18 ms·

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

349 points by Mz 9y ago

bkohlmann 9y ago
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 actually quite likely that at least one of them could have a pretty high probability of experiencing one in 500 chance of something each year. I haven't run the numbers, of course, but we're looking at Houston in isolation - it needs to be seen in the broader context of all possible cities. We're focusing on the individual probability of Houston having this improbable string of events, rather than looking at Houston only being the city that experienced this phenomenon. Of course, the "once in 500 years flood" may also be an inaccurate probability as well, but if its not, then I'm not surprised this is happening SOMEWHERE.
- dsfyu404ed 9y ago
  Nobody notices the 500yr events that happen in the middle of nowhere.
  - deleted 9y ago
    [deleted]
  - Piskvorrr 9y ago
    With water? Unlikely. Water doesn't just drop and disappear on the same spot in the middle of nowhere, it tends to move downstream, into places that are in the middle of somewhere - as we have seen with the Mississippi watershed: first Out In Nowhere, IN, then ORCS, then Lake Ponchartrain. A 500y water event in nowhere land would still propagate; the speed of water release from the drop point (sorry for not using the correct jargon) would mean the difference between a 10y event downstream or a 500y event on the go.
- TallGuyShort 9y ago
  >> Of course, the "once in 500 years flood" may also be an inaccurate probability as well I suspect that's it. Of course I may be falling into the same birthday paradox here, but a similar thing happened when it flooded where I lived now. The x in "x-year Flood" kept increasing (I'm inclinced to blame that on sensationalizing news for ratings), but people kept comparing it to the 3 or 4 similar floods that happened in the last hundred years. It would seem the probably has gone up due to climate change, or that we're noticing the floods more now because there's more damage to be caused to more things that have been built up, and we have better records and better news transmission, but it certainly seems that can objectively say they're no longer 1-in-500 year flood regardless of the reason.
  - mannykannot 9y ago
    There is always considerable political pressure to low-ball the estimates. That is hinted at in this article, and it happened in New York, where the first post-Sandy flood zone maps provoked widespread opposition from those in the newly-expanded zones, and they were rapidly scaled back. There is, of course, some uncertainty in the estimation, but it seems likely that the official maps will be, at best, based on the most favorable possible interpretation of the data. Mother nature does not respect grandfather clauses.
    - x0x0 9y ago
      Because, crucially, your mortgage may require you to buy insurance in flood plains. And it can be quite expensive. see also https://qz.com/1063985/hurricane-harvey-why-85-of-homeowners-in-houston-dont-have-federal-flood-insurance/ https://qz.com/1063985/hurricane-harvey-why-85-of-homeowners... There's gonna be lots of screaming when homeowners in Houston realize how much flood insurance is going to cost them in the future. We all got a preview of this in coastal Florida.
      - cat199 9y ago
        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
- waqf 9y ago
  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%.
  - empath75 9y ago
    And it's not like Houston is the only city having 100 year+ floods regularly.
    - Taniwha 9y ago
      yes, it's almost as if the climate is changing
      - _Tev 9y ago
        Or our assumptions about climate / wheather were wrong.
  - Piskvorrr 9y ago
    ...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.
    - waqf 9y ago
      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.)
- ChadyWady 9y ago
  From what I understand of the article, the "1 in 500 years" measure is based on the estimated probability of a flood on a given year in a local area. So the probability of a "1 in 500 year" flood in Houston is independent of the probability of a "1 in 500 year" flood for another city.
  - twinkletwinkle 9y ago
    Right what the parent comment is saying is that look at a map of the US, how many major cities are there. The odds of two "1 in 500 year" floods happening consecutively are low for any individual city. But if you have enough cities it will happen somewhere. And then you write the article about it. It's kind of analogous to hitting a golf ball into an open field and then exclaiming "Of all the blades of grass, the ball landed on this one!" It's only interesting if it goes in the hole with the flag, otherwise you have a sort of selection bias going.
    - ChadyWady 9y ago
      That makes sense, I was confused since the Birthday problem is more about the pigeonhole principle rather than larger samples being more likely to have individually unlikely outcomes. But for this case, it would still be very unlikely. For any arbitrary 3-year span, the probability of consecutively getting a "1 in 500 year" flood for 3 years is (1/500)^3. If we have N cities, then the probability of none of these cities having 3 consecutive floods is (1-(1/500)^3)^N. For even a vast overestimate such as N=20000, it is still a significantly improbable event. Of course, this doesn't account for "3 consecutive years or more within some year range" and it is a gross simplification, but I think there are probably better explanations than selection bias, such as the inaccuracy of the model or the fact that these events might be temporally dependent on each other.
  - analog31 9y ago
    My understanding is that by definition, a 500 year flood has a probability of 1/500 per year. But I live at the top of a hill in the Midwest, so a 500 year flood in my locale has a much different severity than a 500 year flood in Houston.
- curiouscats 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. 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
- walshemj 9y ago
  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
  - ncallaway 9y ago
    Except there's a very good ability to know precisely the odds of what the ball landing on black 17 are. It's a very complicated question to determine what a 1/500 odds of flooding are. So, it's very possible that this isn't the gambler's fallacy, but rather the model itself is inaccurate.
    - walshemj 9y ago
      or the massive building growth without building the required infrastructure
  - mikeash 9y ago
    It's more analogous to getting black 17 six times in a row. It is possible, but it's weird enough to justify thinking that you've misunderstood the game, or the game has changed.
  - deleted 9y ago
    [deleted]
  - walshemj 9y ago
    really voted down for pointing out probability 101 I would have expected better mathematical knowledge on HN
    - Piskvorrr 9y ago
      Probability 101 begins with a qualifier: "Assuming independent events..." This can mean two things: a) a set of events that is evidently possible, but increasingly improbable b) events that are not independent
- polotics 9y ago
  No it's not the same thing. The 500 was a Houston stat, not a US overall one.
- SideburnsOfDoom 9y ago
  > Of course, the "once in 500 years flood" may also be an inaccurate probability as well Or rather, the odds of that kind of flood increase a bit each year.
- pesfandiar 9y ago
  I understand your argument, but you can't compare floods to the birthday problem. Unlike randomly-sampled people's birthdays, floods in US cities are not independent random variables. For instance, chances of 500-year floods in two Texas cities is not 1 out of 500*500 in a year. If they're geographically similar and physically close, it should be closer to 1 out of 500.
  - saas_sam 9y ago
    In 2 Texas cities, sure, but in every U.S. city? There are going to be several that end up getting a few 500-year floods... and several others that don't flood as expected in several 1000 years.
    - Spooky23 9y ago
      In many US cities. Recall that during hurricane sandy, pretty much any landfill or low lying area was flooded in the NY metro area. That means dozens of cities and towns with 7% of the US population.
- hexane360 9y ago
  Birthday paradox relies on non-independent events. I'd assume floods should be relatively independent between different cities, meaning that the number of 500-year flood events in the U.S. would follow a binomial distribution (https://en.wikipedia.org/wiki/Binomial_distribution https://en.wikipedia.org/wiki/Binomial_distribution). p = 1/500 years = 0.002 For 100 cities, the probability of at least one city having a 500-year flood is 18.14%. For a 3-year run: p = (1/500)^3 = 8e-9 For 100 cities, the probability is 8e-7 of at least one success. For 1000 cities, the probability is still only 8e-6. Wolfram alpha helps in calculating PDFs and CDFs: http://www.wolframalpha.com/input/?i=binomial+distribution+n+%3D+1000,+p+%3D+8e-9 http://www.wolframalpha.com/input/?i=binomial+distribution+n...
  - learc83 9y ago
