Is it certain that hundred year floods have finite variance?
I'm sure a statistics guru over on cross-validated could answer this better for you, but here's some general info on estimating flood frequency fyi.

The 100 year flood has a 1% probability of occurring in any given year. It is typically estimated by fitting some type of an extreme value distribution to observed flood peaks with additional skew adjustments to account for regional influences or upstream regulation.  In the US, the standard approach is to fit a Log Pearson Type III distribution to log transformed observed flood peaks.  Until recently, Bulletin#17B was the FEMA approved method for flood risk hydrology when a sufficient record of flood peaks was available.  It was superseded March 2018 by Bulletin#17C which has some improved statistical methods.

As far as your question regarding finite variance, my understanding is that the LP III distribution asymptotically approaches unity and thus remains unbounded without finite variance since from a risk perspective there could always be a larger flood that has not yet been measured.  There are also methods for estimating a Probable Maximum Flood (PMF) that is meant to represent the most extreme combination of meteorological and hydrologic conditions that are reasonably possible in a catchment.