1 in 100 years is hardly a short term solution.
Unless you have infinite amounts of money, you always have to pick a cutoff point.
I suppose it comes down to a risk/benefit equation which is often very hard to quantify, after all, how much is a life worth?
And how often does a Katrina size storm come around?
The way to look at it is - imagine every year, you have jar with 99 red M&Ms and 1 green M&M. You pick 1 M&M. How long before you pick that green one?
In a previous job, I worked with a team building chemical-process plants, and were looking at a site, 1/100 year flood potential was the bare minimum, and that was for industry - not urban/housing/residential construction.
I’m not a hydrologist, but I have heard that when the Dutch build or repair their levees (to keep the Baltic Sea out) they look at a minimum of of 1/10,000 year event possibility.
When Katrina made landfall it was a category 3 hurricane. Category 3 is fairly common.