount of damage, loss of life and injuries in a tropical cyclone (hurricane).  For example, the worst wind related disaster based on casualties that ever occurred in the US was the 1900 Galveston, Texas hurricane.  Galveston was a Category 4 storm resulting in an estimated 8000 fatalities.  Compare this with  the Category 5 Hurricane Katrina which caused about 1800 deaths.  An expanded and detailed chart of types of damage for various types of structures associated with different hurricane categories can be found in the chart noted below at the National Weather Service – National Hurricane Center website.

 Worst Hurricanes

The worst hurricane is a matter of perspective and depends on what criteria you want to measure such as fatalities and injuries or economic damage.  The links below provide a variety of rankings relative to the worst of the worst:

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