New Orleans' levees garnered much of the blame after the catastrophic consequences of Hurricane Katrina. Now, three and a half years later, a report shows that no levees would have kept New Orleans dry. 

The fifth and last report by the National Academy of Engineering and the National Research Council sums up the lessons of the disaster with a different lesson: Although New Orleans' levees weren't up to spec, any city that becomes over-reliant on levees for survival is doomed, regardless of how high and strong they are.

New Orleans isn't alone among American cities in relying on a levee system to stay dry—we've seen how Fargo, N.D., struggled to keep its levees from breaching as the Red River crested into a flood last month. But it was Katrina that brought their frailty into the forefront of the American consciousness.

However, the report says, the response in Katrina's aftermath has been too focused on the fact that the levees were substandard. What has gotten lost, the authors say, is that while levees are an important part of a coastal area's insurance against storm surge and flooding, but they shouldn't be expected to repel any and all severe events. Quoting an earlier document by the Association of State FloodPlain Managers, the new report says America did just that, building levees "at a time when the nation was convinced it could engineer its way out of flooding."

"There is always the risk—even with well-constructed and maintained structures—of failure," the authors write. Believing otherwise just gives a false sense of security and prevents planners from taking other measures that could help minimize the damage when the storm of the century comes around. One thing the authors suggest would be to build the first floors of buildings up to  the 100-year-flood mark at the very least, ensuring the survival of energy and communication supplies in an emergency. But even the 100-year mark might be not be enough; recent studies suggest that while overall precipitation in the U.S. might diminish in a warming world, the frequency of the most severe storms could increase.

In the end, residents of low-lying areas, be it New Orleans, Venice, the Netherlands or elsewhere, simply have to deal with the fact that there's no foolproof technological solution. The report strongly suggests that cities not build levees to reclaim land that would be flooded already, but that hindsight knowledge is a little late for already-developed metropolitan areas. And the suggestion of voluntary relocation for people in the most dangerous areas probably isn't going to fly for people who lived their whole lives there.

Every place you could live has its dangers, but the lesson of levees seems to be the same one as investing—don't put all your hopes in one basket.