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 or more mechanisms.

#1 Sideways hydrostatic pressure, which essentially “pushes over” the levee from the high water side. This usually means that the levee is not massive enough.

#2 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.

#3 This type of failure is known as overtopping, during which waves splash over the top of the levee and erode them from the outside inwardly.
Water in the river raises higher than the crown of the levee. The water flows over the crown and down the land side of the levee, eroding the levee from the back side.

#4 Surface erosion. During high water when the river water is against the levee, waves generated by wind will erode the water-side of the levee.

#5 Shear failure (slope instability) of the levee. Manifested as large slabs of the levee sides sliding down the side of the levee during or immediately after high-water. Many days of high water will saturate the surface layers of the levee (by phreatic water movement) making them heavier than the underlying layers. The heavier surface layers lose cohesion with the lower layers, resulting in the surface layer sliding down the slope of the levee. In older levees constructed of sand the entire levee can become saturated during a flood resulting in total collapse of the levee.

#6 Piping and underseepage. During a flood the water column exerts a weight upon the levee sides and adjacent floodplain that pushes water into any holes or cracks in the levee structure. If the water reaches the landside of the levee it will flow from the levee and carry levee material with it, resulting in the levee eroding from the inside out. Flood water flows through permeable soil material that underlies the levee. The water arises to the surface near the landside base of the levee in what is termed a ”boil”. Again, the water is flowing, so it is carrying levee foundation material out from under the levee.

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.

Hope that helps. If you feel any of the above is not factual, research the specific matter yourself, and then post your findings. This is how we learn.

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