I have a friend that lives in an area that drains into the Chesapeake bay. He is doing some changes to his property and is not allowed to do anything that would

I have a friend that lives in an area that drains into the Chesapeake bay. He is doing some changes to his property and is not allowed to do anything that would increase water run-off. OK, I said, that seems quite reasonable. Then he tells me it is for 12in of rain in 24hr. Wow, ok, that is a lot but I guess it could happen. Yes, it is a once in a 100yr storm. Then he tells me the rule is you have to handle that, and be prepared for another one within 48hrs. What?? Any water retention has to be drained within 24hrs so that if another storm hits it can be handled. So two, once in a century storms in a total of 72hrs. Kind of changes the definition of once in a century?