Texas A&M University

Why the Levees Broke By Lesley V. Kriewald

Understanding why levees and flood walls failed instead of protecting New Orleans from Katrina’s surging waters is the job of three Texas A&M civil engineers.

Swept Away

Erosion of levees caused by Katrina’s storm surge led to massive flooding that devastated the Big Easy. A Texas A&M expert in bridge scour applied his expertise to studying the quality of soil in New Orleans’ earthen levees.

Dr. Jean-Louis Briaud
Jean-Louis Briaud, professor and holder of the Spencer J. Buchanan Chair in Civil Engineering, tested soil samples from levees all around New Orleans. He says that some levees were made of much more erodible soil than others, leading to disastrous breaches, and that any levees left behind or being rebuilt should be evaluated for erodibility.

Laissez les bons temps rouler (“Let the good times roll”) may be New Orleans’ unofficial motto, but good times in the city have been hard to come by since floodwaters brought by Hurricane Katrina poured through broken levees and devastated the Big Easy in 2005.

As much as eight feet below sea level, New Orleans straddles the Mississippi River and lies south of Lake Pontchartrain. More than 350 miles of levees — earthen embankments or concrete floodwalls that run alongshore to restrain water — protect New Orleans on both sides of the Mississippi River.

Those levees failed catastrophically, and Katrina’s storm surge inundated most of the city. Eventually, more than 450,000 people left New Orleans or were evacuated. Floodwaters damaged or destroyed more than 150,000 buildings in the city, and authorities estimate hurricane-related property damage at almost $23 billion.

Understanding why the levees failed is the goal of the Independent Levee Investigation Team, a National Science Foundation-funded group led by researchers at the University of California, Berkeley, that includes Texas A&M’s Jean-Louis Briaud, holder of the Spencer J. Buchanan Chair in Civil Engineering and a member of the Board of Governors of the American Society of Civil Engineers’ Geotechnical Institute.

Looking for answers

Briaud is an expert in bridge scour, the erosion of soil around bridge supports due to water flow. He is applying this expertise to study the erosion of the levee materials during the hurricane.

Broken Levee New Orleans, La., Sept. 9, 2005 — A blackhawk helicopter loads sandbags into areas where the levee has broken, which allowed neighborhoods throughout the area to be flooded as a result of Hurricane Katrina. Photo by Jocelyn Augustino/FEMA

The storm surge, not the wind, is the most destructive part of a hurricane, and flooding caused by Katrina’s storm surge and accompanying rain flooded parts of the city to depths of 20 feet.

The levees that failed did so because of what engineers call sliding failures due to the force of the water or the water overtopping the levees during the storm surge, leading to erosion of the materials the levees were made of.

In the case of the Mississippi River Gulf Outlet and Industrial Canal levees, the height of the storm surge caused the water to rise and eventually overtop the levees. Some levees had been extended vertically with floodwalls, and the water cascaded over the tops of the floodwalls. When the water hit the earth on the back side of the levee, the overtopping water eroded the foundation of the levee, weakening its support and leading to breaches and flooding.

The power of water

Water applies a force on each soil particle, Briaud says. The faster the water flows, the stronger that force is, and if the force is strong enough, the soil particle dislodges and erosion begins. Briaud says that force can be as weak as the pressure you feel when you blow gently on your hand — or powerful enough to breach levees. Soil resistance to erosion, however, can vary, depending on the degree of compaction, or cementation, of the particles.

FEMA Urban Search and Rescue teams New Orleans, La., Sept. 9, 2005 — FEMA Urban Search and Rescue teams continue search operations into areas affected by Hurricane Katrina. Photo by Jocelyn Augustino/FEMA

To test the soils used to construct the levees, Briaud analyzed samples from several sites — from levees that failed and from those that held. The samples were collected by shoving a hollow metal tube into the soil and then were brought back to Texas A&M, where Briaud and his students tested them using his Erosion Function Apparatus, a patented and licensed device that he invented.

The apparatus tells how fast the surface erodes as a function of water velocity. If the flow rate is slow enough, no erosion occurs. But even though a super-slow erosion rate may seem like nothing, to ignore it would be to ignore the Grand Canyon: The Colorado River may have taken 10 million years to erode the canyon, but it’s a mile deep.

Briaud also performed a second, site-specific soil test by dropping a flow of water from a set height — say, two feet, for starters — and measuring the depth of the hole in the soil cut by the water flow. Then move the height up a set distance and repeat the test.

“We’re trying to quantify how erodible a material is,” Briaud says. “We need a rating system, like the hurricane rating system.”

Briaud says the test results can go a long way to predict erosion. An erodibility score of 1 or 2 means the soil erodes easily, whereas a score of 4 or 5 indicates resistance to erosion. “The rate of erosion is critical,” Briaud says. “If the levees are overtopped but hold, it’s not really a problem. The overtop water is manageable.” It’s the subsequent erosion of the back side of the levee by cascading overtop water that causes breaches.

