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My city's founders had the short-sightedness to place it on the coast. Now, for mysterious reasons, the sea level is rising. The municipal government has decided that building dikes is "too Dutch" and wants to let water claim the city, turning streets into waterways. To do this, they will water-proof the buildings' first few floors. External elevators will deliver goods from the current water level to retrofitted entrances above the highest water point.

My city is densely urban, a mix of one third high-rises (10+ floors), two thirds low-rises (4-9 floors), and the rare 1-3 floor building.

What are the factors they have to watch out for? Are any of these an issue, if so, how major, and how might they be handled?

  • Erosion from salt water damaging the largely concrete buildings
  • Water saturating the ground and compromising foundations, electrical wires, etc
  • Buildings sinking into the ground because it's wet now
  • Something else?

Assume that the city has as much time as it needs to waterproof their stuff, and wants the waterproofing to be effective for a few hundred years before repairs need to be made. The technology level is modern (early 21st century) but this is not a Hard Science question.

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    $\begingroup$ Take a look at the history of Galveston. They lifted all the buildings. $\endgroup$ – JDługosz Dec 21 '16 at 20:05
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    $\begingroup$ Is this modern, past, or future technology level? $\endgroup$ – JDługosz Dec 21 '16 at 20:06
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    $\begingroup$ You might be interested in omniphobic materials. They can repel all kinds of liquids and semi-liquids. $\endgroup$ – Thom Blair III Dec 21 '16 at 22:58
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    $\begingroup$ Um, actually, your city is sinking. That's what you get for building in a marshy area. $\endgroup$ – Xandar The Zenon Dec 22 '16 at 2:39
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    $\begingroup$ The common city folks build dykes against the municipal leaders wishes and all who oppose the city folk are fired,jailed,recall election, or ignored. Maybe a good old lynch mob to change their minds. $\endgroup$ – cybernard Dec 22 '16 at 15:27
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Well, I think the city founders should maybe respect the Dutch a little bit more, because they don't know what they're getting into.

First and foremost, salt water is incredibly corrosive. There's no good way to protect your buildings from it, and any solution will require constant maintenance over time. A lot of resources and manpower are going to go into this.

Second, once the city is flooded, the ground will settle to a far greater degree than right now. Foundations will begin to sink deeper into the muck. Structural instabilities, and maybe outright collapses are to be expected.

Third, and this really compounds the problems stated above, the waves washing over the buildings, the tides coming in and out, etc. will not only corrode the buildings, they will also exert significant forces against those structures. A few cubic meters of water slamming into a building during a storm is not going to end well for your average homeowner.

You might be thinking: but Andrei, Venice is partially submerged, and it's been doing just fine for hundreds of years!

Well, yes, but Venice was not initially built on dry land. It was intended to exist above the water all along, and they didn't sink the entire first floors of their homes into the much, just very strong support pillars which were chemically treated to resist decay.

Your case is a lot more severe, as a significant part of the structure itself will be exposed to the water. For one thing, it means that a much greater surface area of the building is going to be in the way of all that water rushing in/out during storms, and during tides.

