The record 2005 hurricane season and the devastation of New Orleans and Mississippi by an indirect hit from Hurricane Katrina ramped up fears about the possibility of a "super-hurricane" colliding head-on with a major city. But the chances of any one place on the Gulf Coast being hit by such a powerful storm are slim, a new study of past hurricane activity finds.
Though hurricane activity seems to fluctuate in 20 to 30 year cycles, finding patterns of this activity is difficult because historical records only go back about 150 years.
"People were discussing the probability of a Category 5 hurricane making direct impact on New Orleans," said study team leader Kam-biu Liu of Louisiana State University. "That's tricky, because it's never actually happened in history. Even Katrina, though still extremely powerful, was only a Category 3 storm at landfall."
The odds of a particular city, such as Mobile, Alabama, being hit are small, but the chances of a major hurricane striking anywhere along the Alabama coast or along the entire United States would be much greater, Liu told LiveScience, though an exact percentage is difficult to calculate.
Averages for the nation's coasts overall are meaningless in some ways, because most major storms hit specific locations, such as Florida, and hardly ever strike somewhere like Providence, Rhode Island, he said. A national figure would be falsely inflated by the rate of major storms in the southeast. Instead, it makes more sense to calculate the odds for targeted cities or regions. For Fort Myers, Florida, major hurricanes tend to return once every 16 years, he said. In comparison, major hurricanes hit New Orleans once every 31 years.
Sifting through sand
To look beyond the historical record and peer further back in hurricane history on the Gulf Coast, the researchers examined sediment samples from coastal lakes.
"Basically, we worked under the assumption that the storm surge from these catastrophic hurricanes would have the capability to drive sand over beach barriers and into coastal lakes," Liu said. "This is called an overwash event. We believed that pulling sediment cores from coastal lakes and analyzing the sand layers might give us the information we needed."
According to the team's results, published in the March issue of American Scientist, catastrophic hurricanes have only hit each drilling site 10 or 12 times in the past 3,800 years.
"That means the chances of any particular Gulf location being hit by a Category 4 or Category 5 hurricane in any given year is around 0.3 percent," Liu said, though he cautions that the team needs more data to make sure this number is accurate.
The chances of any one place in the Northeast being hit by a major hurricane would likely be even smaller, Liu said.