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LGS Luncheon – September 2017


Title: The Role of Geology in the Failure of the Levees in New Orleans during Hurricane Katrina

Mary L. Gentry, P.G.
Louisiana Department of Environmental Quality

When Hurricane Katrina struck just east of New Orleans on August 29, 2005 it unleased the most costly catastrophic failure of an engineered system in American history. Approximately 80% of the greater New Orleans area was flooded for nearly six weeks with up to 15 feet of water. Damage totals exceeded $110 billion and the death toll in the New Orleans area alone was 1,577. This was not a natural disaster, but rather a man-made engineering failure due to unrecognized geologic hazards. New Orleans is built on geologically young Holocene river deposits in one of the world’s most dynamic, geologically active deltas. Much of the landscape lies within interdistributary basins formed between delta channels and is therefore underlain by highly organic and unconsolidated swamp and marsh deposits. From an engineering standpoint these deposits are poorly suited to support the loads imposed by adding structures such as levees to the surface. A subsurface feature known as the Pine Island Barrier Trend extends along the southeast shoreline of Lake Pontchartrain buried beneath 3-30 feet of delta sediments and is composed of sand, silty sand and seashells. These sediments are not subject to consolidation and shrinkage to the same degree as the surrounding delta sediments making the overlying land prone to differential subsidence, undermining structures built upon it. In addition to organic soils and differential subsidence, deep-seated faults that underlie the area likely contributed to the failure of the levees. Geotechnical engineering failures included errors in the assessment of soil strength by averaging the data, failure to factor in subsidence rates into the structural elevation design, and linearly extrapolating stratigraphic contacts from soil data locations over one mile apart where the land is a product of delta switching and channel growth and abandonment cycles that result in dramatic changes in geology over short distances. Levee safety can best be improved by completing basic geology surveys supplemented by geophysical reconnaissance. Know the science before drawing lines on maps and building hard structures!





Mary Gentry was born and raised in New Orleans and received her B.S. degree in Geology from the University of New Orleans in 1990. She has worked as a geologist in water planning and assessment at the Louisiana Department of Environmental Quality since February of 1991 and is a licensed Professional Geoscientist in the State of Louisiana. She works with communities implementing Source Water/Wellhead Protection Programs statewide, evaluating hydrogeological data and educating water system operators and the public on drinking water protection. She received two awards from the U.S. EPA Region 6 for distinguished service in promoting statewide development of the Wellhead Protection Program. She was a co-developer of Louisiana’s Source Water Assessment Program to assess the susceptibility of public water systems to potential contamination. She also serves on the Advisory Board of the Louisiana Water Resources Research Institute and served on Board of Directors of the Baton Rouge Geological Society.

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