7 ms·

A 500 year flood event is very likely new data that tells us our prediction it was a 1 in 500 year event was wrong. http://engineering.curiouscatblog.net/200

A 500 year flood event is very likely new data that tells us our prediction it was a 1 in 500 year event was wrong. http://engineering.curiouscatblog.net/2008/07/13/500-year-floods/ It is of course possible, it was actually a once in 500 year event. I just believe it is much more likely our previous conclusion was faulty. Our prediction of 500 year floods is not very good, we rely on way less than 500 years of data. Also in most places where this happens the massive changes to the environment (roads, cities, paved over wetlands, constrained rivers...) are not factored in well at all. Add to that global climate change and 500 year flood estimates are likely still poor today. The birthday problem doesn't relate to us getting new data that changes what we used to know. A 500 year flood probably is new data (that gives us a strong indication our previous belief was wrong).

autokad 9y ago
more likely their model is wrong. it reminds me of fukushima: https://ml.berkeley.edu/blog/2017/07/13/tutorial-4/ https://ml.berkeley.edu/blog/2017/07/13/tutorial-4/
- enraged_camel 9y ago
  Their model is wrong, but it's because it assumes climate is stationary. Climatologists are actively debating what the models should look like when global warming is taken into account: https://i.imgur.com/HLxUBim.png https://i.imgur.com/HLxUBim.png edit: not sure why I'm being downvoted. I can only assume the downvotes are political in nature, which is a shame.
  - tedunangst 9y ago
    That's not such a great reference...
  - sdenton4 9y ago
    A screen cap of a discussion forum is a poor reference indeed...
fjfiggiguffuf 9y ago
yea its almost like the underlying model is... warming?
- deleted 9y ago
  [deleted]
bmm6o 9y ago
This is part of the reason few private insurers offer flood insurance. They understand they there isn't enough information to properly price the risk. The historic data is spotty, we don't understand the geography well enough, you have to understand how construction and urbanization affects water absorption over long distances. If everyone who claimed to know what a 500-year storm looked like had to pay out every time one happened, we might have better models.
- dmurray 9y ago
  I don't understand this. Even if they can't assess the risk in any one property, can't they estimate the total amount of flood damage in (say) the US each year, and spread the risk across thousands or millions of properties? If the answer to that is that only people in areas vulnerable to flooding would buy insurance, and the companies would have to pay out too much - well, that means the insurance buyers are pricing the risks better, and the insurance company should hire some of them.
- dv_dt 9y ago
  To re-emphasise what the poster before you said - not only could the model be wrong, but even if it was right once before, you have to keep up the model against all the construction changes in the area that alter the watershed, as well as account for climate change contributions.
- Mz 9y ago
  No, that isn't why. I worked in insurance. Not homeowners insurance, but I had a certificate and processed claims. The reason is that most human settlements occur in flood plains because we cannot live without water. We drink it. We bath in it. We irrigate crops with it. Flood plains have fertile soil. We use rivers and oceans for essential cargo transit. Insurance is about risk management. It is a form of betting. And there is no bet here because there is no question of if it will flood. The question is only when will it flood? That's a fool's bet to say "I will pay you X amount of money if it floods" when it is guaranteed to flood sooner or later. That amounts to charity, not insurance.
  - 9y ago
tedunangst 9y ago
Does 1 in 500 come from assuming a normal distribution? Seems you would also need to accommodate random events like "tropical storm parks itself off the coast" which is more like a poisson process.
xbmcuser 9y ago
One of the things that is expected with global warming is heavier rain as the seas get warmer. This 500 year rain calculation is looking at the past for prediction but the future is different.
kbenson 9y ago
> It is of course possible, it was actually a once in 500 year event. I just believe it is much more likely our previous conclusion was faulty. It seems it could go either way to me, and the way to get a good indication of which is most likely is to look at look at much smaller timeframe models and assess if they are changing over time, and how much. It's entirely possible it's both, but it seems we have a good way of gauging the relative importance of changing climate norms so we should at look at that before discounting it as noise in the bigger problem of poor models in general.
madaxe_again 9y ago
The other problem is that these definitions are time independent - they treat the probability of each event as a roll of die, when in fact each previous event impacts the probability of future events, by impacting surface features, moving centres of production, driving further energy expenditure and altering haloclines and other such variables. It's a positive feedback process.
srean 9y ago
Your comment goes on to show that human intution aligns with an assumption that things are distributed with a thin tail (for example Gaussian like). On the other hand if the phenomena has a fat tail (Pareto for example) then runs of such 'extremes' are not unusual. What is interesting is that one of the motivations (funding wise) for understanding fat tailed distribution, extreme value distributions were indeed flooding and dyke failure events. https://en.wikipedia.org/wiki/Extreme_value_theory#Univariate_theory https://en.wikipedia.org/wiki/Extreme_value_theory#Univariat...
woodandsteel 9y ago
>A 500 year flood event is very likely new data that tells us our prediction it was a 1 in 500 year event was wrong. Sounds like a use for Baye's Theorem.

