Levee and flood wall failure in New Orleans (following hurricane Katrina)
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Hurricane Katrina caused extensive failures of the levees and flood walls protecting New Orleans, Louisiana. The subsequent investigations by civil engineers are attempting to find the underlying reason for the failures.
Based on the predictions of experts on Hurricane preparedness for New Orleans, fears were expressed during the approach of Hurricane Katrina that the levees and floodwalls would be overtopped by a storm surge on both the Mississippi River and Lake Ponchartrain of a hurricane of category 4 or 5. However, the strength of Katrina and its position of its track to the east of New Orleans did not produce a high enough storm surge. Nevertheless several floodwalls lining the shipping and drainage canals in New Orleans collapsed and the resulting flood water from Lake Ponchartrain inundated up to 90% of the area of the city within the two days following Katrina, causing costly damage to buildings and resulting in many deaths. It is estimated that over 700 people died in New Orleans. The majority were drownings or indirect consequences of the flood.
The protection system for New Orleans was sized for hurricanes up to category 3, since protection from a category 4 or 5 hurricane would have cost too much. The failure was initally thought by many to be because effects of Katrina were greater than category 3. Katrina had been category 4 or 5 as it approached land.
However, engineers are now investigating the possibility that a failure in design, construction or maitenance caused much of the flooding. Some investigations point to the possibility of a weakening of the peaty soil beneath the foundations of the flood walls due to storm water. If this is confirmed as the principal cause of the flood wall breaches, then majority of the flood damage within the city will have been as a result of an engineering design failure during the building of the flood walls, and the New Orleans flood may be destined to be recognized as one of the most disasterous civil engineering failures in the USA.
Another major contribution to the flooding may have been overtopping of the levees on the east of the city by the 24 foot storm surge direct from the Gulf which would have been 10 feet higher than the levees there.
See also Effect of Hurricane Katrina on New Orleans
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Sequence of breeches of levees & flood walls
See also Civil engineering and infrastructure repair in New Orleans after Hurricane Katrina which gives a detailed timeline of the failures and subequent repair actions.
The industrial canal breaches occurred first, about 9 a.m. on Aug. 29, the day Katrina hit. The second breach occurred at the 17th Street Canal about 4 p.m. The London Avenue levee did not fail until about midnight.
Preliminary investigations
During the 6 weeks follwing the hurricane, preliminary investigations were carried out by the Army Corps of Engineers, the American Society of Civil Engineers
According to Col. Richard Wagenaar of the Army Corps of Engineers, 3 flood walls were breached in five separate places along the Industrial Canal, London Avenue Canal and the 17th Street Canal. [1] Note: the 17th Street and London Avenue Canals are used for drainage while the Industrial Canal is used for shipping.
Possible failure mechanisms being investigated by engineers include:
- Overtopping by storm surge and consequential undermining of flood wall foundations, or other weakening by water of the wall foundations,
- Storm surge pressures exceeding the strength of the floodwalls, and
- Impact by vessels such as barges which had broken free of their moorings.
A preliminary report by the American Society of Civil Engineers on an independent investigation [2] has concluded that the flooding in the Lakeview neighborhood was caused by the soil of the levees giving way and not by water overtopping the flood walls. The statement said that there was evidence that a section of the levee embankment that supported the flood wall had moved approximately 35 feet laterally. Inspectors also found evidence of the dirt levee moving at the London Avenue breach. However, many miles of levees worked as they should even though the water got over their tops.
Soil borings have been made in the area of the 17th Street Canal breach. According to a report these show a layer of peat which starts about 15 to 30 feet below the surface and ranges from about 5 feet to 20 feet thick. The peat is from the remains of the swamp on which the low area of New Orleans towards Lake Pontchartain were built. The shear strength of this peat was found to be very low, and to have a high water content. According to Prof. Robert Bea from the University of California, Berkeley, this would make the floodwall very vulnerable to the stresses of a large flood.
"At 17th Street, the soil moved laterally, pushing entire wall sections with it. ..... as Katrina's storm surge filled the canal, water pressure rose in the soil underneath the wall and in the peat layer. Water moved through the soil underneath the base of the wall. When the rising pressure and moving water overcame the soil's strength, it suddenly shifted, taking surrounding material -- and the wall -- with it."
