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Houston is experiencing its third ‘500-year’ flood in 3 years

washingtonpost.com

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Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#11

Post by curiouscats »

bkohlmann wrote: post #2

While the article obliquely makes reference to this, I wonder if this is somewhat similar to the "birthday" problem in statistics. Any given person has ~1 in 365 chance of being born on a particular day. Yet, if you have 30 people in a room, there is a greater than 50 percent chance that at least one PAIR has the same birthday. Given the hundreds of cities (thousands?) in the US that may be affected by a flood, it's…

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).

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#12

Post by rlglwx »

Maybe we should change the nomenclature, 500-year flood denotes once every 500 years. As opposed to a 0.2% chance. This, in turn, causes people that don't pay attention to floodplain maps to say, "we had a 500 year storm last year, we are good for another 500 years".

That, my friends, is a prime example of the gambler's fallacy.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#13

Post by idlewords »

rlglwx wrote: post #12

Maybe we should change the nomenclature, 500-year flood denotes once every 500 years. As opposed to a 0.2% chance. This, in turn, causes people that don't pay attention to floodplain maps to say, "we had a 500 year storm last year, we are good for another 500 years". That, my friends, is a prime example of the gambler's fallacy.

The nomenclature is not the problem, the problem is making numerical estimates of low probability events based on just 150 years of data and a raft of modeling assumptions.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#14

Post by idlewords »

devy wrote: post #5

Fort Bend County Judge Robert Hebert stated that Hurricane Harvey is more like a 800-year event, as quoted from the statement from Monday, reported by the same newspaper WaPo[1] [1]: https://www.washingtonpost.com/news/post-nation/wp/2017/08/2...

There is no way to credibly make this kind of claim. We don't have the data to quantify the recurrence rate of low-probablity events like this, nor can we be sure the the underpinning mechanism isn't changing with time.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#15

Post by jmcgough »

diafygi wrote: post #6

Howdy! I work in cleantech, and I guess it's that time again for a what-can-you-do-about-it post :) To start, here's my favorite climate change joke: "They say we won't act until it's too late... Luckily, it's too late!" ==So what can you do about it?== The biggest thing that is most relevant to the HN audience is that you can work at a new energy technology company! Our industries are out of the R&D stage and are cu…

The alternative approach to working directly towards solving the problem is to work at a mature tech company with higher pay, and donate 10% or more of your income every year to a non-profit like the Natural Resources Defense Council.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#16

Post by tertius »

idlewords wrote: post #14
devy wrote: post #5

Fort Bend County Judge Robert Hebert stated that Hurricane Harvey is more like a 800-year event, as quoted from the statement from Monday, reported by the same newspaper WaPo[1] [1]: https://www.washingtonpost.com/news/post-nation/wp/2017/08/2...

There is no way to credibly make this kind of claim. We don't have the data to quantify the recurrence rate of low-probablity events like this, nor can we be sure the the underpinning mechanism isn't changing with time.

If we have a baseline of 500 year flood level at 0.2% and it's well over that then yes there is a basis for making this claim.

I wouldn't make this claim though...

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#17

Post by idlewords »

A similar thing has happened on the Mississippi river, which has had '500 year floods' in at least 2011, 2009, 2008, 1993 and 1927. I may be missing a few.

Assigning probabilities to rare events is meaningless unless you a) have data over a much longer time period than the recurrence time and b) the mechanism causing flooding is not changing.

Neither of these holds in the case of Mississippi flooding, for which we only have about 150 years of observations.

Unfortunately, multi-billion-dollar decisions about flood risk are made using these manufactured numbers. For example, in Missouri, land behind a 100-year levee is not required to have flood insurance.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#18

Post by idlewords »

tertius wrote: post #16
idlewords wrote: post #14

Earlier quoted context omitted.

There is no way to credibly make this kind of claim. We don't have the data to quantify the recurrence rate of low-probablity events like this, nor can we be sure the the underpinning mechanism isn't changing with time.

If we have a baseline of 500 year flood level at 0.2% and it's well over that then yes there is a basis for making this claim. I wouldn't make this claim though...

I don't think I understand what you're saying. To be able to say things about 0.2%/year events, we would need many thousands of years of data on an unchanging system.

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#19

Post by autokad »

curiouscats wrote: post #11
bkohlmann wrote: post #2

While the article obliquely makes reference to this, I wonder if this is somewhat similar to the "birthday" problem in statistics. Any given person has ~1 in 365 chance of being born on a particular day. Yet, if you have 30 people in a room, there is a greater than 50 percent chance that at least one PAIR has the same birthday. Given the hundreds of cities (thousands?) in the US that may be affected by a flood, it's…

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.…

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

Re: Houston is experiencing its third ‘500-year’ flood in 3 years

#20

Post by walshemj »

bkohlmann wrote: post #2

While the article obliquely makes reference to this, I wonder if this is somewhat similar to the "birthday" problem in statistics. Any given person has ~1 in 365 chance of being born on a particular day. Yet, if you have 30 people in a room, there is a greater than 50 percent chance that at least one PAIR has the same birthday. Given the hundreds of cities (thousands?) in the US that may be affected by a flood, it's…

its the gamblers fallacy black 17 has not come up for ages so its more likely to come up - looks like the journalist does not understand probability
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