When it slammed ashore on Aug. 29, Hurricane Katrina generated what might have been a record storm surge with water levels higher than a two-story building and storm waves cresting more than 50 feet above sea level.
“The whole scale of this thing is just staggering,” said Asbury H. Sallenger, an oceanographer with the U.S. Geological Survey’s Center for Coastal and Watershed Studies in St. Petersburg, Fla.“It is horrible,” said Stephen Baig, a surge specialist with the National Hurricane Center in Miami.
Baig has estimated the peak storm surge at 27 feet. That was atop the normal 2-foot tide at that time of the morning.
The estimate is based on a computer calculation, Baig said, and it could take a month or more to verify the figure.
A surge is caused by powerful winds piling up the water.
If accurate, the storm surge would break the old record of 24.4 feet set by Hurricane Camille in 1969, a storm that hit almost the same regions as Katrina. Camille was a powerful Category 5 hurricane, with winds exceeding 155 mph. However, Camille was a much smaller storm than Katrina in terms of areal extent, and Camille’s overall damage was less extensive.
Baig said Katrina’s maximum surge evidently hit a five-mile stretch along Bay St. Louis in Mississippi, near the Stennis Space Center, which remains closed, and a Navy oceanographic facility.
It probably penetrated five miles inland and lasted about four hours, he said.
Katrina’s surge and storm waves also devastated coastal areas to the east and west of Bay St. Louis, including the Chandeleurs Islands, a barrier chain that fronts and protects the southeastern Louisiana coast, and Dauphin Island, Ala.
“The Chandeleurs are basically wiped down to isolated pieces of sand,” Baig said.
The western end of Dauphin Island suffered “a major breach,” according to the U.S. Geological Survey.
Baig said the surge on Dauphin was only 6 to 8 feet, but Katrina was the fourth hurricane to whack the low-lying barrier island in the last three years.
A storm surge essentially is a rapid rise in sea level, over and above regular tide heights. Katrina hit a few hours before high tide, which would have been 2.1 feet above sea level, said James Eberwine, a marine specialist with the National Weather Service in Mount Holly.
Katrina’s surge could have contributed to mammoth waves. “It deepens the ocean and allows waves to come much farther inland than they did before,” Baig said.
Baig said waves can reach heights 80 percent of the depth of the ocean. For example, water that is 5 feet deep can support 4-foot waves. That means waves as high as 23 feet could have formed above Katrina’s surge. Thus along St. Louis Bay, waves could have crested at more than 50 feet above sea level.
The estimates by SLOSH Sea, Lake and Overland Surges from Hurricanes typically are within 20 percent of the actual measurements, Baig said.
In Katrina’s case, the surge range would be 22 to 32 feet. Even if the numbers are in the low range, that would mean waves potentially as high as 40 feet above sea level.
Verifying the surge numbers will be difficult, if not impossible, Baig and Sallenger said. Investigators will be looking for high-water marks, and they will have to differentiate between those made by waves and those by surge.
They said they also confront other obstacles: Katrina knocked out the tidal gages, and any structures bearing the high-water marks might be long gone.
