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You are misunderstanding the engineering language in a term-of-art way. 1 in 500 year storm is a one-year probability of 1/500, based on now. There is no impl

You are misunderstanding the engineering language in a term-of-art way. 1 in 500 year storm is a one-year probability of 1/500, based on now. There is no implied or expected future change that needs to be considered. That's already baked into selecting the design-storm-interval of 1 in 500 vs 1/250 or whatever else. A 1 in 1000 year event need not be, and likely won't be, the same/valid probability in the year 3024. Or even 2074. And that's OK. The engineers know that.

edflsafoiewq 2y ago
How is the one-year probability calculated?
- beerandt 2y ago
  Historical data only. Rule of thumb is usually 30 yrs minimum of records/data from usgs/ nws or similar weather stations. But it's often less. Then it's just a probability eqn, and there are various rules for making adjustments based on things like distance away from the weather gages. But the more fundamental thing is you are setting a static design benchmark, not a future or perpetual target. Once that benchmark is set, you apply factors/probabilities of various failure modes. Or you can just choose a larger design-storm. But you don't say 'I'm designing for what the 2025 or 2074 or 2124 100-yr storm will be'.
nextaccountic 2y ago
> 1 in 500 year storm is a one-year probability of 1/500, based on now. It's more likely to be based on historical data that as of now is outdated and unreliable
- beerandt 2y ago
  It is based on only historical data, by definition. It cannot be 'outdated and unreliable' (due to future expectations), because that's not how it's defined. And it's not how engineering design processes work. (Google LRFD or ASD) 'Now' means the historical data that is current as of design. You are merely setting a design criteria benchmark. Based on historical data. If you want to account for future uncertainty, it goes into the other side of the design equations, via the factors and probabilities. But more fundamentally, picking a 1/500 design storm already takes into account certain aspects of risk levels, and has nothing to do with expected lifetime or survivability. You can and do often pick design storm intervals that greatly exceed design-lifetime, for various reasons.

edflsafoiewq 2y ago

How is the one-year probability calculated?

beerandt 2y ago
Historical data only. Rule of thumb is usually 30 yrs minimum of records/data from usgs/ nws or similar weather stations. But it's often less. Then it's just a probability eqn, and there are various rules for making adjustments based on things like distance away from the weather gages. But the more fundamental thing is you are setting a static design benchmark, not a future or perpetual target. Once that benchmark is set, you apply factors/probabilities of various failure modes. Or you can just choose a larger design-storm. But you don't say 'I'm designing for what the 2025 or 2074 or 2124 100-yr storm will be'.

nextaccountic 2y ago

> 1 in 500 year storm is a one-year probability of 1/500, based on now. It's more likely to be based on historical data that as of now is outdated and unreliable

beerandt 2y ago
It is based on only historical data, by definition. It cannot be 'outdated and unreliable' (due to future expectations), because that's not how it's defined. And it's not how engineering design processes work. (Google LRFD or ASD) 'Now' means the historical data that is current as of design. You are merely setting a design criteria benchmark. Based on historical data. If you want to account for future uncertainty, it goes into the other side of the design equations, via the factors and probabilities. But more fundamentally, picking a 1/500 design storm already takes into account certain aspects of risk levels, and has nothing to do with expected lifetime or survivability. You can and do often pick design storm intervals that greatly exceed design-lifetime, for various reasons.