There is a wikipedia article on the Boscastle Flood of 2004. This was a flash flood event, caused by simultaneous water discharge from several catchments, though land clearances and engineering works no doubt played their part as well.
One rainguage recorded 3.5 inches of rain within one hour, but many others in the region showed less than 0.5 inches.
Like most flash floods, debris dams formed, and upon their breeching, a flash flood followed downstream.
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That is some rainstorm! The heaviest rainfall on earth is only around four times that - in a whole year! The answer is 17.1875 inches.
Rain that fell and filled up one centimeter of the ground
2 inch/hour divided by 60 mins/hr = 1/30 inch per minute 1/30 times x minutes = rainfall in x minutes = x/30
5 mm?
In a year, no. In an hour, possibly - specially if in an area that is not used to much rain.