I believe that calling something a 25 year, century, 500 year, or millennium flood is a poor use of statistics.
If one collects a series of water levels over time, one can get the average water level, and then calculate the standard deviation, and come up with how far any particular flood is from the norm. And thus calculate the frequency of occurrence.
Yet, that ignores the underlying cause of the flood. Not all rain events are the same.
If you once in a while get hit by a Hurricane, then you have to look at the probability of getting hit by a hurricane, rather than the average water level. And not all hurricanes are the same.
Our big local flood was the 1964 Christmas Flood. We have had a few floods, but nothing in my lifetime like that one. So, we're 60 years on, not bad.
en.wikipedia.org
Looking at the cause of the flood, it wasn't that we just had a big rain. Yes, there was that. But, there were 3 primary contributing factors.
1) Hard Early Freeze creating impermeable ground.
2) Heavy Snow storm.
3) Heavy Warm Rain (atmospheric river). Perhaps a "Pineapple Express" storm. That melted the snow which washed off rather than soaking in.
We've also had flood control dams that were built around the 1950's and 1960's, and it is possible the risk is somewhat reduced today, although I wonder if the dam system would be overwhelmed by that flood. Of course there is also an effort to remove the dams that were protecting us.
Any of those events happen from time to time. The problem was the 3 of them occurring together causing catastrophic widespread flooding.
So, to calculate the frequency of a flood like the Christmas flood, one has to not look at the standard deviation of the water levels, but to look at the contributing factors and determine how frequent they occur together. It is also possible that the three events weren't independent, but were part of a single storm.
I am a slightly lower elevation than when I was a kid, but I just don't think we've had hard freezes like when I was younger.
Here on the West Coast we also have typhoons or hurricanes. Most don't make landfall in Oregon, but we've had a few blowing by close. Then there was the California hurricane a couple of years ago.
en.wikipedia.org
We could get directly hit by a hurricane or typhoon that could cause huge floods that would be far outside of any predicted range.
Anyway, one has to pay attention to the cause of the flood and risk factors, not just high water.
