Questions About and Resources on Causes of Levee Failures in New Orleans
by Mary Savina, Professor of Geology
I’ve noticed quite a bit of variability in the stories of the levee breaches, depending mainly on the date of the story and, to a lesser extent, who is being quoted. I’ve tried to summarize the main points as they are known in early March 2006. Further studies and reports will, no doubt, clarify and perhaps alter these conclusions.
Which levees failed during Katrina and why?
This link: http://soundwaves.usgs.gov/2006/01/ includes a good map of the levee breaks in New Orleans and their probable causes.
This link: http://www.nola.com/hurricane/popup/nolalevees_jpg.html is a pre-Katrina (June 2002) New Orleans Times-Picayune graphic that shows the levee systems in more detail. (Note that the entire 5-part series of stories from 2002 (“Washing Away”) can be read online at http://www.nola.com/hurricane/?/washingaway/index.html).
Breaks in the eastern and southern sections of the New Orleans levee system appear to have been caused by overtopping during the storm surge from Katrina, although there is some speculation that these levees might have stayed more intact had they been engineered better (c.f. Raymond B. Seed testimony before Congress at http://www.berkeley.edu/news/media/releases/2005/11/02_levee_testimony.shtml). The levees on 17th Street and London Avenue were not overtopped; instead, they failed through collapse of the levee foundations. It’s important to note that the levee system on the banks of the Mississippi River through the city of New Orleans remained intact through the storm, though there were some river levee failures downstream from the city.
Note that breaks and breaches in the levee system during a storm or a flood have the effect of reducing pressure on the levees in other places. In the 1927 Great Mississippi Flood, levees were purposefully breached to “save New Orleans” (see John Barry, Rising Tide).
What’s a levee, anyway?
A levee is an embankment along a body of water, usually a river or stream. Many rivers construct their own levees: when a river reaches bank-full stage and spills over its banks, the water velocity drops sharply and the river drops whatever sediment it’s carrying. Most of the deposited sediment gets dumped close to the river and, over time, a natural embankment is built up.
Humans have modified levees by building them higher, constructing levees where none existed previously, and connecting levee sections into a continuous string.
What specific soil conditions (below the levee embankments) contributed to the failure of some of the levees?
There’s a great schematic diagram of a cross-section through the levee at 17th Street showing the stresses on the structure from flowing ground water (from Kintisch, cited below, diagram at http://www.sciencemag.org/cgi/content/full/310/5750/953/F2) . Fill used to make the levee itself, the peat layer where the levee is anchored and/or the sand at the base of the levee may have failed. At some of the breaches, there’s evidence of piping, a phenomenon that occurs in silty and sandy soils where water pressure builds up and causes below-ground erosion, forming a open tunnel (the pipe) and eventually leading to the collapse of the embankment. Note that I’m using the word “soil” in the engineering sense of any loose material.
Why are soils in the New Orleans area variable? And what is the consequence of this variability for subsidence and for stability of engineering structures such as artificial levees?
New Orleans is in the Mississippi Delta area, which has a long history of sediment deposition related to rivers, swamps, lake and nearshore ocean environments. Moreover, at times in the past when sea level has been lower (glacial periods), the area was less influenced by the ocean and more by rivers. This diagram http://www.loyno.edu/lucec/images/engingeology.jpg (from Kolb and Saucier, 1982) shows some of the complexities that result. Note, for instance, the extent of natural levees along the Mississippi River channel. Some sediments, such as organic peats, are notably more compactible (prone to subsidence) than others. As noted above, many of the soils supporting the artificial levee structures are weak. This page: http://www.loyno.edu/lucec/mrdliving.html (from Loyola University of New Orleans) has an excellent introductory discussion of the consequences of the soil conditions for building homes and larger structures in New Orleans.
What engineering designs contributed to levee failure?
Many of the levees that were overtopped by the storm surge were constructed of easily erodible materials such as sand and silt. Water overtopping the embankment could easily erode this material. Also, levees proved most vulnerable at transitions points between different kinds of construction (see Peter Nicholson Senate testimony, http://www.ewrinstitute.org/files/pdf/katrinalevees.pdf). Where water overtopped levee walls, the levee foundations scoured out, especially in weaker embankment material, eventually led to collapse of the piling or sheet metal.
Can the levees be rebuilt effectively?
The main questions include a) whether to rebuild the entire levee system or allow parts of the New Orleans area, including parts of the city, to revert to marsh; b) whether to rebuild the levees to the pre-Katrina specifications of a Category 3 hurricane or whether to attempt to build them much higher and stronger to withstand a Category 5 storm; and c) whether and how to develop and enforce regulations on development in particularly flood-prone areas. Nicholson and other engineers and geoscientists have identified ways that the levees should be made stronger and some of these suggestions are relatively inexpensive. There are ethical considerations around any attempt to reclaim (or not) parts of the city that were inhabited. There are other important ethical considerations around use of financial resources to greatly strengthen the levee system.
http://hsgac.senate.gov/_files/Katrina/Preliminary_Report.pdf. USGS/ASCE report on levee breaks during Katrina. (Also see the Senate testimony by many engineers and geoscientists. This testimony is accessible through Lexis/Nexis. Here are a couple of specific web pages:
http://www.ewrinstitute.org/files/pdf/katrinalevees.pdf (testimony of Peter Nicholson, ASCE).
Stokstad, Erik, 2005, After Katrina: Louisiana Wetlands Struggle for Survival: Science, v. 310, p. 1264-1266. (Available on-line through Carleton’s subscription. Erik Stokstad is a 1992 Carleton graduate).
Kintisch, Eli, 2006, Hurricane Katrina: Levees came up short, researchers tell Congress: Science, v. 310, p. 953-955.
Kolb, C. R. and R. T. Saucier, 1982, Engineering geology of New Orleans: Geol. Soc. Amer. Rev. Engineering Geol., v. 5, p. 75-93.
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