This modern myth needs to be put firmly in the pixie-box along with the tooth fairy and santa claus.

You'll note that N'O has now had TWO "hundred year" floods within a decade. It'll probably have 10 more in the next century.

There are two factors that hydraulic engineers (a naturally conservative profession) tend to fall into.

1) they assume the world is constant, other than their flood works
2) ditto
3) ditto

1) It is a well-established fact that the whole Gulf coast is undergoing subsidence due to the Mississippi delta load and the ensuing subsidence and compaction, plus isostatic depression of the crust. This is a regional-scale factor that "geologists" know but "engineers" often ignore.

That subsidence amounts to mm per year, or even a cm/yr locally!. Any structure you build has to be able to withstand a forecast "n-year" flood not just today but in n-years time. So if you build for a 50 year flood, you have to allow for 50 years of subsidence. (3 mm/yr * 50 yrs = 150 mm = 6"). Now six inches may not sound like much, but it's the difference between not overtopping a barrier and overtopping it.

2) How do we know what a "100-year" flood is? Well engineers, being conservative, and eschewing the uncertainties of stochastic models, they like to look at the last 100 years and measure the flood history. That's great, but in that time, major works have entirely changed the profile of the mississippi - dredging of shallows, opening of the main channel, construction of levees and docks that tend to focus the floods downstream, rather than spill tham locally.

At the downstream end of a major developed river, floods are demonstrably worse on a year-by-year basis than the historical record. This correction is often ignored and only the press gets to wail about "another hundred-year flood".

3) Climate change. Ignore it at your peril.

At the end of the last glacial epoch, sea level rose by 100m in about 1000 years due to climate change. That can and will happen again as part of a natural cycle. Perhaps sea level will go down and we can reclaim vast areas of coastal plain from the seas. Or perhaps it will go up and we will lose vast areas of arable land and coastal cities to the tides, like a modern-day Atlantis.

Nothing is stable, change is the only constant.

(There is nothing in this section discussing the CAUSE of climate change - only the observed reality - a bit like the engineer's historical tracking of 100 year floods. I can assure you, as a geologist, the 100,000 year flood is a doozie. between +85 and -120m of sea level change. (for you in the USA, a metre is a beefier and more macho version of a Yard, so that's +280 to -400 FEET.)

As a geologist, I have always lived above the +85 m elevation contour, just in case...