    How does the birthday paradox rely on non-independent events?
    - hexane360 9y ago
      I guess what I meant wasn't "non-independent events". However, there is still a fundamental difference. The probability of a 500-year flood happening next year is still 0.2%. With a birthday paradox, the filling of birthdays is what causes the explosion in probability. Assuming no collisions so far, the 20th person you add to the room has a 19/365 chance of colliding, while the 2nd person only has a 1/365 chance. I'm not sure how to succinctly phrase this idea. Maybe "non-constant probabilities"?
      - thaumasiotes 9y ago
        In the terms of an example: If you put 400 people in a room, the odds that no two of them share a birthday have gone all the way down to zero percent. No matter how many independent cities you measure flooding in, you can never achieve a perfect 0 or 1 probability of (whatever), because the cities are independent. We don't define "interesting" flooding in one city by reference to what happened in another city, so this situation is not described by the birthday paradox.
  - 9y ago
- T-hawk 9y ago
  > it's actually quite likely that at least one of them could have a pretty high probability of experiencing one in 500 chance of something each year. This is the key point here, and it's called the fallacy of multiple endpoints. If there are 500 observable cities, then somewhere among them should occur a "500-year event" every year. It's just that nobody ever notices the big bulk of the population that hasn't yet had any 500-year event in its recorded history.
- kurthr 9y ago
  But storms aren't people with randomly distributed cities to flood so you don't get the combinatorial effect. It's not like there are a fixed number of 500 year storms every 3 years that have to flood a uniformly distributed list of cities. The floods are supposedly independent (even if our estimates of likelihood are bad) so there's no binomial coeeficient. The simple calculation is P=1-(1-1/500^3)^N and the probability is less than 1:10000 assuming independence and even given 6000 cities of a million people (the entire world population is in a similar city). If it was birthday like (every possible 500 year storm for 3 years is in a room choosing out of 100 random million person cities), the probablility would be reasonable. You can use a Poisson distribution to estimate the 3 person birthday problem. P=1-exp(-3choose100/500^2)=48%
  - roenxi 9y ago
    Actually, in the US it is approximately 1 in 250,000 /assuming independence which is a bit of an assumption/. Obviously, nature doesn't know what a year is so it is totally possible that big storms come in batches. Assuming 1 in 500 year floods are independent, note the ~300 cities in the US [0]. The fact that Huston is a big city doesn't seem relevant to me, because the storm doesn't care. Much like picking a specific number plate, it had to land somewhere. The chance of there being a city in the US which has a 1 in 500 year flood is therefore 60% (300/500) each year. The chance of that city having another flood of similar magnitude in Year 2 is 1/500, because the odds are independent and we started monitoring the city in Year 1. The chance of it flooding again the third year brings the odds down to 1/(500 * 500) = 1 in 250,000. [0] https://en.wikipedia.org/wiki/List_of_United_States_cities_by_population https://en.wikipedia.org/wiki/List_of_United_States_cities_b...
- rothbardrand 9y ago
  Having lived in Houston, this isn't a measure of the flood but a measure of how terrible the infrastructure is. They have terrible flood management systems but are "rated" to handle everything but a "100" year flood... yet Houston gets 100 year floods every year- multiple ones- and has for 40 years. It's a scam involving the ratings much like the mooody's ratings scam of the last decade IMNSHO
- danmaz74 9y ago
  How do you count "floods"? If there is one hurricane that causes floods in 3 cities, is that 1 flood, or 3? If it's 1 - which is what I think is meant when saying "a 1 in XXX years flood" - then your comparison doesn't hold, because the number of floods isn't dependent on the number of cities, but on the distribution of cities on the land, and specifically in flood-prone areas.
  - Spooky23 9y ago
    IMO you need to look at something other than city limits. Cities like Houston and LA are huge cities geographically. IMO you need to look at regional and per capita impacts.
- epitomix 9y ago
  I think that this statistic only makes sense if it is specific to the location. Otherwise, it is useless for planning purposes. If it is specific to the location then the birthday paradox does not apply. Interesting note, this is Houstons third 500-year flood in three years [1]. [1] -https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houston-is-experiencing-its-third-500-year-flood-in-3-years-how-is-that-possible https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houst...
- oso2k 9y ago
  The "Birthday Problem" has a simple explanation. Something like nearly 45% of people are born in the Summer months. Ensure that you get people of different ages and your chances of finding someone with similar/same birthday goes up enormously.
  - brewdad 9y ago
    Is this true? Which months are considered "summer"? Northern Hemisphere or Southern or both? This can easily turn into a "40% of sick days are on Monday or Friday" type fallacy.
  - pmalynin 9y ago
    Thats....not how that works. At all.
  - bitwize 9y ago
    The birthday problem still exists even assuming that everyone has an equal chance of being born on any day of the year. It's a phenomenon of the statistical likelihood that given a large enough group of people, the likelihood that any two will have the same birthday goes to almost certainty very rapidly.
  - umanwizard 9y ago
    No, the birthday problem still works if you consider that the probability of being born on a particular day is uniform. It's related to the fact that the probability of two people in a room having a birthday is more closely related to the number of pairs of people in the room (which in turn grows as O(n^2)), than to the number of people in the room. A detailed proof/explanation can be looked up easily with Google, if you have a bit of mathematical maturity.
  - oso2k 9y ago
    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.cdc.gov/nchs/data/nvsr/nvsr66/nvsr66_01_tables.pdf https://www.cdc.gov/nchs/data/nvsr/nvsr66/nvsr66_01_tables.p...
- curiousgeorgio 9y ago
  A more accurate term for what you describe would be a clustering illusion[1], and it certainly could be a valid explanation (among other possibilities) for the observation made in the article. [1] https://en.wikipedia.org/wiki/Clustering_illusion https://en.wikipedia.org/wiki/Clustering_illusion
  - honestoHeminway 9y ago
    Yes, or as with nuclear power, it could be that the base assumption regarding safety just was wrong.
- simonebrunozzi 9y ago
  Ahem, no - your birthdays example can't apply here. When you increase the number of people in the room, each one of them benefits from every new person added, in terms of probability of having been born on the same day. In the flood case, a flood in California and one in Miami don't really increase the chance of another flood simply because they have happened.
- bnegreve 9y ago
  > 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. Not quite the same because Huston is not the only city that has experienced extreme climate these two years, many other cities also have (maybe not 500 years flood, but close). So that would be like: one pair is born the same day, and 20 other people in the room are born a week around that day. Hardly a coincidence. It's more likely that there was a party about 9 months and X years before that day. (I'm not sure how far we can go with this silly birthday analogy:)
danielvf 9y ago
Cue discussion on the works of Tabeb, and what a horrible job humans do at predicting the frequency of rare events.
- kbutler 9y ago
  Parent means Nassim Nicholas Taleb http://www.fooledbyrandomness.com/ http://www.fooledbyrandomness.com/ And yes, basically we don't understand the actual probability of a "500-year storm", so our 0.2% chance per year is unreliable at best, more likely misleading. Just like Europeans didn't know the actual probability of a black swan until they found them in Australia.
  - 21 9y ago
    I think the idea of a black swan is something which you don't even imagine before you encounter it, ie: nobody would even think about computing the probability of a black swan (bird) existing.
    - kbutler 9y ago
      While unimaginability may fit the original conception of a black swan, Taleb's book is "The Black Swan: The Impact of the Highly Improbable" (not unimaginable or inconceivable) and defines a black swan as "an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility." http://www.nytimes.com/2007/04/22/books/chapters/0422-1st-tale.html http://www.nytimes.com/2007/04/22/books/chapters/0422-1st-ta...
- dragonwriter 9y ago
