The costs of protection against extreme natural disasters are juggled with other public priorities. Environmental considerations are debated. Stakeholders from all levels of government and the private sector weigh in.
Ultimately, congressional funding levels largely determine just how high the embankments will reach and what levels of risk will be accepted.
The current system in New Orleans was designed decades ago and has been shaped over time by past storms.
An unnamed hurricane on September 1947 flooded Jefferson Parish, which includes metropolitan New Orleans, to depths of about three feet (one meter). The storm caused 100 million dollars (U.S.) worth of damage.
After the storm, hurricane protection levees were built along Lake Pontchartrain's south shore.
Hurricane Betsy made landfall some 50 miles east of New Orleans on September 10, 1965. Winds in the city reached 125 miles per hour (200 kilometers an hour) and the storm surge neared 10 feet (3 meters). After extensive flooding, the Orleans Levee Board raised existing levees to a height of 12 feet (4 meters).
In 1998 Hurricane Georges triggered a mandatory evacuation of the Crescent City. The storm devastated much of the Caribbean but largely spared New Orleans.
Still, traffic snarls illustrated the difficulty and danger that would accompany evacuation in the face of a more direct hitlike the one delivered by Hurricane Katrina.
Until the day before Katrina's arrival, New Orleans's 350 miles (560 kilometers) of levees were undergoing a feasibility study to examine the possibility of upgrading them to withstand a Category Four or Five storm.
Corps officials say the study, which began in 2000, will take several years to complete.
Upgrading the system would take as long as 20 to 25 years, according to Al Naomi, the Corps' senior project manager for the New Orleans District.
Martin McCann, a civil and environmental engineering professor at Stanford University in California, warns that long-term planning may not account for changes to the risk equation.
"As further development goes on behind levees, over decades you need to revisit the question and say, Are those levees providing us the protection that we wanted?" he said.
"The answer is probably no, because the exposure is probably greater. The number of people and the [amount of] valuable property [behind the levees] is greater."
While New Orleans's levee system is rather unique, such embankments are also critically important to other U.S. cities.
Throughout the Midwest along the upper Mississippi River, communities are protected from destructive floods by levee systems of varying extent and strength.
Another large and possibly vulnerable levee system is housed in the California Delta. The delta provides a vital freshwater conduit from Northern California to the state's Central Valley and on to Southern California.
The area includes a system of islands, some as much as 20 feet (6 meters) below sea level, that are individually protected by more than 1,100 miles (1,770 kilometers) of levees.
"[The islands] house communities, infrastructure, pipelines, road, rail, and agriculture," McCann said. "Because they are below sea level, if even one levee breaks, it fills up an island and sucks salt water in from San Francisco Bay," McCann said.
"The salinty levels get so high that you can't deliver fresh water to the south [parts of the state]," he said.
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