All soils are not equal

Broken Levee Briaud’s Erosion Function Apparatus, a patented and licensed device that he invented, helps determine erodibility of soil by telling how fast surface material erodes as a function of water velocity.

Briaud says that some of the New Orleans levees were made of very erodible material and some of erosion-resistant material. The good news, if any can be found in the aftermath of Hurricane Katrina, is that most of the erodible material in the levees has washed away. Briaud says the levees left behind or being rebuilt need to be evaluated for erodibility.

Briaud says the fight between water and soil can be fierce. Often, the water wins, and people die. That’s what happened in New Orleans when the levees around the city failed, causing catastrophic flooding and devastation in the city of more than 1 million people.

“The problem with levees is that if one component of the levee fails, the whole system fails,” Briaud says. “There’s no backup in case the levees fail. If any 100 feet of a levee fails, there’s no redundancy. To me, that has to change. You have to build some redundancy in those systems because the levees protect more than just people. It’s houses, factories, harbor facilities, warehouses — it’s not just lives.

“It’s mind-boggling to see that water — which if you think about it is such a ‘soft’ material — is able to destroy levees and bridges and lives but also create the Grand Canyon.” end of story

Failure and flood

Numerical simulations and mathematical models are helping engineers paint the big picture of what went wrong in the Big Easy.

Drs. Billy Edge and Patrick Lynett
Billy Edge, holder of the W.H. Bauer Professorship in Dredging Engineering and head of the Coastal and Ocean Engineering Program, and assistant professor Patrick Lynett are helping to piece together exactly what failed and why during Hurricane Katrina.

For four months, one topic of conversation was off-limits to colleagues Billy Edge and Patrick Lynett: the performance of New Orleans’ hurricane protection system during Hurricane Katrina.

Edge, head of the Coastal and Ocean Engineering Program and Bauer Professor, is serving on a committee of the American Society of Civil Engineers tasked with studying the performance of New Orleans’ hurricane protection system during Hurricane Katrina. As part of the committee, he is charged with reviewing the findings of others.

Including assistant professor Lynett’s findings: Lynett did the numerical simulations necessary for making detailed predictions of forces on levees and during overtopping of the levees.

“We don’t have a lot of observation,” Lynett says. “We don’t know what happened other than the devastation. So we have to rely on numerical simulations to tell us what happened.”

Overhead view of Levee New Orleans, La., Sept. 7, 2005 — Neighborhoods on one side of the levee are flooded as one side remains dry as a result of Hurricane Katrina. Photo by Jocelyn Augustino/FEMA

For the numerical simulations, Edge says that Lynett performed about three years of calculations in about three weeks using Texas A&M’s Tensor Cluster, 256 computers purchased with a large National Science Foundation instrumentation grant. Using the computers, Lynett recreated conditions at given times. For instance, in the case of the 17th Street drainage canal, which failed near its entrance, Lynett took predicted water levels and the wave height in the canal to figure out what the water level and the wave height were near the failure, and the forces acting on the structure at various points in time.

“We’re looking at forces at certain points in time,” Lynett says. “Then we give that information to structural and geotechnical engineers and say, ‘If these are the forces and the wave heights, etc., tell us what failed and how.’”

Lynett also looked at the Mississippi River Gulf Outlet levee system. As water overtops a levee, it goes down the backslope of the levee and causes erosion. Using the velocity of the water and the overtopping of the levees at a given number of feet per second, Lynett estimated the erosion rate and compared that with the actual erosion of the levees.

“I’m confident that the simulations recreated the conditions of Hurricane Katrina to a reasonable degree,” he says.

Lynett says there were failures nearly everywhere in New Orleans’ hurricane protection system. The investigators have focused on the failures while also giving thought as to why some parts didn’t fail — for instance, the Orleans Canal, which flooded only at a pump station because of low wall height.

“The Orleans Canal had the largest wave energy but no failure,” Lynett says. “The design worked, nothing failed, so the design elsewhere should work.”

And now it’s Edge’s turn to review the work of Lynett and others investigating the failures.

“Modeling is extremely important to determine what happened because most of the wind-, wave- and water-level measurement devices failed to capture the event,” Edge says. “The models are being compared with many eyewitness accounts where they were available.”

Edge says that the city’s geography gives a big part of the picture.

“New Orleans continues to sink,” Edge says, “but determining how much the city is sinking is almost impossible because the survey monuments are sinking as well. New Orleans is going down so fast, surveyors can’t keep up with it. To accommodate the rate of relative sea-level rise, reference points have to be continually adjusted and protection measures designed accordingly.

“‘Category 1 through 5’ tells how fast the wind was blowing, but it doesn’t tell what happened, and that’s not fair. It’s all about location, location, location  — that’s what makes the difference.” end of story