What does it mean to protect a city from flooding. The Corps seems to think that protection from the biggest storm reasonably expected to hit the area once every one hundred years is good enough. What if you intended to live in New Orleans for 20 years? What would your probability of destruction be? The insurance companies sure want to know. Urban planners look at longer timeframes.

The answer is simple:

• You can expect to avoid destruction for 10 years with a 90% probability
• You can expect to avoid destruction for 20 years with a 82% probability
• You can expect to avoid destruction for 40 years with a 66% probability
• You can expect to avoid destruction for 68 years with a 50% probability
• You can expect to avoid destruction for 100 years with a 37% probability

Another way to say this is that 2 out of every 3 cities with 100 year protection will see their protection overwhelmed once every century. I don't think many Americans would be satisfied with that kind of safety standard. When we invest a couple hundred billion in a city and urge the residents to return what kind of safety do we expect?

The Netherlands adopted a 1 in 10,000 year standard storm for their flood control protection. This gives them a 99% probability of surviving a century without catastrophe and a 90% probability of surviving 1,000 years. I like those odds better than ours.

Protecting against at least a 1,000 year storm should be the weakest level of protection we are willing to accept. That would correspond to a 99% chance of surviving a decade and a 90% chance of surviving a century.

Finally  in March, 2006 the Corps admitted that they did not have an adequate system in place to protect New Orleans against a 100 year storm. Mistakes and low safety margins have proven catastrophic.