John Schuring, PhD and PE, is professor of civil and environmental engineering at New Jersey Institute of Technology (NJIT). He holds several U.S. patents for developing methods of treating polluted soil. He is an expert in pile foundations, differential settlement of structures, and landslides. He has worked on engineering projects for the U.S. Environmental Protection Agency, the U.S. Department of Energy and the New Jersey Department of Transportation.
Schuring can be reached by email (email@example.com) or phone: 908-295-7070 (cell); 908-852-6716 (home); 973-596-5849 (office). A question and answer session about levees with Schuring follows:
How are levees designed and constructed?
There are thousands of miles of levees along the rivers and coastal areas of the United States, and the vast majority of levees are made mainly of earth or soil. Since soil is not waterproof, most levees will leak or seep some water. Such leakage is normal, and engineers control it in various ways. One way is to install an internal drainage system that intercepts and removes the seepage. Another way is to reinforce the soil with concrete walls or sheet pile walls made of steel. It is also common to install clay or cement grout within the levee to reduce seepage. Usually, more than one of these design measures is employed within a levee system. The levee system in New Orleans contains all of these design types.
What causes a levee to fail?
Most levee failures can be attributed to one of three mechanisms. The first is sideways hydrostatic pressure, which essentially “pushes over” the levee from the high water side. This usually means that the levee is not massive enough. The second is seepage of water through or beneath the levee, which leads to “erosion from the inside out.” Such failures usually mean that the levee was not water resistant enough. The third kind of failure is known as overtopping, during which waves splash over the top of the levee and erode them from the outside inwardly.
What caused the levee to fail during Katrina?
An investigation into the cause of the levee failure continues. An initial report attributed the failure to water overtopping and washing the levee away. It is also possible that hydrostatic pressure, or under seepage through the levees, contributed to or even caused the failure. It is my opinion, based on limited data I have seen, is that under-seepage likely played a significant role - weakening the base of the levee.
What are some of the future risks for the levees in New Orleans?
It is almost certain that other sections of the levees have been weakened in addition to those locations where the failures actually occurred. It is thus imperative that the levee system be inspected and repaired before it can be relied upon to function properly and protect the city. Even a storm weaker than Katrina will put significant stress on the compromised levee system and cause more breaches. Reportedly, the city system was originally designed to resist a category 3 hurricane, but it will not sustain that level of storm in its current condition.
Are there any other risks for the levees to Orleans?
In some sections of New Orleans the levees are underlain by weak soils that are slowly compressing or sinking (organic silts). In some areas the city is sinking as much as 5 mm per year, which equates to about 2 inches in 10 years. To complicate matters, the sea level is rising globally, in the range of 2-4 mm per year; that means the city is losing an additional inch, with a water level rise of about 3 inches in 10 years. It is clear that any intermediate to long-term solution must take into account this rise of the water levels. Extending the prediction to 100 years, for example, means a rise in the water level of about 3 feet if current rates continue.
How might the levees be repaired?
Many options and technologies are available to raise, repair and rebuild levee systems. The simplest is to build up the earth embankment. In an urban setting such as New Orleans, however, this alternative is not sound since it requires more space. For example, for every foot that an earth levee is raised, it must extend laterally three or more feet, which encroaches on adjacent right-of-ways. Earth is also heavy, which adds additional weight that can accelerate the aforementioned settlement or sinking. Other options include installing concrete walls within and on top of the levee, similar to what had been done at the 17th Street levee in New Orleans, which failed. Another alternative is to drive heavy steel sheets through the levee down into the underlying soils, and let the steel sheeting rise above the levee to gain more height.
Any suggestions on rebuilding New Orleans?
Without doubt, rebuilding the city to business as usual is not an alternative. That is not to say that New Orleans needs to be abandoned. The many parts of the city at higher elevations that can be protected by new or rebuilt levees can be redeveloped. But those sections of the city that are chronically sinking, and well below sea level, either need to be raised in elevation or in some cases converted to less critical, non-residential land uses. Unfortunately, the weak soils underlying parts of the city will complicate raising the ground surface.
The rebuilding plan needs to include a comprehensive strategy to reconstruct the levees to modern design standards. The city’s levees have been built, rebuilt, extended, and patched many times since the its founding in the 1700's. Now is the time to implement a master strategy, one that will likely include construction of a flood control structure at the mouth of Lake Pontchartrain. Unfortunately, the disaster that the civil engineering community has always feared has happened, and it is now time to rebuild New Orleans, making it a smarter and a better place to live.