Elevated Structures on US Floodplains?

[Kanastrous]

So one thing that has kind of leaped out at me every time there's an oh the Mississippi's overflowing its banks what a surprise oh my we're getting flooded who'd 'a thunk it?! story (like this week) - why does it seem as though practically everyone who traditionally lives in flood/surge prone areas builds their buildings on stilts (Nipa huts, like in the P.I., or really just about anywhere in SE Asia that floods a lot) except in the USA? I mean, after the third or fourth time your house is inundated or washed away, how can one *not* think about sinking a big foundation block with columns that reach above flood-level, and build your damn house on top of *that*? Sure, it's far more expensive than a typical slab or perimeter foundation but still likely cheaper than replacing an entire house, or house interior, plus contents every few years.

It seems like such a painfully obvious solution that I figure there must be compelling reasons the technique's not adopted - but aside from the one-time expense I sure can't work out what those reasons might be.

[Mr Bean]

Because building on stilts means your house is gone the first time a Hurricane or Tornado hits the area which Mississippi is also prone to (At least in some regions of the Delta. Living on the flood planes is a three way hell fest because it floods every few years, there's Tornado's every year and Hurricane's bring floods for those years where there's not enough rainfall.

So it's a cost factor plus the simple fact they can still get housing insurance. If H&R Block refused to insure anyone in the flood planes then much few people would build there.

[Kanastrous]

Doesn't seem as though a house on a level foundation is going to do that much better against a hurricane or tornado. If tornado pressure dynamics are right to explode an elevated house, would a house-at-ground fare any better? And being on stilts can save you from a hurricane storm surge, while the wind pressure a few meters above ground level isn't that much higher than *at* ground level, is it...?

Sure, an arrangement that allows high-velocity wind to pass under a house likely presents problems, but we address those problems in bridges and cantilevered structures, so the solutions must be available.

Maybe part of the problem is being perhaps a little bit stupidly generous when it comes to insurance...

[Simon_Jester]

The expense of the building goes up a lot when you build it on some elaborate arrangement of stilts.

If you use simple stilt construction, the obvious problem is going to be the stilts' ability to handle lateral forces, such as very strong winds pressing against one side of the house. It's not hard to imagine a situation where the wall can handle the pressure but the stilts can't handle the force and start to shear.

[someone_else]

A more interesting question would be why don't make flood control structures, like say detention basins, or some kind of argin.

The likely answer is that it's vastly cheaper to pay an insurance and be happy about it.

Also, the main problem of tornadoes/hurricanes is that they throw shit around. You can make a house able to withstand the tornado itself with some ingenuity, but to survive the thrown shit (that can be pretty big) you need a decent amount of reinforced concrete.
And bunkers are expensive.

[Kanastrous]

If you use simple stilt construction, the obvious problem is going to be the stilts' ability to handle lateral forces, such as very strong winds pressing against one side of the house. It's not hard to imagine a situation where the wall can handle the pressure but the stilts can't handle the force and start to shear.

Yeah, that's why I suggested a slab with columns. By the time a 24" steel-reinforced concrete column gives way, the house on top will be gone anyway.

[Mr Bean]

Having lived through three hurricanes and visited the aftermath's of a dozen hurricane sites I can tell you this much. It all depends on Hurricane strengths. At Cat 2 or below you worry much more about Storm surge than you do about Wind speed. But Cat 3 and above Wind damage becomes a major issue as even if your outside the storm surge wind driven rain will still soak the inside of your house be it on stilts or no since Cat 3 and above Hurricane's can and will break out all of your windows.

Visiting Florida when I was young after Andrew (Which hit shore as a Cat 5) every single house which was on stilts was gone. Not as in gone as in totally destroyed but gone as in missing. You could see the houses on the ground who were foundation based and had walls busted out from the storm surge and half a roof there, some even had intact roofing (Depending on construction) but Stilt houses were torn apart. Hurricane Ivan which was a Cat three storm I lived through again in Florida had an strong storm surge but past half a mile from the beach the only damage was wind damage and there were intact buildings who were foundation based while the Silt buildings where typically ripped in half on their side. In fact if you took the time to particle board up your windows and sandbag in thing like garage doors, some of the houses were quite livable which were foundation based. But stilt base housing was again totally destroyed and normally ripped open as if by giant hands grabbing a corner and pulling in opposite directions.

Stilt houses make sense in the context of dodging flooding or a storm surge but you are much more vulnerable to high winds than a foundation house.

