Benoire:The entire system is built on the concept of a 10% AEP service level, i.e. the surface drainage system can cope with what was considered standard level events but there was always a 10% chance that a storm event could overwhelm the system.
Friend of mine was joking with some other guys that he was helping unblock drains in flooded streets that he must be several hundred years old because he's experienced multiple one-in-a-hundred events...
(And yes, I know how AEP works, and so does he).