[wongarsu]

"500 year flood" just means that in any given year there's a 1/500 chance of it happening. In theory history has nothing to do with it.

In practice it's calculated with models that are tuned with historical data (among other things), so if you fail to notice some important changes and update your models you might be wildly off.

[dsfyu404ed]

He's saying that they were 1/500 events previously, when the frequency was determined to be 1/500, but conditions have changed (climate, development, etc.) and they are no longer 1/500 events.