  Or, more on point, the common disclaimer, “past performance is no guarantee for future results”.
- 21 9y ago
  He just tweeted about it: "When people talk about 500 y floods or 100 year rain, they are klueless abt tail distribution. The empirical distribution is NOT empirical!" (and a picture with math equations) https://twitter.com/nntaleb/status/902632121407299585 https://twitter.com/nntaleb/status/902632121407299585
devy 9y ago
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/2017/08/27/harvey-is-causing-epic-catastrophic-flooding-in-houston-why-wasnt-the-city-evacuated/ https://www.washingtonpost.com/news/post-nation/wp/2017/08/2...
- idlewords 9y ago
  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.
  - tertius 9y ago
    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...
    - idlewords 9y ago
      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.
      - sjg007 9y ago
        This is not true. You can assume a uniform prior and run your model through bayes law and come up with an estimate.
        idlewords 9y ago
        That's called "making things up", but sure.
    - ars 9y ago
diafygi 9y ago
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 currently focused on scale and growth[1], and we need as many smart people as we can get. There are lots of companies hiring software engineers. ==How do I find a job fighting climate change?== I'd recommend browsing the exhibitor and speaker lists from the most recent conference in each sector (linked below). Check out the companies that interest you and see if they are hiring. * Energy Storage[2][3] * Solar[4][5] * Wind[6] * Nuclear[7] * Electric Utilities[8][9] * Electric vehicles[10] Also, if you're in the SF bay area, I'd recommend subscribing to my Bay Area Energy Events Calendar[11]. Just start showing up to events and you'll probably find a job really quickly. [1]: https://techcrunch.com/2017/01/22/energy-is-the-new-new-internet/ https://techcrunch.com/2017/01/22/energy-is-the-new-new-inte... [2]: http://www.esnaexpo.com/ http://www.esnaexpo.com/ [3]: https://www.greentechmedia.com/events/live/u.s.-energy-storage-summit-2016 https://www.greentechmedia.com/events/live/u.s.-energy-stora... [4]: https://www.intersolar.us/ https://www.intersolar.us/ [5]: http://www.solarpowerinternational.com/ http://www.solarpowerinternational.com/ [6]: http://www.windpowerexpo.org/ http://www.windpowerexpo.org/ [7]: https://www.nei.org/Conferences https://www.nei.org/Conferences [8]: http://www.distributech.com/index.html http://www.distributech.com/index.html [9]: https://www.greentechmedia.com/events/live/grid-edge-world-forum-2016 https://www.greentechmedia.com/events/live/grid-edge-world-f... [10]: http://tec.ieee.org/ http://tec.ieee.org/ [11]: https://bayareaenergyevents.com/ https://bayareaenergyevents.com/
- jmcgough 9y ago
  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.
  - sitkack 9y ago
    Work where your skills have the highest impact. Sometimes it translates into labor -> dollars.
rlglwx 9y ago
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.
- idlewords 9y ago
  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.
  - WorldMaker 9y ago
    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 rainfall statistics analysis.
    - tedunangst 9y ago
      I don't know much about the field, but I'm curious. How much are estimates based on normal distributions, and how much does the model consider other distributions? As you say, I don't think we know much about the tail. More than ten inches of rain may be a once a year phenomenon, but that once a year event might be twenty or fifty inches?
      - idlewords 9y ago
        I am not a math, but I remember that if you can't assume a normal distribution and want to know the shape of the tails, you need a pretty ridiculous number of observations.
      - 9y ago
idlewords 9y ago
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.
- maxerickson 9y ago
  Should any land be required to have flood insurance? The NFIP supposedly reduces taxpayer cost (with the shaky risk management you mention), but it currently has huge potential liabilities compared to the premiums it collects (and owes the government a bunch of money from past payouts).
  - cat199 9y ago
    Hey we've got a legal mandate to fund insurers now with our 'public private partnership' healthcare system.. pick this up as your issue and you might have a lucrative career in lobbying lined up..
    - matt4077 9y ago
      "Now"... The National Flood Insurance Act of 1968 established the government-backed flood insurance scheme. It has Ronald Reagan's signature. https://en.wikipedia.org/wiki/National_Flood_Insurance_Act_of_1968 https://en.wikipedia.org/wiki/National_Flood_Insurance_Act_o... The idea was and is twofold: first, it was seen as the only possibility to actually provide such insurance because insurance companies (even together) lack the financial strength to be able to insure risks that are statistically extremely dependent. Something like this flood costs dozens of billions, and it would bankrupt the whole industry. Secondly, it's in the national interest to promote such schemes that allow for more areas to be used productively.
dogruck 9y ago
Is this like when reporters say, "Trump won. We now think that Clinton's lead wasn't as strong as previously reported."
blondie9x 9y ago
The fundamental takeaway from this article is about half way down. Major flood events are occurint more frequently. What was once likely to hit a city every 500 years is happening to more cities on a planetary scale more frequently.
- ars 9y ago
  I know it's easy to blame climate change for anything and everything, but that's simply incorrect. The actual takeaway is that our data is wrong, these floods occur more often than we thought. With only 150 years of good data, it's impossible to calculate "500 year" events accurately. You would need around 5,000 years of data to do that, and we don't have it. So we ended up significantly underestimating how often these events occur.
  - blondie9x 9y ago
    You can see in graph that floods are occurring more often and that clouds are able to absorb more water as temperatures rise. This hurricane would have happened. But this flooding is worsening because of climate change that is irrefutable.
    - interstitial 9y ago
      By irrefutable, what do you mean? Over-fitting and variance should make normal engineers more cautious. Evidently, climate science is far more magical.
  - interstitial 9y ago
    It's almost like you are saying the model was overfit to the data.
  - qqg3 9y ago
    And we failed to incorporate the "concrete jungle" effect
  - honestoHeminway 9y ago
    Shouldnt there be more data in geologic strata? Flooding leaves distinct marks if i remember correctly. https://en.wikipedia.org/wiki/Geography_of_Houston https://en.wikipedia.org/wiki/Geography_of_Houston Would love to ask these gentlefolks that question: http://www.beg.utexas.edu/presentations/2005_presentations/blakeney102505.pdf http://www.beg.utexas.edu/presentations/2005_presentations/b... https://www.osti.gov/scitech/servlets/purl/841632 https://www.osti.gov/scitech/servlets/purl/841632
mbroncano 9y ago
The elephant in the room has been mentioned somewhere else these days: severe lack of planning and crumbling infrastructures. Remarkably how ne of the few non-partisan issues on the table goes under the radar time and time again. Tongue in cheek, we might think only polemic issue get the attention of the public. There is also the notion of national pride. We built our infrastructural base earlier than pretty much anyone else in the world, but we don't invest consistently on them anymore. Today our roads, dams etc. pale in comparison with any western country's but it seems like it's an affront to our national pride to acknowledge that. Houston is ill prepared to face any kind of emergency like this one, and only the bravery and tenacity of the people is preventing a humanitarian disaster in the scale that would keep us awake at night. That's not the American way: planning, preparing and executing is. The sooner we start taking this leap the better.
- pdq 9y ago
  Which other city in the US can handle 40 or 50 inches of rain, and 15 trillion gallons of water, within a one week timespan? That's as much rain as most US cities get in a year. This is not the fault of poor planning or poor infrastructure in Houston. It's freak weather event that shouldn't just be given partisan rhetoric.
  - dlp211 9y ago
    Probably none, but it's not a matter of what city could survive unscathed, it's a matter of what city has minimized the consequences. Houston has allowed for basically unfettered growth constraining their bayou's and preventing flooding infrastructure to be built. Hopefully Houston uses this disaster to rezone and allow for appropriate land management.
  - enraged_camel 9y ago
    Poor planning and poor infrastructure may have contributed to the problem: https://www.washingtonpost.com/graphics/2017/investigations/harvey-urban-planning/ https://www.washingtonpost.com/graphics/2017/investigations/... This is not "rhetoric" and there is also nothing "partisan" about it.
