Report Revises Katrina's Force

Water spills over a levee along a New Orleans canal on Aug. 30, the day after Hurricane Katrina hit.
Water spills over a levee along a New Orleans canal on Aug. 30, the day after Hurricane Katrina hit. (Pool Photo By Vincent Laforet)
By Peter Whoriskey and Joby Warrick
Washington Post Staff Writers
Thursday, December 22, 2005

Hurricane Katrina will go down in the history books as the costliest hurricane in U.S. history, but not by a long shot the most powerful.

The National Hurricane Center released a summary report on Katrina this week that downgraded the storm's intensity at landfall in Louisiana on Aug. 29 from Category 4 to Category 3. The winds in New Orleans, which lay to the west of the storm's center, were probably even weaker than that, at Category 1 or 2 speeds, the report said.

Estimates of Katrina's power are expected to play a central role in the ongoing investigations into what failures led to the catastrophic flooding of New Orleans.

One of the key points of contention is how well the existing levees, which were said to be capable of withstanding a Category 3 hurricane, performed.

Leaders of the U.S. Army Corps of Engineers, the federal agency responsible for New Orleans's levees and ultimately accountable for their failure, initially blamed the problems on the fact that the storm was more powerful than the Category 3 event for which the levees had been designed.

The downgrading of Katrina partly undermines their case.

As recently as last week, federal and Louisiana officials responsible for the region's levees were describing Katrina as a Category 4 or 5 storm. Edmond J. Preau Jr., Louisiana's assistant secretary for public works, told a Senate panel that Katrina essentially delivered a Category 5 blow to the Gulf Coast.

"That storm was the biggest storm ever to enter the Gulf of Mexico," Preau said in testimony before the Senate's Homeland Security and Governmental Affairs Committee. "I think it would be a real disservice to everyone if Katrina goes down in the history books as a '4' because the wind speed dropped at the last minute."

A U.S. Army Corps of Engineers spokesman said yesterday that the portrayal of Katrina as a weakened storm is misleading.

"In the Gulf, Katrina was a Category 5 storm, and the surge was still Category 5 when it hit the ground," said spokesman Susan Jackson. "It's the surge -- the pressure of water against those levee walls -- that's the most important factor, not the winds."

National Hurricane Center scientists similarly noted that though Katrina was probably a Category 3 storm at landfall, with winds of 125 mph, its destructive force was probably amplified by the fact it had been a Category 5 only a day before landfall and whipped the waters up beyond Category 3 levels. While the winds die off quickly, waves do not.

Yesterday's report echoed earlier conclusions by Louisiana State University experts who had gathered their own data on the storm's intensity.

To some local experts, the report was further evidence that human error was primarily to blame for New Orleans's drowning.

"This is a further indictment of the levee system," Ivor Van Heerden, an LSU professor and leader of a team of Louisiana investigators probing the cause of the levee breaches. "It indicates that most of the flooding of downtown New Orleans was a consequence of man's folly."

Other engineering experts agree: Considering Katrina's weakened state at the time it reached New Orleans, the failure of the city's 17th Street and London Avenue canal floodwalls can be explained only as a failure of design or construction, said Robert Bea, a civil engineering professor at the University of California at Berkeley.

"The water level in the canals wasn't that high when the floodwalls breached," said Bea, a member of an investigating team funded by the National Science Foundation. "We had a premature failure of the defense system."

It is not unusual for hurricane scientists to revise hurricane intensity levels months later, once the data from aircraft and radar can be analyzed more completely.

The revision was deemed necessary because scientists had initially assumed that, as is often the case, Katrina's wind speeds on the ground were about 90 percent of what was measured by the hurricane aircraft, about two miles above ground, scientists said.

But a wealth of other data, including an analysis of the amount of sea foam and figures from parachuted instrument packages cast from airplanes, indicated that the ground speeds were probably lower than that.

© 2005 The Washington Post Company