Geology of the New Orleans Area and the Canal Levee Failures

The geologic history of the New Orleans area significantly influences the engineering properties of the foundation soils beneath the levees. Geologic and engineering data gathered from the levee breaches identify a spatially complex geomorphic landscape, caused by Holocene sea level rise, lateral changes in depositional environments, development of Mississippi River delta lobes, and the distributary channels associated with delta development. Overlying the Pleistocene surface beneath New Orleans are predominantly fine-grained, shallow water sediments associated with bay sound (or estuarine), nearshore-gulf, sandy beach, lacustrine, interdistributary, and paludal (marsh and swamp) environments. These environments define the New Orleans area history during the Holocene and comprise the levee foundation beneath the failure areas. A barrier beach ridge is present in the subsurface along the southern shore of Lake Ponchartrain, which blocked the filling of the lake with fluvial-deltaic sediments. This buried beach impacted the supply and texture of sediment being deposited by advancing distributary channels and influenced the engineering properties of these soils. Marsh and swamp soils beneath the failure area at the 17th Street Canal are much thicker in comparison to those beneath the London Avenue Canal failures because of the influence of the beach complex, and are thickest in the Industrial Canal area. Additionally, human activities in the New Orleans area during historic time contributed to the spatial complexity and affected the engineering properties of the foundation soils. These activities include construction of drainage and navigation canals, groundwater pumping, hydraulic filling of the Lake Ponchartrain lake front, and construction of levees to prevent river flooding. Human activities, combined with the geologic setting and subsidence in this region, are responsible for the unique landscape that was impacted by Hurricane Katrina.