  - 9y ago
xexers 9y ago
Anyone notice the comments for this article on WP? There appears to be a poster called "The burning bush" who is commenting with lengthy comments about every 6 minutes. All of the the comments from that user are sceptical of man made climate change. Washington post does not appear to let you sort by anything other than "newest"... which means that this commenter's posts are always the top comment. That's concerning!
- stevenwoo 9y ago
  Every comments section I've seen for newspaper articles even in supposedly blue urban areas contain an outsize proportion of conservative trolls. It's possibly worse than youtube comments.
- 2sk21 9y ago
  Newspaper comments sections are almost uniformly unreadable. HackerNews is, I think, the only site I subscribe to read comments.
- Applejinx 9y ago
  That's normal. You'll find the same thing going on in the HN comments for '24/192K audio is silly': there's a class of people who fight for their arguments not with science or discussion, but through brigading and suppressing other people's arguments to get 'em out of sight. It's very much the case with climate change denialists, and it's also in play with '16 bit audio is enough for every possible person!' audio pundits: they will brigade to suppress opposing arguments to make it seem like their views are unopposed. Pretty sure the WP comments from the climate skeptics are more organized. There's a lot more money in climate denialism, from traditional energy companies with enormous resources and a clear agenda to continue their present mode of operations. I've never understood why the 16-bit audio guys are so similarly quixotic.
- enraged_camel 9y ago
  WaPo is experimenting with a new comment system, which is why it currently only lets you sort by newest. The old system had other sorting types, and the new one will too soon. Don't worry about it too much. :)
dmix 9y ago
This article reminds me of Nate Silver's book [1] which has a far more scientific and in depth look on the failure/success of these types of predictive statistics. The chapters on the many attempts (or many failed hopes) in predicting earthquakes was particularly interesting, including many times the media has bought into hyped up new charlatans who say they finally figured it out but which ultimately failed to survive under basic statistical scrutiny. It also has a useful soft introduction to Bayesian statistics and other useful concepts from the field of prediction that I hope more journalists read about. As this seems to be a very common theme in reporting. Even this journalist couldn't help themselves with this line (combined with some scary looking charts described with an alarmist tone farther down): > Was there some miscalculation of how frequently these massive flooding events occur? Or, most alarmingly, is something else happening that suggests these catastrophic weather events are becoming much more common? The failure to mention the effects of El Nino/El Nina seems like a big oversight in this article, especially when we're just coming out of a particularly strong one. Climate stats are an easy one to get wrong - or to shape into any narrative - especially when timeframes and location are easy things to be viewed too narrowly. [1] https://www.amazon.com/Signal-Noise-Many-Predictions-Fail-but/dp/0143125087/ https://www.amazon.com/Signal-Noise-Many-Predictions-Fail-bu...
MarkMc 9y ago
The problem is that the person who judges the probability at 1-in-500 doesn't have any skin in the game. The judgement of the US meteorological office affects where people can build houses and how much they pay for government insurance. Homeowners have a strong financial interest in underestimating the probability of a severe flood, so there is likely subtle political pressure applied to the met office assessments. For a more accurate assessment, the government should buy a sizeable insurance policy from several reinsurance companies and use it to calculate the implied probability.
- lutorm 9y ago
  Homeowners have a strong financial interest in underestimating the probability of a severe flood I don't know about that. Even if I have flood insurance, it's not clear I'm financially better off by having my house flood. It may be a confluence of "financially not fully liable" and wishful thinking, though.
- tuna-piano 9y ago
  And the reinsurance folks seem to say that there is absolutely no evidence that there has been an uptick in these big disastrous storms. People are constantly thinking of global warming, so when they see a storm, they say "Aha! Global Warming!". Of course there have always been hurricanes, and human perception/remembering is not a replacement for statistics and math. From Warren Buffet (One of the largest insurers of this type in the world) "...climate change had not up until then [2016] produced more frequent nor more costly hurricanes nor other weather-related events covered by insurance."" https://www.cnbc.com/2017/02/27/warren-buffett-global-warming-not-impacting-berkshires-insurance-biz.html https://www.cnbc.com/2017/02/27/warren-buffett-global-warmin...
  - dogruck 9y ago
    Markets also did not collapse when Donny won. But, hey, what do markets know?
    - XorNot 9y ago
lcfcjs2 9y ago
Tell me again how Climate Change isn't real.
twblalock 9y ago
I hope the city will require houses to be elevated when rebuilt.
mirimir 9y ago
> Climatologists say the mechanism by which this is happening is fairly straightforward. “Warmer air can contain more water vapor than cooler air,” according to the 2014 Climate Assessment produced by the U.S. government. “Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming. This extra moisture is available to storm systems, resulting in heavier rainfalls.” This is just so totally obvious. But I've rarely seen anything in news coverage about increased mean atmospheric water concentration. Also, increased water content is (of course) the expected multiplier for increasing CO2. Edit: spelling
- ianai 9y ago
  I wonder if there are maps of predictions of that.
cttet 9y ago
Maximum likelihood ¯\_(ツ)_/¯
nyolfen 9y ago
> The big news in the Galloway report, based on a global review of scientific and engineering data, was this: All of the money the United States had paid for massive public works to control flooding over the previous half-century not only had failed to improve flood safety at all, but the spending and the big public works projects actually were making flooding much worse. > The Galloway report was no global warming screed. Like the Dutch national policy based on many of the same findings, it dealt almost entirely with land use and population growth. Or to put it in more accessible terms, the 'burbs. http://www.dallasobserver.com/news/harvey-not-only-can-happen-in-dallas-it-will-9816590 http://www.dallasobserver.com/news/harvey-not-only-can-happe...
blondie9x 9y ago
https://mobile.nytimes.com/2017/08/29/opinion/harvey-the-storm-that-humans-helped-cause.html https://mobile.nytimes.com/2017/08/29/opinion/harvey-the-sto... On climates impact on extreme flooding.
- mberning 9y ago
  "Some version of Harvey probably would have happened without climate change, and we’ll never know the hypothetical truth." But let me write an op-ed about it anyway.
exabrial 9y ago
Are random events evenly distributed? ......
Steve44 9y ago
I saw a TV programme a year or two ago which was discussing the increase in hurricanes etc in the Gulf of Mexico area. They were saying that the storms aren't increasing due to global warming but are actually regressing back to their natural levels. The previous decades of pollution had been suppressing the storms and by cleaning the air the storm ferocity is heading back to normal. I can't see the article at the moment but I think it was Cloud Lab on the BBC, they floated around on an airship.
staticelf 9y ago
Bad shit happens when you fuck up the earths temperature. Why is anyone surprised? This has been said by climate scientists all over the world many times. Warmer climate will result in more drought on some places and other stronger storms which result in heavy rainfall and floods. https://climate.nasa.gov/effects/ https://climate.nasa.gov/effects/
jaclaz 9y ago
There used to be a calculator by NOAA: http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod that is now here: https://www.weather.gov/epz/wxcalc_floodperiod https://www.weather.gov/epz/wxcalc_floodperiod According to it, the probabilities of a 500 year event in the next 500 years is 63.2%. What I like to call flippism point, i.e. when you have the same chances as flipping a (perfect) coin is next 346 years. A three consecutive years series does seem a bit "queer".
VeronicaJJ123 9y ago
I feel sad when people use statistics like this. It makes no sense to call this 500 year flood because floods do not follow any kind of distribution. Not to mention the human beings have been changing their environments drastically in last 200 years. NN Taleb would say this is fat tail event. It does not follow empirical distribution.

