News for hackers. Stuff that matters.

A comment on Houston is experiencing its third ‘500-year’ flood in 3 years. Parent comment

Re:Houston is experiencing its third ‘500-year’ flood in 3 y... by kurthr on Tuesday August 29, 2017 @05:40PM (#15127821)
But storms aren't people with randomly distributed cities to flood so you don't get the combinatorial effect. It's not like there are a fixed number of 500 year storms every 3 years that have to flood a uniformly distributed list of cities. The floods are supposedly independent (even if our estimates of likelihood are bad) so there's no binomial coeeficient.

The simple calculation is P=1-(1-1/500^3)^N and the probability is less than 1:10000 assuming independence and even given 6000 cities of a million people (the entire world population is in a similar city).

If it was birthday like (every possible 500 year storm for 3 years is in a room choosing out of 100 random million person cities), the probablility would be reasonable. You can use a Poisson distribution to estimate the 3 person birthday problem. P=1-exp(-3choose100/500^2)=48%

[ Parent | on HN ]
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.

[0] https://en.wikipedia.org/wiki/List_of_United_States_cities_b...

[ Parent | on HN ]