A lot of confusion about what a “500-year event” means in relation to #Harvey, and since I evaluate #Risks, and am from #Dickinson , TX …
A 500-year event only means it has been assessed a probability of 0.2% of happening for that location in a year (1% for a 100-yr event) …
When you start stacking events (multiple years or multiple areas) then the probability can get much higher …
See this graph of cumulative probabilities of experiencing such an event with multiple independent* events (*probably a bad assumption) …
(again assuming independence) over a 30-year mortgage, a house would have a 5.8% probability of seeing a 500-yr event (26% of 100-yr event)
Over 50 regions with the same risk, there would be a 9.5% probability of one of them seeing a 500-yr event in a yr (39.4% for 100-yr event)
Over a 79 year lifetime, there is a 14.6% probability of seeing a 500-year event (54.8% of 100-yr event) [again assuming independence] …
*Independence* means that no event informs us on the probability of any other event (i.e. things are not changing)
So, independence assumes no climate change, no increase in flooding due to development, etc. (i.e. prob not true, but makes math easier)
Leave a Reply