349 points by Mz 9y ago

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 actually quite likely that at least one of them could have a pretty high probability of experiencing one in 500 chance of something each year. I haven't run the numbers, of course, but we're looking at Houston in isolation - it needs to be seen in the broader context of all possible cities. We're focusing on the individual probability of Houston having this improbable string of events, rather than looking at Houston only being the city that experienced this phenomenon. Of course, the "once in 500 years flood" may also be an inaccurate probability as well, but if its not, then I'm not surprised this is happening SOMEWHERE.

dsfyu404ed 9y ago
Nobody notices the 500yr events that happen in the middle of nowhere.
- deleted 9y ago
  [deleted]
- Piskvorrr 9y ago
  With water? Unlikely. Water doesn't just drop and disappear on the same spot in the middle of nowhere, it tends to move downstream, into places that are in the middle of somewhere - as we have seen with the Mississippi watershed: first Out In Nowhere, IN, then ORCS, then Lake Ponchartrain. A 500y water event in nowhere land would still propagate; the speed of water release from the drop point (sorry for not using the correct jargon) would mean the difference between a 10y event downstream or a 500y event on the go.
TallGuyShort 9y ago
>> Of course, the "once in 500 years flood" may also be an inaccurate probability as well I suspect that's it. Of course I may be falling into the same birthday paradox here, but a similar thing happened when it flooded where I lived now. The x in "x-year Flood" kept increasing (I'm inclinced to blame that on sensationalizing news for ratings), but people kept comparing it to the 3 or 4 similar floods that happened in the last hundred years. It would seem the probably has gone up due to climate change, or that we're noticing the floods more now because there's more damage to be caused to more things that have been built up, and we have better records and better news transmission, but it certainly seems that can objectively say they're no longer 1-in-500 year flood regardless of the reason.
- mannykannot 9y ago
  There is always considerable political pressure to low-ball the estimates. That is hinted at in this article, and it happened in New York, where the first post-Sandy flood zone maps provoked widespread opposition from those in the newly-expanded zones, and they were rapidly scaled back. There is, of course, some uncertainty in the estimation, but it seems likely that the official maps will be, at best, based on the most favorable possible interpretation of the data. Mother nature does not respect grandfather clauses.
  - x0x0 9y ago
    Because, crucially, your mortgage may require you to buy insurance in flood plains. And it can be quite expensive. see also https://qz.com/1063985/hurricane-harvey-why-85-of-homeowners-in-houston-dont-have-federal-flood-insurance/ https://qz.com/1063985/hurricane-harvey-why-85-of-homeowners... There's gonna be lots of screaming when homeowners in Houston realize how much flood insurance is going to cost them in the future. We all got a preview of this in coastal Florida.
    - cat199 9y ago
      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
waqf 9y ago
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%.
- empath75 9y ago
  And it's not like Houston is the only city having 100 year+ floods regularly.
  - Taniwha 9y ago
    yes, it's almost as if the climate is changing
    - _Tev 9y ago
      Or our assumptions about climate / wheather were wrong.
- Piskvorrr 9y ago
  ...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.
  - waqf 9y ago
    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.)
ChadyWady 9y ago
From what I understand of the article, the "1 in 500 years" measure is based on the estimated probability of a flood on a given year in a local area. So the probability of a "1 in 500 year" flood in Houston is independent of the probability of a "1 in 500 year" flood for another city.
- twinkletwinkle 9y ago
  Right what the parent comment is saying is that look at a map of the US, how many major cities are there. The odds of two "1 in 500 year" floods happening consecutively are low for any individual city. But if you have enough cities it will happen somewhere. And then you write the article about it. It's kind of analogous to hitting a golf ball into an open field and then exclaiming "Of all the blades of grass, the ball landed on this one!" It's only interesting if it goes in the hole with the flag, otherwise you have a sort of selection bias going.
  - ChadyWady 9y ago
    That makes sense, I was confused since the Birthday problem is more about the pigeonhole principle rather than larger samples being more likely to have individually unlikely outcomes. But for this case, it would still be very unlikely. For any arbitrary 3-year span, the probability of consecutively getting a "1 in 500 year" flood for 3 years is (1/500)^3. If we have N cities, then the probability of none of these cities having 3 consecutive floods is (1-(1/500)^3)^N. For even a vast overestimate such as N=20000, it is still a significantly improbable event. Of course, this doesn't account for "3 consecutive years or more within some year range" and it is a gross simplification, but I think there are probably better explanations than selection bias, such as the inaccuracy of the model or the fact that these events might be temporally dependent on each other.
- analog31 9y ago
  My understanding is that by definition, a 500 year flood has a probability of 1/500 per year. But I live at the top of a hill in the Midwest, so a 500 year flood in my locale has a much different severity than a 500 year flood in Houston.
curiouscats 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. 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
walshemj 9y ago
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
- ncallaway 9y ago
  Except there's a very good ability to know precisely the odds of what the ball landing on black 17 are. It's a very complicated question to determine what a 1/500 odds of flooding are. So, it's very possible that this isn't the gambler's fallacy, but rather the model itself is inaccurate.
  - walshemj 9y ago
    or the massive building growth without building the required infrastructure
- mikeash 9y ago
  It's more analogous to getting black 17 six times in a row. It is possible, but it's weird enough to justify thinking that you've misunderstood the game, or the game has changed.
- deleted 9y ago
  [deleted]
- walshemj 9y ago
  really voted down for pointing out probability 101 I would have expected better mathematical knowledge on HN
  - Piskvorrr 9y ago
    Probability 101 begins with a qualifier: "Assuming independent events..." This can mean two things: a) a set of events that is evidently possible, but increasingly improbable b) events that are not independent
polotics 9y ago
No it's not the same thing. The 500 was a Houston stat, not a US overall one.
SideburnsOfDoom 9y ago
> Of course, the "once in 500 years flood" may also be an inaccurate probability as well Or rather, the odds of that kind of flood increase a bit each year.
pesfandiar 9y ago
I understand your argument, but you can't compare floods to the birthday problem. Unlike randomly-sampled people's birthdays, floods in US cities are not independent random variables. For instance, chances of 500-year floods in two Texas cities is not 1 out of 500*500 in a year. If they're geographically similar and physically close, it should be closer to 1 out of 500.
- saas_sam 9y ago
  In 2 Texas cities, sure, but in every U.S. city? There are going to be several that end up getting a few 500-year floods... and several others that don't flood as expected in several 1000 years.
  - Spooky23 9y ago
    In many US cities. Recall that during hurricane sandy, pretty much any landfill or low lying area was flooded in the NY metro area. That means dozens of cities and towns with 7% of the US population.
hexane360 9y ago
Birthday paradox relies on non-independent events. I'd assume floods should be relatively independent between different cities, meaning that the number of 500-year flood events in the U.S. would follow a binomial distribution (https://en.wikipedia.org/wiki/Binomial_distribution https://en.wikipedia.org/wiki/Binomial_distribution). p = 1/500 years = 0.002 For 100 cities, the probability of at least one city having a 500-year flood is 18.14%. For a 3-year run: p = (1/500)^3 = 8e-9 For 100 cities, the probability is 8e-7 of at least one success. For 1000 cities, the probability is still only 8e-6. Wolfram alpha helps in calculating PDFs and CDFs: http://www.wolframalpha.com/input/?i=binomial+distribution+n+%3D+1000,+p+%3D+8e-9 http://www.wolframalpha.com/input/?i=binomial+distribution+n...
- learc83 9y ago
  How does the birthday paradox rely on non-independent events?
  - hexane360 9y ago
    I guess what I meant wasn't "non-independent events". However, there is still a fundamental difference. The probability of a 500-year flood happening next year is still 0.2%. With a birthday paradox, the filling of birthdays is what causes the explosion in probability. Assuming no collisions so far, the 20th person you add to the room has a 19/365 chance of colliding, while the 2nd person only has a 1/365 chance. I'm not sure how to succinctly phrase this idea. Maybe "non-constant probabilities"?
    - thaumasiotes 9y ago
      In the terms of an example: If you put 400 people in a room, the odds that no two of them share a birthday have gone all the way down to zero percent. No matter how many independent cities you measure flooding in, you can never achieve a perfect 0 or 1 probability of (whatever), because the cities are independent. We don't define "interesting" flooding in one city by reference to what happened in another city, so this situation is not described by the birthday paradox.
- 9y ago
T-hawk 9y ago
> it's actually quite likely that at least one of them could have a pretty high probability of experiencing one in 500 chance of something each year. This is the key point here, and it's called the fallacy of multiple endpoints. If there are 500 observable cities, then somewhere among them should occur a "500-year event" every year. It's just that nobody ever notices the big bulk of the population that hasn't yet had any 500-year event in its recorded history.
kurthr 9y ago
But storms aren't people with randomly distributed cities to flood so you don't get the combinatorial effect. It's not like there are a fixed number of 500 year storms every 3 years that have to flood a uniformly distributed list of cities. The floods are supposedly independent (even if our estimates of likelihood are bad) so there's no binomial coeeficient. The simple calculation is P=1-(1-1/500^3)^N and the probability is less than 1:10000 assuming independence and even given 6000 cities of a million people (the entire world population is in a similar city). If it was birthday like (every possible 500 year storm for 3 years is in a room choosing out of 100 random million person cities), the probablility would be reasonable. You can use a Poisson distribution to estimate the 3 person birthday problem. P=1-exp(-3choose100/500^2)=48%
- roenxi 9y ago
  Actually, in the US it is approximately 1 in 250,000 /assuming independence which is a bit of an assumption/. Obviously, nature doesn't know what a year is so it is totally possible that big storms come in batches. Assuming 1 in 500 year floods are independent, note the ~300 cities in the US [0]. The fact that Huston is a big city doesn't seem relevant to me, because the storm doesn't care. Much like picking a specific number plate, it had to land somewhere. The chance of there being a city in the US which has a 1 in 500 year flood is therefore 60% (300/500) each year. The chance of that city having another flood of similar magnitude in Year 2 is 1/500, because the odds are independent and we started monitoring the city in Year 1. The chance of it flooding again the third year brings the odds down to 1/(500 * 500) = 1 in 250,000. [0] https://en.wikipedia.org/wiki/List_of_United_States_cities_by_population https://en.wikipedia.org/wiki/List_of_United_States_cities_b...
rothbardrand 9y ago
Having lived in Houston, this isn't a measure of the flood but a measure of how terrible the infrastructure is. They have terrible flood management systems but are "rated" to handle everything but a "100" year flood... yet Houston gets 100 year floods every year- multiple ones- and has for 40 years. It's a scam involving the ratings much like the mooody's ratings scam of the last decade IMNSHO
danmaz74 9y ago
How do you count "floods"? If there is one hurricane that causes floods in 3 cities, is that 1 flood, or 3? If it's 1 - which is what I think is meant when saying "a 1 in XXX years flood" - then your comparison doesn't hold, because the number of floods isn't dependent on the number of cities, but on the distribution of cities on the land, and specifically in flood-prone areas.
- Spooky23 9y ago
  IMO you need to look at something other than city limits. Cities like Houston and LA are huge cities geographically. IMO you need to look at regional and per capita impacts.
epitomix 9y ago
I think that this statistic only makes sense if it is specific to the location. Otherwise, it is useless for planning purposes. If it is specific to the location then the birthday paradox does not apply. Interesting note, this is Houstons third 500-year flood in three years [1]. [1] -https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houston-is-experiencing-its-third-500-year-flood-in-3-years-how-is-that-possible https://www.washingtonpost.com/news/wonk/wp/2017/08/29/houst...
oso2k 9y ago
The "Birthday Problem" has a simple explanation. Something like nearly 45% of people are born in the Summer months. Ensure that you get people of different ages and your chances of finding someone with similar/same birthday goes up enormously.
- brewdad 9y ago
  Is this true? Which months are considered "summer"? Northern Hemisphere or Southern or both? This can easily turn into a "40% of sick days are on Monday or Friday" type fallacy.
- pmalynin 9y ago
  Thats....not how that works. At all.
- bitwize 9y ago
  The birthday problem still exists even assuming that everyone has an equal chance of being born on any day of the year. It's a phenomenon of the statistical likelihood that given a large enough group of people, the likelihood that any two will have the same birthday goes to almost certainty very rapidly.
- umanwizard 9y ago
  No, the birthday problem still works if you consider that the probability of being born on a particular day is uniform. It's related to the fact that the probability of two people in a room having a birthday is more closely related to the number of pairs of people in the room (which in turn grows as O(n^2)), than to the number of people in the room. A detailed proof/explanation can be looked up easily with Google, if you have a bit of mathematical maturity.
- oso2k 9y ago
  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.cdc.gov/nchs/data/nvsr/nvsr66/nvsr66_01_tables.pdf https://www.cdc.gov/nchs/data/nvsr/nvsr66/nvsr66_01_tables.p...
curiousgeorgio 9y ago
A more accurate term for what you describe would be a clustering illusion[1], and it certainly could be a valid explanation (among other possibilities) for the observation made in the article. [1] https://en.wikipedia.org/wiki/Clustering_illusion https://en.wikipedia.org/wiki/Clustering_illusion
- honestoHeminway 9y ago
  Yes, or as with nuclear power, it could be that the base assumption regarding safety just was wrong.
simonebrunozzi 9y ago
Ahem, no - your birthdays example can't apply here. When you increase the number of people in the room, each one of them benefits from every new person added, in terms of probability of having been born on the same day. In the flood case, a flood in California and one in Miami don't really increase the chance of another flood simply because they have happened.
bnegreve 9y ago
> 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. Not quite the same because Huston is not the only city that has experienced extreme climate these two years, many other cities also have (maybe not 500 years flood, but close). So that would be like: one pair is born the same day, and 20 other people in the room are born a week around that day. Hardly a coincidence. It's more likely that there was a party about 9 months and X years before that day. (I'm not sure how far we can go with this silly birthday analogy:)