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    $\begingroup$ "..the waves ... will also exert significant forces.." It's not just hydrodynamic forces at certain times in the surface zone, it's also hydrostatic forces all of the time. I live in a flood-prone region where many folks try to "water-proof" their basements, only to discover that their houses are MORE prone to collapse because the immense weight of the water in the flooded earth is not balanced by an similar weight/pressure of water in their basements. Lots of house have been lost this way. $\endgroup$ – RBarryYoung Dec 21 '16 at 22:14
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    $\begingroup$ Typo: "...into the much..." $\endgroup$ – jpmc26 Dec 22 '16 at 0:30
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    $\begingroup$ "A few cubic tonnes ..." - whatever ungodly liquid this is, water it is not. $\endgroup$ – Nij Dec 22 '16 at 2:49
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    $\begingroup$ @Nij A 3x3 meter section of wall at around 6m under water should have about 70 metric tons of pressure against it (assuming all the various conversions and roundings that I did were correct). I calculated at a 25 foot depth, assuming three floors at 10 feet/floor. At 25', water pressure is approx 10.84psi, so approx 1561 pounds per square foot. A 10x10 foot square is approx 3m^2. so 156100 lbs total pressure, so about 78 tons, so about 70 metric tons of hydrostatic pressure when the water is still. Tides would be much more. And most walls are >3m^2. $\endgroup$ – user2338816 Dec 22 '16 at 12:42
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    $\begingroup$ @user2338816: Nij is just pointing out that "cubic tonnes" doesn't really make sense; "tonnes" or "cubic meters", sure, but not "cubic tonnes". (At least, not in this context.) $\endgroup$ – ruakh Dec 22 '16 at 22:25
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This is an incredibly bad idea, for all the reasons other people have mentioned, but I will add one more thing (seen often here in Missouri when we get floods):

Sewage, Water, and other utility lines are designed to be above water. Bad things happen when these things get flooded. Most electrical lines are secured and insulated well enough to withstand some water, but most of your breaker boxes will be in the lower floors of buildings, as will outlets on the first few floors.

If you seal off the first few floors of your buildings so that they are watertight, you will need to be very careful with plumbing as well. The funniest thing I've seen is a building surrounded by sandbags that got flooded out because they didn't seal off a toilet and the flood water level went above the toilet's water level (flooding the building not only with water, but sewer water, requiring a lot of cash spent in renovations).

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This will be expensive.

  • Increase plot lines to intrude into the public right of way, let's say about two to three meters (10'ish).

  • Extend interest-free loans from the government for developers to extend their structure to that lot line. Incentivise the addition of two more floors of real-estate wrapping the base of the buildings. This would require the formation of some kind of urban development council.

  • Ensure there is adequate height to mitigate against storm surges, which should be more likely in your underwater future.

  • The new spaces can be used for residential, commercial, you-name-it. This would help offset the cost of the new construction, afterall people in single family homes are going to rapidly begin taking an interest in new housing!

Encroaching into the public realm (streets and sidewalks) shouldn't be a problem, since you won't be using them anyway. This is a very, very quick sketch (don't judge):

enter image description here

This is where it gets more expensive. There are a lot of precedents for structures under water, from Sea World to research stations. But it isn't cheap to build underwater safely. There's no magic to proofing against salt water, it's commonly done with cement and steel for oil rigs and other structures (steel can be protected by adjacent placement to a dissimilar metal, such as zinc to protect it from corrosion.) But it's not cheap.

A stormwater drainage system is one of the more expensive utility networks. Sewerage is up there, too. Instead of rebuilding the entire network elevated, begin installing pumping stations to elevate what is drained typically by gravity in your system. Begin building structures to access all underground utilities through the new water system.

Finally, if you're expanding every building in the downtown by one-to-ten percent 'usable ground floor area', make sure all of your utilities can handle the new load. That's ten percent more electricity, water, etc. used in downtown.

This is just addressing the buildings, there's a WHOLE lot else you have to consider. Maybe make park space can be made on the top of your extensions, etc.

[EDIT] This doesn't even begin to address your other issues, such as transportation of goods and people, accessibility, property rights, etc. (can you tell I'm an urban planner), but your question was about the buildings specifically, so there ya go. Maybe a good urban planning question out there for you to ask in addition on to this.

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Build Like A Bridge Footing

If you have a modern urban environment and you are not going to use a dyke of some kind, probably your best option is to use what amounts to a "mini-dyke" for each major building you want to save. I say major building because you are probably not going to have the resources to save every building.

Bridges, some docks, and certain other major engineering projects use large masses of cement blown into underwater forms. There are even types of cement that can cure underwater nowadays. A bridge footing might have 10+ feet of "waterproof" cement surrounding the steel pylon that anchors part of the bridge. To really waterproof something, there will be multiple layers of different materials surrounding anything that could be corroded.

