- Storm surge is partly determined by underwater topography.
- While Gulf Coast hurricanes have massive surges, storms elsewhere can generate big waves.
To understand what Irma could do to Florida, look underwater.
The varied topography of the seafloor that surrounds Florida matters when considering what kind of impact the storm will have.
The National Hurricane Center has issued storm-surge warnings and maps for much of the state's southern coastal region, with especially severe potential surge along the southwest coast, as of Friday afternoon. Forecasters have predicted a storm surge 6 to 12 feet high, according to the NHC.
This pales in comparison to the highest surge levels seen during Hurricane Katrina, which brought a record-setting surge of close to 30 feet.
"If you had a Hurricane Katrina that tracked onto south Florida, you would never have the kind of storm surge you got in Louisiana," said Scott Hagen, director of the Center for Coastal Resiliency at Louisiana State University. "By the same token," he added, "what you have in Florida that you don't have in the Gulf is big waves."
This has to do with the depth of the water. The seafloor in the Gulf of Mexico and much of the western coast of Florida descends very gradually. On parts of the southern coast, and along the Florida Keys — as well as many Caribbean islands — the seafloor drops off far more rapidly.
It is the shallow, slowly deepening water in the Gulf that allows these massive surges to build, so they have the potential to become far more severe than those seen elsewhere.
In contrast, it is the steeper declines in depth that allow big waves to form. And they can do a great deal of damage.
A single cubic yard of water weighs 1,700 pounds, and the energy of the waves can pummel both natural structures, like beaches, trees and dunes, and man-made structures.
"You have these waves that are able to get really big in mass before they break," Hagen said. "And that can be a real problem for the Keys, and a problem for south Florida."