Hurricane Irma: Danger of storm surges

A life-threatening storm surge is expected for parts of Florida, especially if Hurricane Irma's winds push seawater ashore at high tide.

"This will cover your house," Governor Rick Scott said. The National Hurricane Centre forecast water levels up to 4m above ground for the Florida Keys island chain and parts of the state's Gulf coast, along with up to 63cm of rain in the Keys.

The flooding threat extended far beyond the path of Irma's eye. The Atlantic coast from Miami to Isle of Palms, South Carolina, could see up to 2m of storm surge.

What is storm surge?

It's not a wall of water or a tsunami. Simply put, hurricane winds push water towards shore. It can happen quickly and far from a storm's centre, inundating areas that don't typically flood. Storm surge doesn't just come from the ocean. It can come from sounds, bays and lakes, sometimes well inland.

The categories for hurricanes measure wind speeds, and don't say anything about storm surge. The flooding risk will not drop just because Irma's winds might weaken, said Jamie Rhome, head of the hurricane centre's storm surge unit. Large hurricanes tend to create greater storm surge over a broader area, and coastal features such as bays can act like funnels and back water up into rivers and canals, Rhome said. "This is going to sneak up on people."

What's at risk?

About 1600km of coastline from Tampa Bay to the mid-South Carolina coast could see storm surge. Much of that landscape lies less than 3m above sea level, and the surge from Irma could be higher in some areas. Much of Florida's southwest coast is uninhabited swampland, including a large section of Everglades National Park. "The Everglades won't stop the potential flooding to inhabited areas," Rhome said.

North of the Everglades lies Naples. The hurricane centre's storm surge maps, showing deep inundation for Naples, worried Colorado State University hurricane researcher Phil Klotzbach. "Look at Naples, the entire town of Naples is underwater."

Farther north is the Tampa Bay region, with about three million people.

Deadly waters

Storm surge has accounted for half the US deaths from hurricanes, tropical storms and cyclones over the last half a century, according to a hurricane centre study.

The surge helped destroy nearly half the structures along a 64km stretch of the Florida Keys during the 1935 Labour Day Hurricane, which killed over 400 people.

Storm surge flooding up to 8m above normal tide levels were associated with Hurricane Katrina in 2005, directly or indirectly causing at least 1500 deaths.

- AP