The Army Corps of Engineers has done something smart (for a change) - it has started telling the people of New Orleans just how risky it is to live there, neighborhood by neighborhood, street by street.
The biggest, most glaring oversight in the Katrina disaster was that nobody knew with any precision how likely or unlikely a big flood disaster was. To put it another way, if you lived there, the idea that your own house would end up under water seemed fuzzy, comfortably remote. Not even the Corps, whose job it was to protect the city, could put a number on the risks. Its numbers were vague, based on outdated projections. And, as it turned out, those numbers were also wrong - the Corps seriously underestimated the danger. And New Orleans is still paying for that mistake.
Now the Corps has put in some serious time on supercomputers, figuring out what the flood risks are, and put some of the results online. There's good news, which we basically already knew: the Corps's post-Katrina levee repairs have made some areas safer, most notably the Lakeview area where the 17th Street canal floodwall breached. The Times-Picayune has worked the information into a useful set of graphics (.pdfs, unfortunately).
But the big picture here is still, well, scary.
The Corps is mapping the risks of 100-year floods (i.e., the odds of such a flood in any given year are 1/100), and as it turns out such floods can still get quite deep in many parts of the city. And while 100-year floods may sound like rare events, they happen more often than you think. Nobody wants to live in a 100-year floodplain. (Not to mention that with global warming, which may mean both bigger storms and higher sea levels, today's once-a-century flood can quickly turn into a once-a-decade event or worse.)
What the maps really show is a city protected by a somewhat bigger band-aid than before. It's great to have that information laid out - it will help clarify the debate, or at least help homeowners decide what to do with their property. But if the city is to survive over the long haul, it needs more than a band-aid and a map showing where it goes.