RE:The Road & River Diversion Program Continues.Er, I don't think that means what you think it means, but I could be wrong.
As a rather low ranking combat engineer to me designing for a 100 year versus 30 year event means taking into consideration maximum extreme conditions experienced in any typical 100 year window as opposed to a 30 year one. Hence, tolerance for more extreme flooding, more powerful earthquakes, etc.
Lifespan is increased mostly by maintenance, but you want that initial design, whatever it is, to be able to take at least one REALLY hard hit without failing.