The peat layer appears to be about 1000 feet wide. It is not clear if it was properly taken into account when the levees were built. The flood walls consist of a concrete cap on a sheet pile base driven 17.5 feet deep at 17th Street Canal. A deeper piling would have anchored the flood wall in much stronger soil.
Investigation of the floodwall design
Investigators are focusing on the 17th Street and London Avenue canals because evidence shows they were breached even though water didn't flow over their tops. That could indicate a design or construction flaw. Eyewitness accounts and other evidence shows that levees and flood walls in other parts of the city, such as along the Industrial Canal, were topped by floodwaters first, then breached or eroded.
A report of statements made by the head of a team from the American Society of Civil Engineers, says that, when the geological borings were made in 1981 prior to the construction of the 17th St Canal, they revealed to both the Americal Corps of Engineers officials and the contractors who designed and built the wall the nature of the weak layer of soft soil that would lie under the base of flood walls' foundation of steel piling.
"According to the analysis, they've got the soil strength test. It doesn't show exactly the input for the analysis, but assuming they used it and came out with factors of safety, it's showing the numbers are safe. So it leaves an open-ended question," as to why the flood wall failed.
The original design for the steel sheet foundations for the flood walls showed a proposed depth of 10 feet, and the design documents show calculations were made with the wall base at 12.8 feet. The depth was apparently increased later, to 17-foot depth, and this is what was built, according to a New Orleans engineer.
According to Professor Robert Bea, a geotechnical engineer from the University of California, Berkeley, it is likely that the Katrina storm surge created unusual subterranean pressures under the walls. Those pressures appear to have made soil under the sheet pile weaker so that it gave way, moving the steel sheet-pile-and-concrete walls along with it. But engineers studying the levees also say that other, unknown factors, including structural problems in the walls, could also have contributed to the breaches.
Prof. Bea has said that the design firm, New Orleans-based Modjeski and Masters, could have followed correct procedures in calculating the safety factors for the flood walls. But he added that design procedures of the Army Corps may not account for changes in soil strength caused by the changes in water flow and pressure during a hurricane flood.
The size of the design saftey margins has also been questioned by Prof. Bea. He said the Corps had applied a 30% margin over the maximum design load. A more typical margin used for highway bridges, dams, off-shore oil platforms and other public structures would be a doubling of strength. There were also indications that sub standard concrete may have been used at the 17th Street Canal.
Overtopping of levees in the east
According to Professor Raymond Seed of University of California, Berkeley, a surge of water estimated at 24 feet — about 10 feet higher than the height of the levees along the city's eastern flank — swept into New Orleans from the Gulf of Mexico, and caused most of the flooding in the city. He said that surge from Lake Borgne travelling up the Intracoastal Waterway caused the breaches on the Inner Harbor Navigation Canal or "Industrial canal".[3]
Aerial evaluation revealed damage to 90 percent of some of the levee systems in the east which should have protected St Bernard.
Investigation of levee maintenance
According to a report in the LA Times poor maintenance practices were found along miles of other levees, where burrowing animals created large tunnels that undermined already weak foundations. Maintenance and inspection are the responsibility of local levee boards.
The Army Corps is investigating whether a large oak tree was knocked down by the wind, causing its large root ball to heave up and undermine the foundation of the levee at the 17th St. Canal. A similar scenario may have played out on the London Avenue Canal.
However, local levee board officials expressed sceptism whether trees or inadequate maintenance had played a part.
Senate Committee Hearings
The evidence and reports which were presented at the hearing of the Senate Committee on Homeland Security and Governmental Affairs on November 2, 2005, and have been published generally confirm the preliminary investigations described above.
In his written evidence to the committee Ivor van Heerden, Louisiana State University, concluded,
"Most of the flooding of New Orleans was due to man’s follies. Society owes those who lost their lives, and the approximately 100,000 families who lost all, an apology and needs to step up to the plate and rebuild their homes, and compensate for their lost means of employment. New Orleans is one of our nations jeweled cities. Not to have given the residents the security of proper levees is inexcusable."
Independent investigation by the National Academy of Sciences
A new independent investigation was announced by Defense Secretary Donald Rumsfeld on October 19. This will be under the direction of the National Academy of Sciences and will study the results provided by two existing teams of experts already probing the failures, and will issue a final report in 8 months time.
