# 1000 year storm **URL:** \ **Category:** Factual Questions\ **Created:** [October 6, 2015, 2:23pm UTC](https://boards.straightdope.com/t/1000-year-storm/733713 "2015-10-06T14:23:41Z")\ **Posts on this page:** 6\ **Page:** 3
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 9, 2015, 7:52pm UTC](https://boards.straightdope.com/t/1000-year-storm/733713/41 "2015-10-09T19:52:56Z")
> [@Stranger\_On\_A\_Train](#): > > I’ll admit to not having knowledge about how these event estimates are formulated, but the notion of taking the period over which an event is expected to occur **not more than once** and dividing it into 100% to get a per annum probability of occurance, or vice versa, is completely wrong in even a naive sense. Applying a statistial distribution to even a modest population of historical data, however, can give a credible estimate and is the the typical way of estimating a probability of occurring over a specified time interval, assuming that the phenomenon is normally distributed and understanding that events at the extremes will certainly deviate from the normal. > > Stranger Emphases Mine If you add that language in, then yes, you’d be wrong. It’s a 1% chance of occurring in any _given year_. That means 1% this year, 1% next year, 1% the following, etc etc etc. Indeed, there’s nothing stopping this event in South Carolina from happening again in a week or two, the atmosphere has no memory. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 9, 2015, 9:19pm UTC](https://boards.straightdope.com/t/1000-year-storm/733713/42 "2015-10-09T21:19:34Z")
My point is that calling an event with a 1% chance of occuring per annum as a “[once in] 100 year” event is nonsensical. If an even has a 1% chance of occuring per annum, ~there is a 63% chance it will occur at least once in a 100 year period. Stranger ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [October 9, 2015, 11:17pm UTC](https://boards.straightdope.com/t/1000-year-storm/733713/43 "2015-10-09T23:17:02Z")
> [@Stranger\_On\_A\_Train](#): > > My point is that calling an event with a 1% chance of occuring per annum as a “[once in] 100 year” event is nonsensical. If an even has a 1% chance of occuring per annum, ~there is a 63% chance it will occur at least once in a 100 year period. And it may occur twice or more. The point is, _it will occur on average once per 100 years._ And what’s so bad about the “63% probability”? If you’d prefer 100% you’ll need to make that time period VERY long. 😃 The really interesting case is where there’s inadequate historical evidence to guess the probability of a rare event. So far there’s been zero response to my post about Hurst and Mandelbrot’s power-law. Was my post wrong? Do I really need to explain why the central limit theorem just doesn’t model these sorts of events? ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 10, 2015, 2:24am UTC](https://boards.straightdope.com/t/1000-year-storm/733713/44 "2015-10-10T02:24:27Z")
> [@Stranger\_On\_A\_Train](#): > > My point is that calling an event with a 1% chance of occuring per annum as a “[once in] 100 year” event is nonsensical. If an even has a 1% chance of occuring per annum, ~there is a 63% chance it will occur at least once in a 100 year period. > > Stranger Ah … I see what you’re saying … yes, in any given _100 year_ period, the chances of a 100-year event in 63%. That can be calculated from the 1% per year number. It still boils down to someone looking at a hundred years of river gauge data and pointing to the highest and calling that the 100-year flood, and feel lucky to have a hundred years data. Weather data tends to have very large standard deviations, so if it’s a “1,000-year flood”, that’s plus or minus 500 years or so. ---
**Author:** ![Stranger\_On\_A\_Train](https://avatars.discourse-cdn.com/v4/letter/s/13edae/32.png) [@Stranger\_On\_A\_Train](https://boards.straightdope.com/u/Stranger_On_A_Train)\ **Post date:** [October 10, 2015, 3:06am UTC](https://boards.straightdope.com/t/1000-year-storm/733713/45 "2015-10-10T03:06:25Z")
Well, sort of. If the odds of occurance are 1% per annum, then yes, the likelyhood of occurrance in a 100 year period is 63%. But, perhaps incorrectly on my part, I would interpret a “100 year event” as an event that would not occur more than once in any 100 year period to some statistical degree of probability (e.g. a single tailed three sigma value). Saying that an event with a 1% per annum probability of occurance is a “100 year event”, as if the per annum probability is addative, is really nonsensical. Stranger ---
**Author:** ![watchwolf49](https://avatars.discourse-cdn.com/v4/letter/w/e9c0ed/32.png) [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)\ **Post date:** [October 10, 2015, 12:06pm UTC](https://boards.straightdope.com/t/1000-year-storm/733713/46 "2015-10-10T12:06:34Z")
> [@Stranger\_On\_A\_Train](#): > > Well, sort of. If the odds of occurance are 1% per annum, then yes, the likelyhood of occurrance in a 100 year period is 63%. But, perhaps incorrectly on my part, I would interpret a “100 year event” as an event that would not occur more than once in any 100 year period to some statistical degree of probability (e.g. a single tailed three sigma value). Saying that an event with a 1% per annum probability of occurance is a “100 year event”, as if the per annum probability is addative, is really nonsensical. > > Stranger Yes indeed, that’s the reason I wanted to get the [actual definition](http://pubs.usgs.gov/gip/106/) posted early in this thread: > [@](#): > > The term “100-year flood” is part of the national lexicon, but is often a source of confusion by those not familiar with flood science and statistics. [Previous page](https://boards.straightdope.com/t/1000-year-storm/733713.md?page=2)