Conspiracy of the levees: the latest battle of New Orleans.

• Author: Colten, Craig E.

Standing on St. Claude Avenue in New Orleans' Bywater neighborhood, a completely familiar place, on September 5, 2005, produced an utterly foreign experience. The levees that New Orleans relied on had failed on August 29, and the city swarmed with recently arrived National Guard units and Coast Guard choppers churning through the muggy air overhead. The city that care forgot had just become the object of global caring.

Even though much of New Orleans (including the Bywater) is a bit decrepit, I had never seen the city in such disarray. Street lights and fallen oak trees littered the median and every car along the curb had a window broken out and its trunk popped. Abandoned city buses, parked at odd angles, mocked the failed evacuation effort. Two empty schools had all their windows thrown open to permit ventilation for the evacuees who had huddled there in the dark during the perilous days before we arrived. Toward Lake Pontchartrain, water stood up to a meter deep in the streets. Facing south toward the Mississippi River, the streets were largely dry.

St. Claude Avenue and the Bywater had endured high winds and rising water as Hurricane Katrina pounded ashore the preceding Monday, but its residents and landscape did not suffer like its neighbor across the Industrial Canal, the Lower 9th Ward. To the uninitiated, New Orleans appears topographically barren, a featureless surface. No ridge, no mountains, not so much as a hill, other than the human-made levees that encircle the city. Although seemingly flat, there is sufficient relief in the city to direct the movement and collection of water when it overwhelms the massive barriers built to protect the city. The story of flooding in New Orleans following Hurricane Katrina is thoroughly tied up in the story of the flood protection levees.

In short, the levees provided a false sense of security, contributed to the subsidence of neighborhoods that they helped protect, and ultimately encouraged inappropriate development in the most vulnerable neighborhoods. Thus they conspired with the drainage works, subsiding soils, public officials, and developers to create a situation exploited by a powerful hurricane.

An Inverted Landscape

Long before Katrina arrived, Native Americans and French settlers faced floods from the river and from hurricanes. Unlike city sites in Europe, where land typically slopes up from a river's bank, the highest ground in New Orleans was immediately adjacent to the river. At the location selected for the colonial capital in 1718, the river had built a shoulder of high ground that was four or five meters above sea level along both banks. Regular murky floods slowed as they escaped the channel, dropped larger sediments on this land (known as the natural levee), and added ever so slightly to its height. As the annual flood waters slowed even more away from the river, they deposited finer sediments, creating the "backswamp." This natural sorting produced a gentle incline that sloped away from the river toward the cypress swamp, which graded to marsh near sea level and Lake Pontchartrain. Thus, the best farm land, urban soils, and highest land crowded near the source of its sediments--the river.

Although unaccustomed to this peculiar terrain, the French learned from the native Americans that it made great sense to build on the high ground near the river. Jean-Baptiste Le Moyne de Bienville, the colonial governor, selected a site for his capital that was a portage location for Native travelers and that linked the river with the lake. He nonetheless admitted the flood hazard, writing in 1720, "the river which overflows almost every year, is cause of great inconvenience and damage to many houses built too close to the waters."

To fend off the regular inundations, the colonial government began building low earthen barriers, which they called levees. Linear mounds of dirt a meter or so high soon stood as a somewhat wishful impediment to river floods. Until the levees reached the first high ground, about 160 kilometers upstream at the site of present-day Baton Rouge, floods escaping the channel upstream could still creep into the city's rear quarters from the...