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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

#151

Post by enraged_camel »

pdq wrote: post #90
mbroncano wrote: post #77

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 any…

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.

Poor planning and poor infrastructure may have contributed to the problem:

https://www.washingtonpost.com/graphics/2017/investigations/...

This is not "rhetoric" and there is also nothing "partisan" about it.

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

#152

Post by enraged_camel »

21 wrote: post #87
Clubber wrote: post #40

Earlier quoted context omitted.

Sounds like the city fell asleep at the wheel, or rolled the dice and didn't want to pay for proper drainage. Now the citizens get to pay. Sounds like the Judge is preparing the country for the fact that nothing will still be done about it.

There was an article from 2016 also on HN today about the risk. Three different experts were quoted saying something along the lines "nothing will be done now, we need to wait for the big flood and 2-4 years after we will build a wall". The "wall" (actually a more complex thing) was projected to cost 8 bil. A bad flood like the one from today was projected to cost 70+, but people were hoping that we have around 10-20…

Models actually showed a disaster of this scale as far back as 2005: http://www.chron.com/news/houston-texas/article/Models-show-...

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

#153

Post by beala »

hexane360 wrote: post #43
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…

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 ). 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/…

This is actually a really interesting problem because depending on how you set it up, you'll get very different answers. To illustrate, notice the difference between these questions:

1) Suppose I flip a coin 3 times and repeat that experiment on 10 different occasions. What's the probability that I get all heads at least once?

2) Suppose I flip a coin 30 times (same total number of flips). What's the probability that I get at least 1 string of at least 3 heads?

(1) is more like the binomial calculation you've set up, but I think (2) is closer to the question we want to be asking.

This is important because, as it turns out, (2) is much more likely than (1). You can tell intuitively that there are more sequences of coin flips that would satisfy (2). For example "THH HTT ..." would satisfy (2) but not (1).

So how should we pose the hurricane question? I propose: What's the probability that in a 100 year period, across 100 cities, there will be at least 1 city that experiences a 500-year flood for at least 3 contiguous years?

Solving this analytically is non-trivial, but it's easy to simulate:

  // Scala

  val rand = scala.util.Random
  def coinFlip(p: Double): Boolean = rand.nextDouble  if(cur){acc + 1} else {acc}}

  // Estimated probability for a single city.
  val p = count/simulations.toDouble
  
  // The probability it will happen *at least* once in a population of
  // 100 cities.
  // This calculates the probability if *won't* happen 100 times, and
  // then takes the complement, giving us the final probability
  1 - Math.pow(1 - p, 100)
The final answer I get is 9.5E-5, so still very unlikely, but 100x more likely than the binomial calculation.

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

#154

Post by nyolfen »

> 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-happe...

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

#156

Post by thaumasiotes »

hexane360 wrote: post #144
learc83 wrote: post #62

Earlier quoted context omitted.

How does the birthday paradox rely on non-independent events?

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…

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.

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

#157

Post by mberning »

blondie9x wrote: post #155

https://mobile.nytimes.com/2017/08/29/opinion/harvey-the-sto... On climates impact on extreme flooding.

"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.

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

#158

Post by vacri »

21 wrote: post #78
danielvf wrote: post #3

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

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

The problem isn't the understanding of statistics, the problem is a grossly-misleading name.

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

#160

Post by 2_listerine_pls »

pdq wrote: post #90
mbroncano wrote: post #77

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 any…

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.

You cannot compare a normal city to Houston without considering hurricanes, dams, rivers and sea level.
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