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Comment by ghshephard | Hacker News Reader

The challenge is that it just doesn't flood often enough to justify investing in significant defenses. If this were the type of event that occurred annually, or multiple times a year - I'm guessing we would see investments in the sorts of thing that Tokyo builds such as https://www.bbc.com/future/article/20181129-the-underground-...

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I was disappointed to hear the term "100-yr flood" with the latest storem because I remember at least two other such floods in the area in my life -- one was in 2012 (Hurricane Sandy) And one was ~1992 (a Nor'Easter.) So that is three in thirty years. Is our distribution incorrect, or is "100-yr flood" just a media term? Or is it a way to make something sound rare so we can justify unpreparedness?

Of course, all three of these were different things -- one a Nor'Easter, one a Hurricane, and the latest was the effects of a Hurricane down south.

It's kind of an abuse of the term. The technical definition is a storm that according to historical records has a 1% chance of happening in any given year. So you'd expect to see it at least once in 100 years. As climate change increases the severity of all types of weather those level of storms will get more common than in the past in a hard to predict way. The storm percentages are also based on the historical record instead of future predictions so it will inherently lag behind the expected increasing storm frequency/intensity.

I wish they'd talk about it in these terms. It's easier to think about the cost/benefit of investing in mitigations when faced with "you have an n% chance of incurring m billions in damage this year" vs "x billion dollar damage storms should only occur once every y years".

We do it all the time with e.g. cloud SLAs.

A probability of 1% per year means 1 - .99^100 = 63.4% probability of at least 1 storm in a hundred years.

The expected value of how many years until such a storm occurs is 100 years.

"A common misunderstanding is that a 100-year flood is likely to occur only once in a 100-year period. In fact, there is approximately a 63.4% chance of one or more 100-year floods occurring in any 100-year period." [1]

Which is different than "You'd expect to see it at least once in 100 years."

However, yes, the expected value of the number of 100-year floods occurring in any 100-year period is 1.

[1] https://en.wikipedia.org/wiki/100-year_flood#Probability

I think the rain in Newark was so unprecedented it broke all expectations of what was possible. It wasn't a one-in-anything flood.

I realize it was rain (and not a hurricane) but we had very similar flooding back in 2012 (just 9yrs ago) with Hurricane Sandy. https://en.wikipedia.org/wiki/Effects_of_Hurricane_Sandy_in_...

I'm not as familiar with NJ as I am with NYC -- but in NYC we had something similar in 2012 and again in ~1992 with a Nor'Easter.

I'm just wondering if the risk distributions were' using are unrealistic?

The flooding from Sandy was due to extreme high tide, not due to rain. Lots of places away from the sea (like Queens) were bad this year but fine in Sandy, while places close to the sea shore (like Staten Island) were devastated with Sandy but no problem this year. So multiple threats which is bad.

The US has built similar systems in other places and they can't keep up with the rain.

https://slate.com/business/2019/01/chicagos-deep-tunnel-is-i...

When I visited home it just happened to be around the time of Sandy. It wasn’t just Sandy itself but some other tropical remnant that dumped 7 inches of rain in two or three days? I tell people in the SF Bay Area, 7 inches is about a third of what we usually get in a year. These kinds of storms seem to be happening every ten years now rather than a hundred. (Meanwhile the West is in a constant state of drought).

Another way of saying the same thing is enough people just havent died to justify the cost. We can't keep reasoning about how to safely build cities by how much money is justifiable. It's madness.

It is a city on the ocean, surrounded by bays and lakes. The investment is justified. The political will, however, remains to be seen.

EDIT: clarity

Why do you believe this? It seems it depends heavily on

- How many people would be injured or killed by the event.

- How much financial damage the event would cause.

For example, if nobody ever dies, and 10 million in damage/problems occurs each time the event does, then it's almost certainly not worth spending 1 billion dollars to to fix the problem.

Obviously, those numbers are ridiculous, but the point is there. The numbers need to be considered.

And yes, human life is a number, too. It's just harder to quantify because there's a lot of opinion involved. But if it the expectation is that such an event will kill 1 person each time, and it would cost 1 Trillion dollars to fix/prevent... it's almost certainly not worth fixing. It's harsh, but spending a Trillion dollars for a single life would be unreasonable.

Now those numbers are actually pretty reasonable. If Nobody ever dies, there's 10 million in damages each time an event like this occurs (around once every 10 years now), and with climate change we can expect the intensity + regularity to increase over time, then 1 billion is likely a good price to pay for solving the problem.

Now with mitigations like those that would be used here (like the storm water systems used in Tokyo), the sooner you implement them, the easier/cheaper they'll be to implement/maintain long term.

At the end of the day the best time to start adding the mitigations is now and expand later as necessary rather than wait until the cost is high enough and go into a mad dash to complete them before the next disaster.

Edit: For context, the Metropolitan Area Outer Underground Discharge Channel in Tokyo cost 2 Billion USD to build and is able to easily handle this volume of floodwater.

I was the original poster suggesting the Tokyo System - but I did the math and I was wrong.

The total underground capacity of the Tokyo consists of the Cans, (250K cubic meters), Cistern (248K cubic Meters) and Tunnels (575K cubic meters) - for a total underground capacity of 1.07 million cubic meters.

New York City was hit by 35 Billion gallons of rain during the flash flood, which = 132 M cubic meters. The Tokyo System has .38% of the capacity of the rainstorm, and is capable of draining at 200 cubic meters/second which would take 7-8 days to drain all that water.

What happened in New York was epic - and no underground system imaginable would have been able to handle it. The only solution to this type of problem occurring in the future is going to be around things like zoning (don't build where it's going to flood), landscape (lots more greenery), architecture (lots more void decks, first floor resilience to flooding, no basement suites, etc...)

I believe this because when I see people dying from living in flooded basement apartments -- a *well known*, documented risk of flooding from natural disasters -- I don't see "the cost was not justifiable", because it would have cost almost nothing to save their lives.

I'm not saying we can prevent any/all deaths -- that is impossible. I am saying I want responsible planning and policy that centers human life, not money, as the very point of all this planning and spending in the first place. But some of the deaths, such as the basement situations, from this disaster were very needless and do not cost trillions to prevent.

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