autokad 9y ago

more likely their model is wrong. it reminds me of fukushima: https://ml.berkeley.edu/blog/2017/07/13/tutorial-4/ https://ml.berkeley.edu/blog/2017/07/13/tutorial-4/

enraged_camel 9y ago
Their model is wrong, but it's because it assumes climate is stationary. Climatologists are actively debating what the models should look like when global warming is taken into account: https://i.imgur.com/HLxUBim.png https://i.imgur.com/HLxUBim.png edit: not sure why I'm being downvoted. I can only assume the downvotes are political in nature, which is a shame.
- tedunangst 9y ago
  That's not such a great reference...
- sdenton4 9y ago
  A screen cap of a discussion forum is a poor reference indeed...

fjfiggiguffuf 9y ago

yea its almost like the underlying model is... warming?

deleted 9y ago
[deleted]

bmm6o 9y ago

This is part of the reason few private insurers offer flood insurance. They understand they there isn't enough information to properly price the risk. The historic data is spotty, we don't understand the geography well enough, you have to understand how construction and urbanization affects water absorption over long distances. If everyone who claimed to know what a 500-year storm looked like had to pay out every time one happened, we might have better models.

dmurray 9y ago
I don't understand this. Even if they can't assess the risk in any one property, can't they estimate the total amount of flood damage in (say) the US each year, and spread the risk across thousands or millions of properties? If the answer to that is that only people in areas vulnerable to flooding would buy insurance, and the companies would have to pay out too much - well, that means the insurance buyers are pricing the risks better, and the insurance company should hire some of them.
dv_dt 9y ago
To re-emphasise what the poster before you said - not only could the model be wrong, but even if it was right once before, you have to keep up the model against all the construction changes in the area that alter the watershed, as well as account for climate change contributions.
Mz 9y ago
No, that isn't why. I worked in insurance. Not homeowners insurance, but I had a certificate and processed claims. The reason is that most human settlements occur in flood plains because we cannot live without water. We drink it. We bath in it. We irrigate crops with it. Flood plains have fertile soil. We use rivers and oceans for essential cargo transit. Insurance is about risk management. It is a form of betting. And there is no bet here because there is no question of if it will flood. The question is only when will it flood? That's a fool's bet to say "I will pay you X amount of money if it floods" when it is guaranteed to flood sooner or later. That amounts to charity, not insurance.
- 9y ago

tedunangst 9y ago

Does 1 in 500 come from assuming a normal distribution? Seems you would also need to accommodate random events like "tropical storm parks itself off the coast" which is more like a poisson process.

xbmcuser 9y ago

One of the things that is expected with global warming is heavier rain as the seas get warmer. This 500 year rain calculation is looking at the past for prediction but the future is different.

kbenson 9y ago

> It is of course possible, it was actually a once in 500 year event. I just believe it is much more likely our previous conclusion was faulty. It seems it could go either way to me, and the way to get a good indication of which is most likely is to look at look at much smaller timeframe models and assess if they are changing over time, and how much. It's entirely possible it's both, but it seems we have a good way of gauging the relative importance of changing climate norms so we should at look at that before discounting it as noise in the bigger problem of poor models in general.

madaxe_again 9y ago

The other problem is that these definitions are time independent - they treat the probability of each event as a roll of die, when in fact each previous event impacts the probability of future events, by impacting surface features, moving centres of production, driving further energy expenditure and altering haloclines and other such variables. It's a positive feedback process.

srean 9y ago

Your comment goes on to show that human intution aligns with an assumption that things are distributed with a thin tail (for example Gaussian like). On the other hand if the phenomena has a fat tail (Pareto for example) then runs of such 'extremes' are not unusual. What is interesting is that one of the motivations (funding wise) for understanding fat tailed distribution, extreme value distributions were indeed flooding and dyke failure events. https://en.wikipedia.org/wiki/Extreme_value_theory#Univariate_theory https://en.wikipedia.org/wiki/Extreme_value_theory#Univariat...

woodandsteel 9y ago

>A 500 year flood event is very likely new data that tells us our prediction it was a 1 in 500 year event was wrong. Sounds like a use for Baye's Theorem.