In other words, if a severe hurricane struck, the city's flooding and abandonment was not what would happen if the plan failed. It was the plan.

New Orleans is built between a lake, a river, and the Gulf of Mexico, and it is lower than the surrounding waters. It was kept dry by an extensive system of levees and pumps. That system was itself contributing to the slow subsidence of the city.

The levee system was largely designed in the early 1960s. By the standards of their day, the levees were built conservatively, but within certain constraints. In particular, they were built to withstand a Category 3 hurricane.

Hurricanes come in two jumbo sizes: Category 4 and, most severe but rarest, Category 5. A storm of either magnitude could deliver a surge that would overtop or breach the levees. The city would be flooded, to depths as great as 20 feet. It would become a lake. Much of it would be destroyed, and many people would die.

All of this was well known. Press accounts and public officials have been quite open about it for years. "Evacuation is the only way to protect New Orleanians," reported the Philadelphia Inquirer last year. It quoted Terry C. Tullier, the New Orleans director of emergency preparedness, as saying, "It's only a matter of time." Col. Peter Rowan, the commander of the New Orleans District of the Army Corps of Engineers, told the Inquirer that the city was "at the mercy of chance for the foreseeable future." Media coverage was rife with such warnings.

What could be done? "It's possible to protect New Orleans from a Category 5 hurricane," Al Naomi, a senior project manager with the Corps, told the Inquirer. "To do nothing is tantamount to negligence." In that interview, he estimated that hurricane-proofing the levees and building floodgates at the mouth of Lake Pontchartrain might cost $1 billion and take 20 years. In other interviews, Naomi estimated the cost at $2 billion to $2.5 billion and said the project could be completed in three to five years.

"The point is to eliminate that storm-surge threat with one of these plans," Naomi told Riverside, a Corps of Engineers magazine. "The philosophy of what we do during a hurricane would change. We could spend more time protecting our homes and less time trying to get out of the city in these desperate evacuations."

In 1999, reports the Chicago Tribune, Congress authorized the Army Corps to conduct a $12 million study to determine the cost of protecting New Orleans. But the study was not set to get under way until 2006, and it has so far received funding of only $100,000 to $200,000. "It was not clear why the study has taken so long to begin," the Tribune reported. Meanwhile, Congress and the White House consistently and sharply cut requests for levee-improvement funds.

Katrina came ashore as a Category 4 storm. The levees failed and the city, only partially evacuated, was swamped. "The intensity of this storm simply exceeded the design capacity of this levee," Lt. Gen. Carl Strock, the commander of the Corps of Engineers, told reporters on September 2.

Told so barely, the tale suggests shocking imprudence. But hindsight is 20/20. Remember, the odds of a Category 4 or 5 hurricane hitting New Orleans any given year were small. Strock told reporters, "We figured we had a 200- or 300-year level of protection. That means that an event that we were protecting from might be exceeded every 200 or 300 years. So we had an assurance that, 99.5 percent, this would be OK. We, unfortunately, have had that 0.5 percent activity here."

Remember, too, that reinforcing the levees was a multibillion-dollar project. An ancillary project to restore the protective marshes of the Mississippi Delta, which would have reduced the force of storm surges reaching the city, would cost something like $14 billion over three decades. For that kind of money, there are always competing priorities, some of them urgent.

The question, then, is not whether the failure to improve New Orleans's flood protection was a mistake in hindsight—obviously, it was—but whether it was a reasonable choice in foresight, based on the probable odds and costs as they appeared at the time.

Weighing low-probability, high-cost events is, as it happens, something economists and engineers know a bit about. W. Kip Viscusi, an economist at Harvard Law School and the editor of the Journal of Risk and Uncertainty, points out that the Corps of Engineers was among the first to develop and apply what has become a common cost-benefit template.

Using the more cautious of Strock's figures, assume the odds are that a storm surge would overtop or breach the existing New Orleans levees once every 200 years. This seems, if anything, optimistic, given that Category 4 storms hit the city in 1915 and 1947; that a Category 5 storm (Camille) narrowly missed in 1969; and that the devastating Katrina itself was not a direct hit. Still, assume it. Assume also that officials could reasonably expect the city's inundation, abandonment, and partial destruction to cost, ballpark, $200 billion in direct and indirect economic losses.

In any given year, then, figure that the expected economic cost of the swamping of New Orleans is $1 billion (divide the $200 billion cost over 200 years). A $2 billion levee project could be expected to pay for itself, probabilistically speaking, in two years; a $14 billion Delta restoration project, in 14 years.

But wait. New Orleans's 200-year flood might take place a century from now instead of right away (remember, this analysis is from a pre-Katrina standpoint), and money lost in the future matters less to us than money lost today. At an interest rate of 3 percent, Viscusi says, the present value of averting $1 billion in expected annual damage forever is $33 billion; at 5 percent, $20 billion; at 10 percent, $10 billion. Any of those numbers is higher than the estimated cost of hurricane-proofing the levees, and all but the smallest are higher than restoring the Delta.

Now, recall that those calculations reflect only tangible monetary cost. They do not account for inconvenience, pain and trauma, lives uprooted, and, above all, lives lost. Even a superbly organized evacuation would leave thousands of people behind. Moving nursing home patients, emptying hospitals, and losing control of the streets are dangerous at best. To all of which, add the psychic and cultural blow of leaving one of the country's most historic cities an empty ruin.

Strock told reporters that decisions about the levees were based on "whether it's worth the cost to the benefit, and then striking the right level of protection." Unless one uses very optimistic assessments of hurricane odds and economic costs, and also places a low value on human costs, New Orleans did not strike the right level of protection. Even in foresight, Naomi's characterization of New Orleans's vulnerability as "tantamount to negligence" appears justified. A far larger flood-prevention program should have been under way.

"This was not a close call," Viscusi says. "It's a no-brainer that you do this."

The immediate problem is to identify and bury the dead, tend to the refugees, and decide whether and how to rebuild. ("Whatever rebuilding is done in New Orleans, nothing very fancy should go there," says Richard A. Posner, a federal appeals court judge and the author of last year's book Catastrophe: Risk and Response.) After that should come a revision of America's disaster strategy no less sweeping than the post-9/11 revision of America's security strategy.

For example, Congress should create an independent Disaster Review Board to perform and publish an annual inventory of catastrophic vulnerabilities, highlighting in red all the places where, as in New Orleans, more prevention or mitigation makes sense. The board should prioritize spending and send an overall disaster budget to Congress every year for an up-or-down vote, forcing politicians to confront the issue. If population centers lie over the San Andreas Fault, in the shadow of Mount Rainier (an active volcano that could devastate the Seattle area), or on the floodplains of the Mississippi, the disaster board should be able to propose protecting them, requiring them to protect themselves, or encouraging them to move.

If there is another New Orleans out there, the public should know about it and should have to think about it. Katrina should change American habits of mind forever.