‘100-year floods’ may happen every three to 20 years

[Staggerwing]

Mr Bigglesworth,
Were/are you a WG forumite as well? Some folks are using new names here so I'm losing track.

Ex KevlarSocks
8)

K-Socks!
That explained it! I knew there was something eerily familiar about you... ;)

[Epee1]

Should be a rule, no new names, I want to keep a list of all the commies on the board. (kidding , just kidding)

[Windigo]

Other reasons that 100 year floods may occur more often - without the sky falling.

1/.  We have longer periods of record which means more accurate probablistic - predictions. (this actually gives us better volume yield and streamflow discharge figures for a given event. So a more accurate calculation gives us a higher OMFG flood event number... remember this is based on historical events that are used to predict future flood magnitudes)
2/.  Changes in land-use that increase the runoff rates from rain events (so for a given size rainstorm we get less water soaking into the ground and more ending up in the stream; e.g., forests soak up more rain than crop fields which soak up more rain than a city parking lot)