Also costs (From a quick search) are roughly 80% higher than a traditional house depending on size/model with it going as low as 25% more expensive to as much as 200% more depending on size/number of floors.

[Ryan Thunder]

Why not just build with something sturdier like, say, concrete? Then restrict windows below a certain height off the ground or above sea level, whichever makes more sense.

[Starglider]

Also costs (From a quick search) are roughly 80% higher than a traditional house depending on size/model with it going as low as 25% more expensive to as much as 200% more depending on size/number of floors.

I believe that if the basis for comparison is typical US houses built out of matchwood and plastic siding, but such housing is a false economy in the first place. Plenty of commercial property is built on concrete or brick columns and withstands both flooding and hurricanes. There is a large hotel built on 2 meter brick-clad concrete columns (on a flood plain) near here.

[Kanastrous]

Why not just build with something sturdier like, say, concrete? Then restrict windows below a certain height off the ground or above sea level, whichever makes more sense.

Because that doesn't help much in terms of water intrusion when there's a flood or a surge. Residential doors aren't going to hold back the force of floodwater, and you'll end up with a water-soaked concrete box wherein the contents are ruined (and water getting into the electrical and air systems isn't going to be good either).

[Vehrec]

If I was building on a floodplain, I'd try to do it on an artificial Hillock. Of course, that would be hugely expensive, and as it subsized my house would break apart. So I live on top of a hill instead.

[Lord of the Abyss]

What about building houses that can float, sort of a houseboat that normally just rests on the ground? Chained to some sort of foundation so they don't drift off when it floods naturally. Would that work, or would the hurricanes just pick them up and smash them (I presume a tornado would smash any normal construction)?

[Broomstick]

And... in addition, much of the Mississippi lies along the New Madrid fault. It doesn't go off very often, but when it does, can be in the 8-9 point range on the earthquake scale. So you'd need stilt homes that can survive both high winds AND a huge earthquake to really disaster-proof the housing.

[Kanastrous]

I'm really thinking in terms of one (well, two) catastrophes at a time. Yeah, the quake's coming but most likely not the primary problem with which people will be dealing for the next few decades.

[Elheru Aran]

Stilt houses are doable in other parts of the world because they're basically huts or shacks. Light, cheap, easily constructed. American codes don't allow for any of that. Is it doable within American codes? Yes, but there are cost and practicality issues.

A floating house is an interesting idea, but I don't believe it's tenable; how would you run the utilities, for example? Are you going to put flexible hoses between every drain (toilets, sinks, bath/showers) and the foundation pipes? Run out extra wire and how much? Etc? Just going with the simple expedient of breaking the utilities if it floats isn't a great idea... once the water comes down, how are you going to make sure the house lines up with the foundation and the utilities reconnect? Might as well just put a houseboat on shore.

[Guardsman Bass]

So it's a cost factor plus the simple fact they can still get housing insurance. If H&R Block refused to insure anyone in the flood planes then much few people would build there.

I think that's a huge part of it: the federal government subsidizes flood insurance. You'd still get people living there even if private flood insurance was only available at gigantic costs, but not as many people.

[Kanastrous]

So maybe it has less to do with engineering and more to do with subsidizing stupid...

[Crossroads Inc.]

So maybe it has less to do with engineering and more to do with subsidizing stupid...

Thats it in a nutshell.
I know land is important, but if there is an area where your at risk of being flooded out more then, well "X" times a year, people should just be told not to build there, OR ban people from being insured in those areas.

[Simon_Jester]

Nah.

Don't ban it, just remove the subsidy. There's no logical reason I shouldn't be allowed to build a house and gamble on needing to replace it every ten years. It's inefficient, but there's no reason to ban it.

But I should have to pay something like appropriate gambling costs commensurate with the flood-resistance of my building.

Here, I think a market solution is good enough.

[Sea Skimmer]

I believe that if the basis for comparison is typical US houses built out of matchwood and plastic siding, but such housing is a false economy in the first place. Plenty of commercial property is built on concrete or brick columns and withstands both flooding and hurricanes. There is a large hotel built on 2 meter brick-clad concrete columns (on a flood plain) near here.

Something reasonable like 2 meters is not sufficient to make a difference vs. a flood of this scale you get if you are close to the Mississippi. You'd need more like 15-20ft because the levees are already that tall, so if they fail the flood is very deep. Just think about how massive that would be, two story tall silts is putting your building on top of a major bridge at that point. It’s not realistic; and generally pointless for the scale that would be required compared to just building better river control structures. The US government already has a program to buy out the very worst affected areas.

