What 100 year flood means

“And this is where you first start getting a sense that maybe engineering hydrology isn’t the most exact of sciences. …none of the Imperial units for these values agree, but if you do the math, the conversion factor is just about one, so we ignore it. We just leave it off! This is not something we take out to the fifth decimal place.”

https://www.youtube.com/watch?v=EACkiMRT0pc