I want to make a story in our world, where there is a sudden rain everywhere, and this rain is pretty average but it's permanent. (This is not a raining cycle, because the origin of the rain is surnatural. The amount of water is actually added, everywhere, not transfered.) What I'd like to know is how much time is needed for the rain to actually fill the sewers and become a flood ? Especially in a university (main place of story). There's a lot of concrete and roads in there, not so much grass fields.
Then, more importantly, how much would the flood height increase in 24H of raining ? Is there a formula about this? That would be perfect.