I think that's a huge part of it: the federal government subsidizes flood insurance. You'd still get people living there even if private flood insurance was only available at gigantic costs, but not as many people.

Well a lot of years the Federal Flood Insurance Program has no cost to the taxpayer, and 75% of policies are not subsidized in the first place. Liability for specific structures is limited so you can’t get your mansion replaced anyway, only pretty reasonably small homes. It isn’t that much of a subsidy compared to the economic value it protects, 800 billion dollars of insured property, and encourages by allowing tens of thousands of square miles of flood plain to be utilized. Every five or ten years we have to pump some serious money into the program, but we easily make that back indirectly. Below are the numbers the GAO put on it; the huge spike at the end is Hurricane Katrina; hardly a usual event. Looking at what this came from, GAO report, we spend about 1.3 billion on average to subsidies stuff. Not much compared to what is thrown at farmers not to grow stuff.

Nah.

Don't ban it, just remove the subsidy. There's no logical reason I shouldn't be allowed to build a house and gamble on needing to replace it every ten years. It's inefficient, but there's no reason to ban it.

But I should have to pay something like appropriate gambling costs commensurate with the flood-resistance of my building.

Here, I think a market solution is good enough.

The Flood Insurance Program already works something like that. In fact you cannot get flood insurance from the feds in your community at all unless it already agrees to begin taking specific planning steps to reduce flood liability. This has included silts requirements in certain marginally affected areas when it could make a difference. It’s also integral to the buy out and relocation efforts. This has reduced the US vulnerability to flooding by tens of billions of dollars in property value in total vs. how we see unregulated towns develop.

[Kanastrous]

Thanks for that, Sea Skimmer.

[Alferd Packer]

Most houses in the Florida Keys are built on stilts of varying heights. The older ones are only about four to six feet off the ground, while anything built in the last 20 years is about 12-15 feet up. Supposedly, their construction is built to withstand a moderate-to-strong hurricane, and, of course, the storm surge. Of course, since tornadoes are significantly stronger than hurricanes, stilting probably won't help too much. But it would guard against flooding pretty effectively.

[Crossroads Inc.]

Thanks a lot for those charts and numbers Sea Skimmer. I didn't think about it till yesterday but I felt a little ashamed about my post, almost sound liking a "AR! Wasted taxpayer dollars on stupid people!" person when it turns out there is NOT a lot of govt money spent on people in Flood areas.

Any money that is spent is just in FEMA and rescue efforts.

[Zaune]

A floating house is an interesting idea, but I don't believe it's tenable; how would you run the utilities, for example? Are you going to put flexible hoses between every drain (toilets, sinks, bath/showers) and the foundation pipes? Run out extra wire and how much? Etc? Just going with the simple expedient of breaking the utilities if it floats isn't a great idea... once the water comes down, how are you going to make sure the house lines up with the foundation and the utilities reconnect? Might as well just put a houseboat on shore.

Something along these lines is actually used in tidal marshlands in the Netherlands, and from what I recall the solution was to run the anchor-chains and utility connections inside telescoping pillars to keep lateral movement to a minimum.

[Sea Skimmer]

Something along these lines is actually used in tidal marshlands in the Netherlands, and from what I recall the solution was to run the anchor-chains and utility connections inside telescoping pillars to keep lateral movement to a minimum.

Yeah that's the thing; you need pillars to hold the house in place... but a big Mississippi class flood will be washing thousands of trees, cars and other heavy debris downriver. Those will hit the pillars and rip them apart unless you install some really heavy duty stuff. Even if the pillar holds, the foundation will likewise need to be very substantial to avoid being undermined by the current scouring around it. A tidal marsh has a lot less big debris and lots of space to bring in heavy pile drivers. This idea could work in known slack water areas of flood plains, but not just anywhere. I can see doing it in a marsh because it'd be nice and private, but not worth it just to live with a bunch of other poor people in the US mid west.

Houseboat moorings have the same general problem, using a chain instead of a pillar is no guarantee of anything and once one house boat breaks free it will hit others moored with it.

If people want the ultimate solution to flooding.. forget about houses or levees. Concepts exist for boring a series of massive underground tunnels from points along the Mississippi and tributaries to reservoirs that would feed Texas farms, reclaiming the desert, as well as draining to points far out in the deep water of the gulf of Mexico (to allow for the slope). Course dead ‘serious’ concepts also exist for diverting the Yukon river as far south as Mexico.