"Assigning probabilities to rare events is meaningless unless you a) have data over a much longer time period than the recurrence time"
This is probably the most important factor nationwide in the US. There simply aren't written records of the weather in the vast majority of land in North America which cover the past 500 years. I wouldn't even be surprised if more than half the land in NA didn't have meaningful data over 200 years old.
"b) the mechanism causing flooding is not changing."
This is probably the biggest factor in Houston's extraordinary reoccurence of "500 year" floods. While Harvey's level of rainfall is truly unprecedented in all of American history, the other floods can easily be blamed on the fact that the rapid growth of Houston's sprawl has turned a gargantuan amount of land area into a concrete swimming pool. Like other posters have mentions, undeveloped land is very effective at slowing and absorbing water but a gutter is nothing but an aqueous superhighway. Flooding isn't just caused by the total volume of rainfall, it's caused by the speed with which that volume reaches natural outflows (rivers, creeks, estuaries, etc).
This is probably the most important factor nationwide in the US. There simply aren't written records of the weather in the vast majority of land in North America which cover the past 500 years. I wouldn't even be surprised if more than half the land in NA didn't have meaningful data over 200 years old.
"b) the mechanism causing flooding is not changing."
This is probably the biggest factor in Houston's extraordinary reoccurence of "500 year" floods. While Harvey's level of rainfall is truly unprecedented in all of American history, the other floods can easily be blamed on the fact that the rapid growth of Houston's sprawl has turned a gargantuan amount of land area into a concrete swimming pool. Like other posters have mentions, undeveloped land is very effective at slowing and absorbing water but a gutter is nothing but an aqueous superhighway. Flooding isn't just caused by the total volume of rainfall, it's caused by the speed with which that volume reaches natural outflows (rivers, creeks, estuaries, etc).