So, if we take something like a skyscraper, we might try this: We can sink extra support pylons around the skyscraper (this will be complicated because of all the service connections to things like sewers, water mains, electrical conduits, and even transportation tunnels to subways depending on the city, but since we will have to find and seal ALL of these things anyway, we might as well get to work). This will be useful to keep the building from becoming the leaning tower of Pisa when the ground underneath liquifies over time. In most cities, major buildings like skyscrapers are anchored to bedrock, but we can't count on that and there are plenty that are built on fill.

Next, we would build a containment wall in a similar way to a bridge footing all the way around the skyscraper. This wall would be probably 12+ feet thick, be made of "waterproof" cement, and have multiple layers of impermeable materials that will block water seeping in. There have been examples of walls like this being used to actually create a dry "tunnel" right down to the bottom of a harbor or bay during major construction. To save costs, we can bring the containment wall up to a reasonable height over where the water level is going to be soon. We can keep adding to the wall as the water level rises. This wall can be built as both a water "shell" and as a strongback to support the building itself if necessary, since we can expect things to get wonky once the fill dirt washes away and waves begin to beat on our structure.

If all goes well and we have a strong enough structure, we could go a ways up before we run into major trouble. Of course, the lower levels of the building will become creepy, dark, damp, mold-smelling places that nobody wants to be, and we will be constantly fighting moisture with dehumidifiers, pumps, and even mops (moisture will be inevitable, no matter what).

With any kind of large city, the problem is that there is no way to put that much effort and money into every single building in the city. In addition, smaller buildings would basically just disappear into the man made dry hole we create if the water level rises too much. We could create areas to preserve neighborhoods by making big water walls around groups of smaller buildings, but in that case, we are back to a dyke.

Basically, if you want it to last as long as you say, build it like a bridge footing.

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    $\begingroup$ all concrete cures underwater. Curing is a chemical process, not simply drying out! (As a matter of fact, curing underwater produces stronger concrete because it has longer to cure without becoming dehydrated). $\endgroup$ – Wayne Werner Dec 21 '16 at 21:26
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    $\begingroup$ @WayneWerner That's hydraulic cement (generally Portland cement). Non-hydraulic cement does not set or dry in the presence of water. Though it is true that most (maybe all?) cement used in construction today is some type of Portland cement or other hydraulic cement. $\endgroup$ – RBarryYoung Dec 21 '16 at 22:23
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Fiberglass and resin.

If the buildings are already on the coast they likely have foundations that are salt water resistant. But you may as well make sure.

Excavate the foundations of any suspect buildings and cast a couple feet of fiberglass resin around them with some fiberglass pilings for reinforcement. Now do the same thing around the exteriors of the buildings past the first few floors. Fill the lower floors with concrete so the buildings don't try and float away.

Also coat any exposed concrete which contains steel reinforcement. The corrosion of rebar is the first thing to destroy concrete structures in a process called salt spalling.

You could also optionally use something like acrylic so inspection is easier in the future, this would also allow you to leave some small passages free of concrete in the lower floors so that existing windows can be used for underwater viewing ports.

Finally, build a breakwater. You don't want to actually be in the ocean, even if you're ok with being underwater.

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    $\begingroup$ Great point on the breakwater. This makes the canals safe to use and prevents any damage inflicted by waves. $\endgroup$ – Lu22 Dec 22 '16 at 5:25
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You are thinking of a city in terms of flat land. Property values could be significantly affected by elevation within the city, and a hill of dry ground would be immensely more valuable.

