Severe flood frequency is one of the elements of flood prediction and planning that's taken a huge punch in recent years.
Now, those terms aren't really scheduling terms, they reflect the likelihood of such a flood event happening in a given year. So, the short term way of looking at a 100 year flood is that it's a flood that has a 1% chance of happening in any given year, not that you'll only get one such flood every 100 years. But, even then....the prediction of frequency is often way off in recent years.
And every meteorologist and planner knows why. We can't talk about it, or even include discussions of it in government planning documents now, because Cloak Room, but trust me....engineers who matter DGAF about Cloak Room issues. They go where the data is taking them when it comes to planning. It takes a good bit of time to catch up, of course. And development and construction that has already happened is there, and isn't going away. But yes, these catastrophic-level events are happening more frequently, and to greater extremes. And our infrastructure and planning is not caught up to the threat, and we seem to be doing all we can socially and politically to ensure that they do not do so.