http://en.wikipedia.org/wiki/Lake_Pontchartrain

Raising and reinforcing the levees to resist a Category 5 hurricane might take 25 years to complete. Some estimates place the cost at $25 billion.

A hurricane in September, 1947 flooded the city, most of which is below sea level (and sinking). After the storm, hurricane-protection levees were built along Lake Pontchartrain's south shore to protect the city. When a storm surge of 10 feet (3 meters) from Hurricane Betsy left much of the city under water in 1965, the levees encircling the city and outlying parishes were raised to heights of 14 to 23 feet (4-7 meters). Due to cost concerns, the levees were built to protect against only a Category 3 hurricane.

Experts using computer modeling at Louisiana State University subsequent to Hurricane Katrina have concluded that the levees were never topped but rather faulty design, inadequation construction, or some combination of the two were responsible for the flooding of most of New Orleans.

Funding

Congress failed to fully fund an upgrade requested during the 1990s by the Army Corps of Engineers, and funding was cut in 2003-04 despite a 2001 study by the Federal Emergency Management Agency warning that a hurricane in New Orleans was one of the country's 3 most likely disasters.