To plan for the changes, the city planners should:

  • Ignore/demolish all buildings less than 6 stories tall, limiting the number of buildings IN THE FLOOD ZONES to less than 50%
  • Retrofitted structures to use an interconnected pylon structure with a deep foundation
  • Ignore waterproofing the bottom few floors and actually work to separate them from the building (relying on the pylons to provide the structural support)
  • Design sky bridges between buildings around the 8th-10th floor (depending on elevation)
  • All utilities are moved into the flooring of the sky bridge
  • One level above the sky-bridge is the only official form of public transit... elevated electric trolley tracks
  • Boats are mostly used for commercial transport (manufactured goods)

As far as society is concerned:

  • Society status would be based on how far you live/work from the water
  • Even though the sky-bridge level is still above water by 4 to 6 stories, it is now considered the commerce level and redubbed as Level 0 (with the level immediately below redubbed as Level -1, etc)
  • The rich would live on dry ground
  • Middle class would live in the upper levels of the high-rise buildings
  • Lower class would live several levels below the sky-bridge
  • The poorest would be required to live in floating communities scattered among the pylons (cold, dark and literally out-of-sight)
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    $\begingroup$ Aw man, I was hoping to keep the subway running. $\endgroup$ – SPavel Dec 22 '16 at 3:31
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As Andrei says, go Dutch.

Your existing buildings cannot be cost effectively retrofitted to survive the forces involved. You can build wave breaks around the town to remove the hydrodynamic forces, but the hydrostatic forces are still higher than anything the town is built for.

Either you build a dyke around the town or you effectively build a dyke around the bottom few floors of every building in the town. A solid concrete barrier a few feet thick, probably dug down at least one story and up as far as you expect the water to rise. You now raise the entrance to each building up to either the second or third floor. This is of course higher than some buildings, but that's what you've signed up for.

You should build elevated walkways at this level. If you want to build elevated roads/mass transit as well that's up to you but anything at ground level is effectively lost.

Sections of metro system that were built below the water table may survive if well enough pumped as the tunnel linings could possibly be good enough. That's a lot of possibly, maybe and perhaps for one sentence, but the Victorians tended to build well above required specs. You will of course need to spend a fortune on raising every entrance tunnel and building to the new walkway level.

New buildings can either be built to the new waterproof specs or built to float. Floating buildings will offset any future water level rises if given enough flex on services. In terms of cost for new buildings, most companies will probably opt to build somewhere else. Your city will probably die. Venice effectively consists only of tourists and tourist attractions now, almost nobody lives there any more.

Or you could build a dyke. I'd recommend a dyke.

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You can solve the hydrostatic pressure problem by not trying to keep the water out, per se. That is, let your buildings get flooded, too. Now, water-proofing reduces to isolating things which really can't get wet (electrical, data, etc.). Just go to an aquarium and look how thick the glass is to convince yourself that building windows are a terrible idea underwater. This will basically turn the underwater floors of your buildings into footings/pylons, and not particularly good ones, at that. It's an engineering nightmare, so the best idea is to just give up on the submerged floors altogether and fill them in with concrete, or at least reinforce them liberally with concrete pillars.

If you choose to leave the existing utility runs in place, they will be insanely expensive to maintain. You would need to seal up every buried electrical/data cable, every manhole used to access them, and install expensive hatch systems so that they could be accessed later by maintenance workers (think: submarine airlock). It's probably cheaper to just start over and rebuild all that infrastructure from scratch above water.

The biggest tragedy will be the high-rises. A lot of tall buildings (and shopping malls) have underground garages. You will be losing 1-8 floors of parking space, because none of the underground garages I have seen have a car exit 3 floors above ground (usually the garage is either underground or aboveground, rarely if ever both). If most of your citizens switch to boats for transport, that might be ok. But consider that previously, they were used to commuting in their own cars, which is why you needed many levels of garage to house all of them. I've never seen a multi-level boat garage (but yes, they do have long-term elevated boat storage), so you will need a good public transport system, and your people will have to accept using it.

You will also need to clear out all your traffic lights, street lights, urban trees, etc., which will become hazards for your commuter boats, and/or possibly install new ones to manage boat traffic. The same can be said of pedestrian overpasses, some highways/on-ramps/off-ramps, etc. Your citizens will lose most of their parks, waterfront beaches, etc. But on the other hand, recreational divers will have a heyday. The whole idea feels rather spooky to me.

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