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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

#31

Post by maxerickson »

idlewords wrote: post #17

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 on…

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).

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

#32

Post by kbutler »

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.

Parent means Nassim Nicholas Taleb 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.

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

#33

Post by pixl97 »

TallGuyShort wrote: post #7
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…

>> 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 i…

>. It would seem the probably has gone up due to climate change

That probably has a small reason to do with the increase, but your next line highlights the issue.

> 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

No, not exactly. The more stuff you build, the faster water runs off. Unincorporated land has a pretty high water slowing capability. It takes more time for water to run through tall grass, and puddles, and log jams, and loose dirt. All things that occur naturally. Humans build houses that have all the water run into gutters, that run into a well cut lawn and down a sidewalk. Once it hits the road it's like a water expressway. It can travel quickly to a local stream. A stream that likely has a concrete lining so the water moves swiftly to a lake or reservoir in hours rather than days.

As a place develops new flood plans must be studied every 5 to 10 years. A new development of 100 hours up the watershed from you can mean your risk of flooding increases dramatically. Places like Houston have had 10,000's of houses build in the surrounding areas meaning what was a 1 in 100 year flood could be as low as a 1 in 10 year flood. All this stuff being built is not taking into account its effect on downstream rainfall runoff.

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

#34

Post by dragonwriter »

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.

Or, more on point, the common disclaimer, “past performance is no guarantee for future results”.

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

#35

Post by peeters »

rlglwx wrote: post #12

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

Honestly I think it's easier to feel irrationally safe about a 0.2% chance, especially if you leave out the "per year". "500-year flood" at least emphasizes that it's not a question of "if", but "when".

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

#36

Post by lukeschlather »

idlewords wrote: post #13
rlglwx wrote: post #12

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

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

I don't think it's accurate to say we only have 150 years of data. We only have about 150 years of data collected in real time by humans, but we have plenty of other data. (Soil/rock strata,core samples, tree rings, etc.)

In some ways the geological and archeological evidence is more trustworthy than the evidence we've written down in the past century or two.

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

#37

Post by rsync »

devy wrote: post #5

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

"Fort Bend County Judge Robert Hebert stated that Hurricane Harvey is more like a 800-year event"

500 years, 800 years ... even 1000 years - none of those are timetables that planning for city ^H^H^H^H region the size of Houston should be operating on.

A city that large, that populous and with that much economic activity that is crucial to the world should be using risk models to 10k year events.

This is not unprecedented - Holland's dykes are designed and built to withstand 10k year events:

https://www.nytimes.com/interactive/2017/06/15/world/europe/...

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

#38

Post by TallGuyShort »

pixl97 wrote: post #33
TallGuyShort wrote: post #7

Earlier quoted context omitted.

>> 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 i…

>. It would seem the probably has gone up due to climate change That probably has a small reason to do with the increase, but your next line highlights the issue. > 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 No, not exactly. The more stuff you build, the faster water runs off. Unincorporated land has a pretty high water slowing capability…

I actually hadn't thought about that aspect at all. I was thinking more along the lines of 'more development -> more stuff to get destroyed', but that's kinda backwards anyway because a flood would probably wipe out a bigger portion of infrastructure hundreds of years ago than it does now.

But faster run-off sounds like a good thing to prevent the flood - are you saying that more development upstream means more flooding downstream because all the run-off arrives with much lower latency and higher bandwidth, to use IT terms?

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

#39

Post by jessaustin »

lukeschlather wrote: post #36
idlewords wrote: post #13

Earlier quoted context omitted.

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

I don't think it's accurate to say we only have 150 years of data. We only have about 150 years of data collected in real time by humans, but we have plenty of other data. (Soil/rock strata,core samples, tree rings, etc.) In some ways the geological and archeological evidence is more trustworthy than the evidence we've written down in the past century or two.

Do floods show up on the "geological and archaeological" records? Sure we can say that 50M yrs ago there was an ocean here, but that's a different statement than "in the last 1000 yrs there have been 14 floods that reached this height above the river".

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

#40

Post by Clubber »

rsync wrote: post #37
devy wrote: post #5

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

"Fort Bend County Judge Robert Hebert stated that Hurricane Harvey is more like a 800-year event" 500 years, 800 years ... even 1000 years - none of those are timetables that planning for city ^H^H^H^H region the size of Houston should be operating on. A city that large, that populous and with that much economic activity that is crucial to the world should be using risk models to 10k year events. This is not unpreced…

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.

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