danielvf 9y ago

Cue discussion on the works of Tabeb, and what a horrible job humans do at predicting the frequency of rare events.

kbutler 9y ago
Parent means Nassim Nicholas Taleb http://www.fooledbyrandomness.com/ http://www.fooledbyrandomness.com/ And yes, basically we don't understand the actual probability of a "500-year storm", so our 0.2% chance per year is unreliable at best, more likely misleading. Just like Europeans didn't know the actual probability of a black swan until they found them in Australia.
- 21 9y ago
  I think the idea of a black swan is something which you don't even imagine before you encounter it, ie: nobody would even think about computing the probability of a black swan (bird) existing.
  - kbutler 9y ago
    While unimaginability may fit the original conception of a black swan, Taleb's book is "The Black Swan: The Impact of the Highly Improbable" (not unimaginable or inconceivable) and defines a black swan as "an outlier, as it lies outside the realm of regular expectations, because nothing in the past can convincingly point to its possibility." http://www.nytimes.com/2007/04/22/books/chapters/0422-1st-tale.html http://www.nytimes.com/2007/04/22/books/chapters/0422-1st-ta...
dragonwriter 9y ago
Or, more on point, the common disclaimer, “past performance is no guarantee for future results”.
21 9y ago
He just tweeted about it: "When people talk about 500 y floods or 100 year rain, they are klueless abt tail distribution. The empirical distribution is NOT empirical!" (and a picture with math equations) https://twitter.com/nntaleb/status/902632121407299585 https://twitter.com/nntaleb/status/902632121407299585

devy 9y ago

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/2017/08/27/harvey-is-causing-epic-catastrophic-flooding-in-houston-why-wasnt-the-city-evacuated/ https://www.washingtonpost.com/news/post-nation/wp/2017/08/2...

