I ran across an interesting analysis of the flood issues in New Orleans in "Civil Engineering" - written well before the current disaster. There's a ton of information in this about the danger that faced the city, and what the answers had been over the years. The bottom line on this is, the plan for anything as powerful as Katrina was "pray it doesn't happen". Consider:

In 1999 the Corps was authorized by Congress to study the feasibility of various proposals for protecting the city against such devastating storms. An obvious possibility would be to raise the current levees to a height deemed acceptable by an AdCirc analysis. That, however, would also require widening the levees, which may not be possible in many areas because of the proximity of homes. Among other alternatives, Naomi will investigate the possibility of creating an immense wall between Lake Pontchartrain and the gulf to keep water out of the lake during a severe storm. Such a project would involve constructing massive floodgates at the Rigolets and Chef Menteur passes, where storm surge would enter the lake.

According to Naomi, any concerted effort to protect the city from a storm of category 4 or 5 will probably take 30 years to complete. And the feasibility study alone for such an effort will cost as much as $8 million. Even though Congress has authorized the feasibility study, funding has not yet been appropriated. When funds are made available, the study will take about six years to complete. “That’s a lot of time to get the study before Congress,” Naomi admits. “Hopefully we won’t have a major storm before then.”

Forget levees for a moment, and consider the other problem that is busily creeping toward New Orleans - the loss of coastal marshland. The taming of the Mississippi has meant no new floods as bad as the 1927 horror, but it's had a nasty side effect - the protective buffer of marshland (the bayou) south of the city is disappearing, fast. At current rates, New Orleans will be a gulf coast city within 30 years. That would mean that - notwithstanding the current flooding - Lake Ponchartrain would be the least of the problems facing the city - a similar storm hitting in 30 years would visit the storm surge that wiped out Biloxi and Gulf Port on downtown New Orleans:

“We’re trying to enforce human decisions on a natural process,” says Naomi. “What we’re trying to do is take a snapshot of geologic time and say, ‘This is what we want; this is where we want to live.’ The question is, Is it going to be feasible in the long term?”

Naomi says this question will not be answered with levee feasibility studies alone. It will also require a more complete understanding of the natural processes at work in and around New Orleans. For example, the wetlands of coastal Louisiana, which would act as a buffer and slow any storm during its approach to the city, are dying because the freshwater and nutrients that historically flooded into them from the Mississippi can no longer escape the river. At the same time, the sediment deposited here by the river long ago is subsiding, and no new sediment is overflowing to replenish it.

That's not the worst of it, believe it or not. The entire southern portion of Louisiana is sinking:

The true situation, however, is almost too grave to consider. “Coastal Louisiana is sinking under its own weight,” Dokka says. “The ground in Louisiana is ultimately going to go under.”

Indeed, the state is subsiding so quickly that the National Oceanic and Atmospheric Administration’s National Geodetic Survey (NGS) considers the orthometric markers in Louisiana surveyed every decade for the North American Vertical Datum (NAVD) to be “obsolete.” Dokka and his colleagues, together with experts from the NGS, are now using high-powered transponders and numerous Global Positioning System satellites to develop “true” elevation points in the state on the basis of their relation to the center of the earth.

Using a rate of subsidence measured at a tidal gauge off the coast of Grand Isle adjacent to an original NAVD marker, Dokka was able to calibrate rates of subsidence at hundreds of other markers around the state. His results indicate that the elevations of some areas have dropped as much as 2 ft (0.6 m) since they were last surveyed for the NAVD. Based on Dokka’s “true” elevations, some of the Corps’s levees in New Orleans may be more than 1 ft lower than their posted elevation.

Which means that the entire levee system is just so much whistling past the graveyard.

New Orleans has been in an exposed condition for well over a hundred years. This time, the city's luck ran out. The main thing we can do now? Visit this FEMA page, which lists charitable organizations that are helping out. Donate to or volunteer with one of them.