The mythical 100 year flood, 100 year wind, 100 year earthquake, whatever

Everybody who designs things, has to think about extreme events.  My main experience is earthquakes, but the same physics applies to flood, wind, fire, etc.

So, in the 30's when most cities were designed, it was a big thing to go with the 100 year event.  That's what you designed for.  Skyscrapers, drainage channels, etc.  In 1954 Toronto was hit with a 12 inch slug of rain and subsequent drainage was designed for that.  However, Toronto has been been hit by several slugs of about 4 to 6 inches, and the sewer system has been unable to take it.  But these were not major flooding events.

The problem with an initial design is that events overtake it.  Houston was theoretically designed for a 12 inch slug, but then they paved the entire watershed.  They get their 100 year flood almost every year now.  Hurricane Harvey with a 50 inch slug of rain was way over the top.

Because of rampant development, nearly all North American cities are getting frequent flooding.  The initial design is irrelevant because they did not stick to a plan.

ps.  I believe that a city should be able shrug off the 100 year event, and recover quickly from a one in 500 year event.  That goes for all causes, including earthquakes.  Yet, time and time again cities are destroyed by the 500 year event.