idlewords 9y ago
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.
- tertius 9y ago
  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...
  - idlewords 9y ago
    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.
    - sjg007 9y ago
      This is not true. You can assume a uniform prior and run your model through bayes law and come up with an estimate.
      - idlewords 9y ago
        That's called "making things up", but sure.
  - ars 9y ago

diafygi 9y ago

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 currently focused on scale and growth[1], and we need as many smart people as we can get. There are lots of companies hiring software engineers. ==How do I find a job fighting climate change?== I'd recommend browsing the exhibitor and speaker lists from the most recent conference in each sector (linked below). Check out the companies that interest you and see if they are hiring. * Energy Storage[2][3] * Solar[4][5] * Wind[6] * Nuclear[7] * Electric Utilities[8][9] * Electric vehicles[10] Also, if you're in the SF bay area, I'd recommend subscribing to my Bay Area Energy Events Calendar[11]. Just start showing up to events and you'll probably find a job really quickly. [1]: https://techcrunch.com/2017/01/22/energy-is-the-new-new-internet/ https://techcrunch.com/2017/01/22/energy-is-the-new-new-inte... [2]: http://www.esnaexpo.com/ http://www.esnaexpo.com/ [3]: https://www.greentechmedia.com/events/live/u.s.-energy-storage-summit-2016 https://www.greentechmedia.com/events/live/u.s.-energy-stora... [4]: https://www.intersolar.us/ https://www.intersolar.us/ [5]: http://www.solarpowerinternational.com/ http://www.solarpowerinternational.com/ [6]: http://www.windpowerexpo.org/ http://www.windpowerexpo.org/ [7]: https://www.nei.org/Conferences https://www.nei.org/Conferences [8]: http://www.distributech.com/index.html http://www.distributech.com/index.html [9]: https://www.greentechmedia.com/events/live/grid-edge-world-forum-2016 https://www.greentechmedia.com/events/live/grid-edge-world-f... [10]: http://tec.ieee.org/ http://tec.ieee.org/ [11]: https://bayareaenergyevents.com/ https://bayareaenergyevents.com/

jmcgough 9y ago
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.
- sitkack 9y ago
  Work where your skills have the highest impact. Sometimes it translates into labor -> dollars.

rlglwx 9y ago

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.

idlewords 9y ago
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.
- WorldMaker 9y ago
  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 rainfall statistics analysis.
  - tedunangst 9y ago
    I don't know much about the field, but I'm curious. How much are estimates based on normal distributions, and how much does the model consider other distributions? As you say, I don't think we know much about the tail. More than ten inches of rain may be a once a year phenomenon, but that once a year event might be twenty or fifty inches?
    - idlewords 9y ago
      I am not a math, but I remember that if you can't assume a normal distribution and want to know the shape of the tails, you need a pretty ridiculous number of observations.
    - 9y ago

idlewords 9y ago

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.

maxerickson 9y ago
Should any land be required to have flood insurance? The NFIP supposedly reduces taxpayer cost (with the shaky risk management you mention), but it currently has huge potential liabilities compared to the premiums it collects (and owes the government a bunch of money from past payouts).
- cat199 9y ago
  Hey we've got a legal mandate to fund insurers now with our 'public private partnership' healthcare system.. pick this up as your issue and you might have a lucrative career in lobbying lined up..
  - matt4077 9y ago
    "Now"... The National Flood Insurance Act of 1968 established the government-backed flood insurance scheme. It has Ronald Reagan's signature. https://en.wikipedia.org/wiki/National_Flood_Insurance_Act_of_1968 https://en.wikipedia.org/wiki/National_Flood_Insurance_Act_o... The idea was and is twofold: first, it was seen as the only possibility to actually provide such insurance because insurance companies (even together) lack the financial strength to be able to insure risks that are statistically extremely dependent. Something like this flood costs dozens of billions, and it would bankrupt the whole industry. Secondly, it's in the national interest to promote such schemes that allow for more areas to be used productively.

dogruck 9y ago

Is this like when reporters say, "Trump won. We now think that Clinton's lead wasn't as strong as previously reported."

blondie9x 9y ago

The fundamental takeaway from this article is about half way down. Major flood events are occurint more frequently. What was once likely to hit a city every 500 years is happening to more cities on a planetary scale more frequently.

ars 9y ago
I know it's easy to blame climate change for anything and everything, but that's simply incorrect. The actual takeaway is that our data is wrong, these floods occur more often than we thought. With only 150 years of good data, it's impossible to calculate "500 year" events accurately. You would need around 5,000 years of data to do that, and we don't have it. So we ended up significantly underestimating how often these events occur.
- blondie9x 9y ago
  You can see in graph that floods are occurring more often and that clouds are able to absorb more water as temperatures rise. This hurricane would have happened. But this flooding is worsening because of climate change that is irrefutable.
  - interstitial 9y ago
    By irrefutable, what do you mean? Over-fitting and variance should make normal engineers more cautious. Evidently, climate science is far more magical.
- interstitial 9y ago
  It's almost like you are saying the model was overfit to the data.
- qqg3 9y ago
  And we failed to incorporate the "concrete jungle" effect
- honestoHeminway 9y ago
  Shouldnt there be more data in geologic strata? Flooding leaves distinct marks if i remember correctly. https://en.wikipedia.org/wiki/Geography_of_Houston https://en.wikipedia.org/wiki/Geography_of_Houston Would love to ask these gentlefolks that question: http://www.beg.utexas.edu/presentations/2005_presentations/blakeney102505.pdf http://www.beg.utexas.edu/presentations/2005_presentations/b... https://www.osti.gov/scitech/servlets/purl/841632 https://www.osti.gov/scitech/servlets/purl/841632

mbroncano 9y ago

The elephant in the room has been mentioned somewhere else these days: severe lack of planning and crumbling infrastructures. Remarkably how ne of the few non-partisan issues on the table goes under the radar time and time again. Tongue in cheek, we might think only polemic issue get the attention of the public. There is also the notion of national pride. We built our infrastructural base earlier than pretty much anyone else in the world, but we don't invest consistently on them anymore. Today our roads, dams etc. pale in comparison with any western country's but it seems like it's an affront to our national pride to acknowledge that. Houston is ill prepared to face any kind of emergency like this one, and only the bravery and tenacity of the people is preventing a humanitarian disaster in the scale that would keep us awake at night. That's not the American way: planning, preparing and executing is. The sooner we start taking this leap the better.

pdq 9y ago
Which other city in the US can handle 40 or 50 inches of rain, and 15 trillion gallons of water, within a one week timespan? That's as much rain as most US cities get in a year. This is not the fault of poor planning or poor infrastructure in Houston. It's freak weather event that shouldn't just be given partisan rhetoric.
- dlp211 9y ago
  Probably none, but it's not a matter of what city could survive unscathed, it's a matter of what city has minimized the consequences. Houston has allowed for basically unfettered growth constraining their bayou's and preventing flooding infrastructure to be built. Hopefully Houston uses this disaster to rezone and allow for appropriate land management.
- enraged_camel 9y ago
  Poor planning and poor infrastructure may have contributed to the problem: https://www.washingtonpost.com/graphics/2017/investigations/harvey-urban-planning/ https://www.washingtonpost.com/graphics/2017/investigations/... This is not "rhetoric" and there is also nothing "partisan" about it.
- 9y ago

xexers 9y ago

Anyone notice the comments for this article on WP? There appears to be a poster called "The burning bush" who is commenting with lengthy comments about every 6 minutes. All of the the comments from that user are sceptical of man made climate change. Washington post does not appear to let you sort by anything other than "newest"... which means that this commenter's posts are always the top comment. That's concerning!

