Generally, the failed sections of earthen levee around St. Bernard and east Orleans Parishes do not appear to have included sheetpiling or other core construction. In a very few instances, evidence of a sheetpile core is now exposed and in general, these occurrences tend to be present where older pre-existing canals and natural waterways cut across the levee's intended location, probably due to recognition that otherwise seepage and percolation would be more of a problem at these locations.
As a result of its elevation near sea level, the Lake Pontchartrain Basin is quite vulnerable to tropical storms and hurricanes. Hurricanes are categorized by their windspeed in miles per hour (mph). Hurricanes have affected the Louisiana coastline with a frequency that peaks in September. Hurricanes with significant monetary or human loss are memorialized by retiring their name.
In addition to its separation from the coast, the topography of the land in the city of New Orleans is adverse. The city is surrounded by a river levee system 25 feet high along its southern boundary, and by hurricane protection levees about 15 feet high along the remaining boundaries. Most of the land in the city is below sea level, with much of the northern half of the city more than 5 feet below sea level. About one half of the population of the city can't and won't evacuate during a hurricane. Many people, about 200,000, do not have automobiles or access to an automobile. There are an additional 20,000 special needs people that cannot be easily moved. Finally, there several hundred thousand people that will not evacuate because of the difficulty of actually evacuating and finding suitable shelters. The hurricane protection levees surrounding the city are designed to protect the city from a category slow 2 or fast category 3 hurricane. Thus for any slow category 3, or category 4 or 5 hurricanes, the possibility exists for flooding the metropolitan area of New Orleans. The city of New Orleans averages 1.8 m (6 ft) below sea level, resembling a shallow depression surrounded by levees and water.
The levee system in New Orleans is one of the most extensive in the world. Levees are earthen structures, made of clay (sedimentary particles smaller in diameter than sand and silt), in cross section forming a truncated triangle. The base is commonly 10 times as wide as the height. Floodwalls are concrete and steel walls, built atop a levee, or in place of a levee, often where space is insufficient for a levee's broad base.
Orleans Levee District, a quasi-governmental body, is resposponsible for 129 miles of earthen levees, floodwalls, 190 floodgates, 2 flood control structures, and 100 valves. The governor appoints six of the board's eight members,