Societies built their cities in the flood plain and put themselves in harm's way.

The 100-yr flood is used for design.

This implies the possibility of a flood larger than 100-yr recurrence interval.

Ultimate flood control design uses the equivalent of 10,000-yr recurrence interval [the PMP and associated PMF].

The 100-yr design flood is a working number [four generations].

It implies the possibility of having to rebuild the flood-control infrastructure approximately every four generations.