stevenwoo 9y ago
Every comments section I've seen for newspaper articles even in supposedly blue urban areas contain an outsize proportion of conservative trolls. It's possibly worse than youtube comments.
2sk21 9y ago
Newspaper comments sections are almost uniformly unreadable. HackerNews is, I think, the only site I subscribe to read comments.
Applejinx 9y ago
That's normal. You'll find the same thing going on in the HN comments for '24/192K audio is silly': there's a class of people who fight for their arguments not with science or discussion, but through brigading and suppressing other people's arguments to get 'em out of sight. It's very much the case with climate change denialists, and it's also in play with '16 bit audio is enough for every possible person!' audio pundits: they will brigade to suppress opposing arguments to make it seem like their views are unopposed. Pretty sure the WP comments from the climate skeptics are more organized. There's a lot more money in climate denialism, from traditional energy companies with enormous resources and a clear agenda to continue their present mode of operations. I've never understood why the 16-bit audio guys are so similarly quixotic.
enraged_camel 9y ago
WaPo is experimenting with a new comment system, which is why it currently only lets you sort by newest. The old system had other sorting types, and the new one will too soon. Don't worry about it too much. :)

dmix 9y ago

This article reminds me of Nate Silver's book [1] which has a far more scientific and in depth look on the failure/success of these types of predictive statistics. The chapters on the many attempts (or many failed hopes) in predicting earthquakes was particularly interesting, including many times the media has bought into hyped up new charlatans who say they finally figured it out but which ultimately failed to survive under basic statistical scrutiny. It also has a useful soft introduction to Bayesian statistics and other useful concepts from the field of prediction that I hope more journalists read about. As this seems to be a very common theme in reporting. Even this journalist couldn't help themselves with this line (combined with some scary looking charts described with an alarmist tone farther down): > Was there some miscalculation of how frequently these massive flooding events occur? Or, most alarmingly, is something else happening that suggests these catastrophic weather events are becoming much more common? The failure to mention the effects of El Nino/El Nina seems like a big oversight in this article, especially when we're just coming out of a particularly strong one. Climate stats are an easy one to get wrong - or to shape into any narrative - especially when timeframes and location are easy things to be viewed too narrowly. [1] https://www.amazon.com/Signal-Noise-Many-Predictions-Fail-but/dp/0143125087/ https://www.amazon.com/Signal-Noise-Many-Predictions-Fail-bu...

MarkMc 9y ago

The problem is that the person who judges the probability at 1-in-500 doesn't have any skin in the game. The judgement of the US meteorological office affects where people can build houses and how much they pay for government insurance. Homeowners have a strong financial interest in underestimating the probability of a severe flood, so there is likely subtle political pressure applied to the met office assessments. For a more accurate assessment, the government should buy a sizeable insurance policy from several reinsurance companies and use it to calculate the implied probability.

lutorm 9y ago
Homeowners have a strong financial interest in underestimating the probability of a severe flood I don't know about that. Even if I have flood insurance, it's not clear I'm financially better off by having my house flood. It may be a confluence of "financially not fully liable" and wishful thinking, though.
tuna-piano 9y ago
And the reinsurance folks seem to say that there is absolutely no evidence that there has been an uptick in these big disastrous storms. People are constantly thinking of global warming, so when they see a storm, they say "Aha! Global Warming!". Of course there have always been hurricanes, and human perception/remembering is not a replacement for statistics and math. From Warren Buffet (One of the largest insurers of this type in the world) "...climate change had not up until then [2016] produced more frequent nor more costly hurricanes nor other weather-related events covered by insurance."" https://www.cnbc.com/2017/02/27/warren-buffett-global-warming-not-impacting-berkshires-insurance-biz.html https://www.cnbc.com/2017/02/27/warren-buffett-global-warmin...
- dogruck 9y ago
  Markets also did not collapse when Donny won. But, hey, what do markets know?
  - XorNot 9y ago

lcfcjs2 9y ago

Tell me again how Climate Change isn't real.

twblalock 9y ago

I hope the city will require houses to be elevated when rebuilt.

mirimir 9y ago

> Climatologists say the mechanism by which this is happening is fairly straightforward. “Warmer air can contain more water vapor than cooler air,” according to the 2014 Climate Assessment produced by the U.S. government. “Global analyses show that the amount of water vapor in the atmosphere has in fact increased due to human-caused warming. This extra moisture is available to storm systems, resulting in heavier rainfalls.” This is just so totally obvious. But I've rarely seen anything in news coverage about increased mean atmospheric water concentration. Also, increased water content is (of course) the expected multiplier for increasing CO2. Edit: spelling

ianai 9y ago
I wonder if there are maps of predictions of that.

cttet 9y ago

Maximum likelihood ¯\_(ツ)_/¯

nyolfen 9y ago

> The big news in the Galloway report, based on a global review of scientific and engineering data, was this: All of the money the United States had paid for massive public works to control flooding over the previous half-century not only had failed to improve flood safety at all, but the spending and the big public works projects actually were making flooding much worse. > The Galloway report was no global warming screed. Like the Dutch national policy based on many of the same findings, it dealt almost entirely with land use and population growth. Or to put it in more accessible terms, the 'burbs. http://www.dallasobserver.com/news/harvey-not-only-can-happen-in-dallas-it-will-9816590 http://www.dallasobserver.com/news/harvey-not-only-can-happe...

blondie9x 9y ago

https://mobile.nytimes.com/2017/08/29/opinion/harvey-the-storm-that-humans-helped-cause.html https://mobile.nytimes.com/2017/08/29/opinion/harvey-the-sto... On climates impact on extreme flooding.

mberning 9y ago
"Some version of Harvey probably would have happened without climate change, and we’ll never know the hypothetical truth." But let me write an op-ed about it anyway.

exabrial 9y ago

Are random events evenly distributed? ......

Steve44 9y ago

I saw a TV programme a year or two ago which was discussing the increase in hurricanes etc in the Gulf of Mexico area. They were saying that the storms aren't increasing due to global warming but are actually regressing back to their natural levels. The previous decades of pollution had been suppressing the storms and by cleaning the air the storm ferocity is heading back to normal. I can't see the article at the moment but I think it was Cloud Lab on the BBC, they floated around on an airship.

staticelf 9y ago

Bad shit happens when you fuck up the earths temperature. Why is anyone surprised? This has been said by climate scientists all over the world many times. Warmer climate will result in more drought on some places and other stronger storms which result in heavy rainfall and floods. https://climate.nasa.gov/effects/ https://climate.nasa.gov/effects/

jaclaz 9y ago

There used to be a calculator by NOAA: http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod http://www.srh.noaa.gov/epz/?n=wxcalc_floodperiod that is now here: https://www.weather.gov/epz/wxcalc_floodperiod https://www.weather.gov/epz/wxcalc_floodperiod According to it, the probabilities of a 500 year event in the next 500 years is 63.2%. What I like to call flippism point, i.e. when you have the same chances as flipping a (perfect) coin is next 346 years. A three consecutive years series does seem a bit "queer".

VeronicaJJ123 9y ago

I feel sad when people use statistics like this. It makes no sense to call this 500 year flood because floods do not follow any kind of distribution. Not to mention the human beings have been changing their environments drastically in last 200 years. NN Taleb would say this is fat tail event. It does not follow empirical distribution.