Re:Houston is experiencing its third ‘500-year’ flood in 3 y...
by roenxi on Tuesday August 29, 2017 @05:55PM
(#15127937)
Actually, in the US it is approximately 1 in 250,000 /assuming independence which is a bit of an assumption/. Obviously, nature doesn't know what a year is so it is totally possible that big storms come in batches.
Assuming 1 in 500 year floods are independent, note the ~300 cities in the US [0]. The fact that Huston is a big city doesn't seem relevant to me, because the storm doesn't care. Much like picking a specific number plate, it had to land somewhere.
The chance of there being a city in the US which has a 1 in 500 year flood is therefore 60% (300/500) each year.
The chance of that city having another flood of similar magnitude in Year 2 is 1/500, because the odds are independent and we started monitoring the city in Year 1. The chance of it flooding again the third year brings the odds down to 1/(500 * 500) = 1 in 250,000.
Assuming 1 in 500 year floods are independent, note the ~300 cities in the US [0]. The fact that Huston is a big city doesn't seem relevant to me, because the storm doesn't care. Much like picking a specific number plate, it had to land somewhere.
The chance of there being a city in the US which has a 1 in 500 year flood is therefore 60% (300/500) each year.
The chance of that city having another flood of similar magnitude in Year 2 is 1/500, because the odds are independent and we started monitoring the city in Year 1. The chance of it flooding again the third year brings the odds down to 1/(500 * 500) = 1 in 250,000.
[0] https://en.wikipedia.org/wiki/List_of_United_States_cities_b...