[Polar Bear]

FWIW...

Most storm systems in Florida are designed to handle storms of nine inches of rainfall in a 24 hour period, called a 25 year design storm, in a routine and simple manner with little impact. They're also designed to handle storms of twelve inches of rainfall in a 24 hour period, called a 100 year design storm, in such a manner as to avoid a major disaster. Irma will drop much, MUCH more rain than that on many areas in a much shorter amount of time. So with respect to the drainage system...which includes streets, pipes, inlets, ponds, surface areas, everything...all bets are off. It's simply impossible to design for such an event. This is when emergency preparedness kicks in.

Your question, op, cannot be answered in as precise a way as you likely wish. Even the design storms listed above are approximations and based upon historic records. I can tell you from experience that a 25 year storm occurs much more frequently than once every 25 years. Same for the 100 year storm. That's simply the names they've had given to them over the years and they've stuck, giving some sense of magnitude even though the names are not particularly accurate.

Bottom line...12" of rain in 24 hours is about the maximum any normal drainage system is designed to handle. Irma will laugh at such numbers. But with adequate preparation, we can all still stay safe even as the drainage systems are overloaded for a period.

Polar Bear, P.E.