Katrina fact-check: Guesstimate of Katrina’s flooding in New Orleans was correct

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U.S. Army Corps of Engineers

During Hurricane Katrina, the floodwall and levee on the 17th Street Canal collapsed without being overtopped. It was one of about 50 breaches in the flood protection system that was supposed to repel "the most severe storm" that could be expected in the area.

During Hurricane Katrina, the floodwall and levee on the 17th Street Canal collapsed without being overtopped. It was one of about 50 breaches in the flood protection system that caused widespread flooding in the city.

U.S. Army Corps of Engineers

“Gas leaks began erupting into flames, and looking at the city, now at least 80 percent under water, it was hard not to think of last year’s tsunami, or even ancient Pompeii.”
New York Times editorial, Aug. 31, 2005

“With 80 percent of the city flooded and police officers working out of their cars, crime lab employees struggled in the aftermath of Katrina to keep even a skeletal operation online.”
The New Orleans Advocate, May 12, 2015

Eighty percent of the city of New Orleans flooded in Hurricane Katrina: That’s one of the undisputed facts about the storm and the subsequent levee failures. It’s cited by journalists, politicians, government agencies and nonprofit groups, usually without a source. (We’ve done it, too.)

As the 10th anniversary approaches, we decided to take another look at that figure. Did 80 percent of the city really flood from Katrina?

Like everything else with the storm, it seems, the soundbite doesn’t quite capture what happened on the ground.

In short, 80 percent is almost certainly an accurate description of how much of New Orleans flooded — but officials latched on to that figure practically through a lucky guess.

There are caveats, however. That includes the unpopulated eastern New Orleans wetlands outside the levees, which can be inundated even from modest tropical storms. That marshy area is a third of the city’s 180 square miles.

In the city’s core, surrounded by levees, about 70 percent of the land area flooded. But that includes Algiers, the small portion of the city on the other side of the Mississippi River. It was unaffected by the levee breaks caused by Lake Pontchartrain, swollen by Katrina’s storm surge.

Excluding the land outside the levees and Algiers, the flooding works out to 79 percent.

Interestingly, the statement that gained currency was a guesstimate, not the result of a scientific study. It came from a FEMA employee who eyeballed the flooding from a helicopter the day Katrina made landfall — before flooding from the levee failures reached its full extent.

The mayor repeated the figure in a TV interview, and it stuck.

One reason it’s hard to check this figure is that Katrina caused two types of flooding.

Its storm surge washed over those wetlands in the far eastern part of the city. But the flooding you saw in all the photos, with people on the roofs of their houses surrounded by water, occurred when levees and floodwalls protecting the city buckled.

When most people talk about how Katrina flooded New Orleans — a shorthand that aggravates many locals, by the way — they’re referring to the flooding caused by the levee failures.

However, the phrase repeated over and over is that 80 percent of the city was flooded, not 80 percent of the portion within the levees.

How it started: a ‘guy named Marty’

On the morning of Aug. 30, 2005, the day after Katrina struck the Gulf Coast, national media reported that New Orleans had been spared the worst of the storm. The narrative changed over the course of the morning.

Early that morning, CNN’s “American Morning” host Soledad O’Brien had news from the mayor.

“Speaking with our affiliate WWL during the night, New Orleans Mayor Ray Nagin says he has, quote, ‘a heavy heart.’ He has no good news, he says, for his residents,” O’Brien said.

She went to a tape of Nagin’s interview, where he said, “We probably have 80 percent of our city underwater. With some sections of our city, the water is as deep as 20 feet.”

Throughout the day, CNN reported that 80 percent of the city was flooded. The figure appeared in an Associated Press story that day. From then, news stories cited it as fact.

In the interview, Nagin said he got the information from a “briefing that I got from a guy named Marty from FEMA.”

He was referring to Marty Bahamonde, a Federal Emergency Management Agency official who had come to New Orleans before the storm to monitor preparations.

In testimony before a U.S. Senate committee in October 2005, Bahamonde said he surveyed the damage from a Coast Guard helicopter on Aug. 29.

