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Comment by TallGuyShort | Hacker News Reader

>> 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.

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>. 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.

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?

Yes, lower latency is a pretty good example.

Also people tend to drain marshes and any other shallow stagnant waterways to avoid mosquitoes having breeding places. It can save us from terrible diseases, but it can make flooding worse.

There is one additional thing to note. Soil has a certain sponge like quality to absorb water. So a runoff curve number was created to "predict runoff or infiltration"

The tables on the wiki pages shows "curve numbers" for different types of soils. These numbers are based on experimental data which is then used to model runoff. (Or used to be. Its been 20 years since I've done runoff modeling, so maybe things have changed)

https://en.wikipedia.org/wiki/Runoff_curve_number

Sounds like an effective, cheap solution would be water butts on every gutter.

Up to a point. To continue the bandwidth analogy, having a buffer is cheap - unless the input link fills it faster than the output link can handle, for a prolonged period. The buffer gives you some extra time for processing, but when it fills, you are back to step 1.

An additional on flooding:

When Rainfall * Area >= SoilAbsorption(See note) + WetlandAbsorption + ArtificialDrainage the end result is flooding.

Essentially the left hand side of the inequality has been slightly increasing because of climate change, but we're chipping away constantly at the right side of the equation in three ways.

(1) More and more soils in American metro areas have been covered by impervious surfaces for development. (2) More wetlands are being destroyed for development. (3) Artificial drainage is being under-built. Drainage is generally built for 1 in 100 year events but the rate of urbanization is increasing the magnitude of flooding. (Drainage is only a problem for the developer that hasn't sold what they're building.)

Note: Soil has a finite capacity for water infiltration.

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.

Because, crucially, your mortgage may require you to buy insurance in flood plains. And it can be quite expensive.

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.

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

I am aware that flood insurance itself is subject to government influence - e.g. the National Flood Insurance Program, and possibly state regulation (even in Texas) - but I don't know the details. Does it result in premiums being affected by official flood-zone designations?

As you appear to be in or near Houston, I hope things are OK with you and your family.

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

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

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