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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

#211

Post by waqf »

Piskvorrr wrote: post #207
waqf wrote: post #8

Earlier quoted context omitted.

Good point, but the chance of a given city having a 500-year flood in three given successive years is only (0.002)³ = 8 × 10⁻⁹. So even if we allowed for 100 U.S. cities, 10 different natural disasters and 10 different three-year periods we're still talking about a probability of less than 0.01%.

...the assumption being that the risks are separate. This is clearly false - you are not rolling dice here, the probabilities of the same disaster at geographically "close" cities are linked.

That is true but it's an error in the unproblematic direction: fixing it makes the argument even stronger.

That is, if the probability for each city flooding is 0.002 but the different cities are positively correlated, then the probability of at least one city flooding is lower than it would be if they were uncorrelated. (Because the probability of no cities flooding is higher, because of positive correlation.)

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

#212

Post by oso2k »

quesera wrote: post #205
oso2k wrote: post #202

Earlier quoted context omitted.

For those who down voted me, you do not know your sample set or your test subjects. Go to the empirical data sources. Human behavior certainly affects the real probabilities. As of 2015 in the US [0], a person is more likely to be born in July, August or September. They're also more likely to be born during the middle of the work week rather than the weekends. Draw your own conclusions from the data. [0] https://www.…

Your information is interesting, but your inference is incorrect. The Birthday Paradox has absolutely nothing to do with procreation, and everything to do with statistical probability.

Probability is just a model in lieu of the actual data with many built-in assumptions. Probability is a shortcut finding, collecting or generating data. My point is, if you have the data or a very large faction thereof, use it. Don't guess using probabilities.

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

#213

Post by woodandsteel »

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.…

>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.

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

#214

Post by cat199 »

mannykannot wrote: post #179
cat199 wrote: post #102

Earlier quoted context omitted.

Pretty much everyone knows it's a good idea to get flood insurance in Houston.. We still get lots of localized downpours, floodplane or no, which can easily overwhelm local drainage, etc. Whether they actually do it or not is another idea

I am aware that flood insurance itself is subject to government influence - e.g. the National Flood Insurance Program, and possibly state regulation (even in Texas) - but I don't know the details. Does it result in premiums being affected by official flood-zone designations? As you appear to be in or near Houston, I hope things are OK with you and your family.

non homeowner, and haven't spoken to many, so no clear inisght.

all well here, many people I know or are aware of are affected, thanks.

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

#215

Post by quesera »

oso2k wrote: post #212
quesera wrote: post #205

Earlier quoted context omitted.

Your information is interesting, but your inference is incorrect. The Birthday Paradox has absolutely nothing to do with procreation, and everything to do with statistical probability.

Probability is just a model in lieu of the actual data with many built-in assumptions. Probability is a shortcut finding, collecting or generating data. My point is, if you have the data or a very large faction thereof, use it. Don't guess using probabilities.

Right, but it's really not about birthdays at all.

It's specifically about coincidences in a randomly distributed set.

It just says "birthday" in the common name because that's a relatable, if imperfect, example.

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

#216

Post by SomewhatLikely »

WorldMaker wrote: post #28
idlewords wrote: post #13

Earlier quoted context omitted.

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.

The problem is not making numerical estimates of low probability events or how such events are modeled: it's completely ignoring the statistical probability distributions of the model. All of the models are extremely "long tail" distributions and just about entirely ignoring the long tail. We shouldn't be referring to this as a low-probability flood but as a high sigma flood. ETA: Disclaimer: Day job includes rainfal…

Pretty much the same errors that were made in the 2008 financial crisis.

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

#217

Post by bmm6o »

Mz wrote: post #94
bmm6o wrote: post #58

Earlier quoted context omitted.

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 tim…

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. A…

> 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.

The bet is more like, "I will pay you X amount of money if your damages from flooding exceed your deductible within the next year." It's not at all given that it will flood at all this year, or that your losses will exceed the deductible.

It's true that it would require substantial diversification or re-insurance, since flooding is a clustering event.

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