“My initial flyover lasted about 10 minutes and even in that short time I was able to see that approximately 80 percent of the city was underwater, and I confirmed the 17th Street Canal levee break,” he testified. Shortly after, he went up again for about 45 minutes.

However, some parts of the city had yet to flood on Monday evening. Over the next two days, water poured through the breaches and spread through the city. It stopped when the level within the city equalized with Lake Pontchartrain.

Studies of flooding

In most American cities, determining the extent of the flooding would be simple: Use satellite photos to determine how much of the city was underwater inside its borders.

But a third of the area inside New Orleans’ city limits is wetlands, outside the levee system.

We sought surveys of flooding in the city from FEMA, the U.S. Army Corps of Engineers, the city of New Orleans, the Louisiana Governor’s Office of Homeland Security and Preparedness and the U.S. Geological Survey.

We found two government-issued reports and a private study done by a consulting firm.

None of them address the flooding outside the levees in eastern New Orleans, but by all accounts it was underwater. That area is about a foot above sea level, depending on the tide. Katrina’s storm surge was marked at 12 feet in one part of that area called Irish Bayou, according to the corps. It appears to be underwater in an aerial photo from Aug. 30.

What about the part of the city within the levees?

In a 2007 report, Dean Gesch, a scientist with the U.S. Geological Survey, estimated the extent of flooding on Sept. 2, four days after Katrina made landfall. The Times-Picayune reported that water started to recede on Aug. 31, but the corps didn’t start pumping water out until Sept. 3.

Gesch estimated that about 75 square miles of land were inundated. However, that included flooding in portions of St. Bernard and Jefferson parishes.

Gesch said his report was not an official estimate by the agency. “The numbers in the report are the result of a relatively quick and limited analysis,” he wrote in an email to The Lens.

A private study done in 2005, provided by Greg Rigamer of consulting firm GCR and Associates, estimated that 69 percent of the most populated portions of the east bank of New Orleans — for those of you who aren’t from around here, that’s actually on the north side of the river — was flooded on Sept. 2. That’s about 56 square miles.

In 2007, the Interagency Performance Evaluation Task Force published an exhaustive study of the failure of the city’s flood protection system. The task force was convened by the U.S. Army Corps of Engineers, which built the levees.

The task force estimated that floodwaters covered about 82.8 square miles. That’s about 46 percent of the entire city, and 68 percent of the portion inside the levee system.

Those estimates group one part of the city — the Lower 9th Ward, which suffered some of the worst flooding — with neighboring St. Bernard Parish, which isn’t part of New Orleans. Most of the Lower 9th Ward flooded, which would push the share within the levee system close to 70 percent.

If you add the 60 square miles outside the levee system, almost all of which certainly flooded, the total comes to about 143 square miles. That’s 79 percent of the city.

There’s another way to think about how much of New Orleans flooded — not land area, but the number of residents affected. That’s particularly meaningful in a city where it seems almost everyone lost something in the storm.

A 2005 Congressional Research Service report estimated that 77 percent of New Orleanians lived in flood-damaged areas. That tracks closely with a study by consulting group GCR and Associates, which estimated that about 75 percent of the city’s population had at least one foot of water on their property.

Heckuva job, Marty

Even sliced a few different ways, that 10-year-old conventional wisdom is correct: 80 percent of New Orleans flooded in Hurricane Katrina.

The only way it doesn’t hit 80 percent is when you look only at the neighborhoods of the city inside the levees, which is often used in referring to the flooding. Then it drops to 70 percent.

We wouldn’t be true New Orleanians if we didn’t remind you that this flooding was caused by the failure of levees built by the federal government.

Seems that “guy named Marty” had a pretty good eye.


Sources

Topography-based Analysis of Hurricane Katrina Inundation of New Orleans,” U.S. Geological Survey

“Hurricane Katrina: Social-Demographic Characteristics of Impacted Areas,” Congressional Research Service

GCR and Associates population projections, 2005

GCR and Associates inundation map, 2005

“Performance Evaluation of the New Orleans and Southeast Louisiana Hurricane Protection System,” Interagency Performance Evaluation Task Force

New Orleans land area based on 2000 Census

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