Hurricane Harvey is a 1 in 500 year flood, according to some sources. Well, that's comforting - it means it most likely won't happen again for 500 years. Harvey dumped 40 inches of rain on Texas and has caused (so far) 51 confirmed deaths. Economic loss estimates range from $10 billion to $160 billion.
Do you remember Tropical Storm Allison, in 2001? That was a 1 in 500 year flood.
40 inches of rain, 55 dead, $9 billion damage
Hurricane Ike; 2008, 195 killed, 145 mph winds, $37.6 billion damage.
Hurricane Rita, 2005. 16 inches rain, 180 mph winds. 97-125 killed, $12 billion damage.
I don't think you need to be a statistician to deduce that four such deadly storms in 16 years means that we should be talking about a once in four year event, not once in 500.
Because if it only happens once in 500 years, there's not a good reason to take significant action. But if huge storms like this happen every four years, maybe it's worth spending a bit to put up defences.
Because climate change couldn't possibly be part of the problem, whereby warmer ocean water increases the amount of moisture that a storm can carry, and Americans believe that climate change is a Chinese hoax.
Fortunately, none of this affects us here in England.