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In a near future where super-cell storms drop massive amounts of water (300 l/m²) in the blink of an eye, cities - built upon the cities of old - have adapted to these tsunami-like flash floods. But how? How do buildings and cellars look? What happens to subways, cars, and canal systems? What prevents buildings (containing air) from becoming pontoon boats when the floors of valleys turn into temporary lakes? What does the warning system look like? Please use a typical concrete city as a example wherever possible.

Addition: As stated in the comments, there are examples of industrialized nations in Asia (India, China, Bangladesh) that have this problem. However, due to the rainfall significantly increasing, even this industrialized nations are not properly prepared. Brand new cities in the tropical regions get regular flooding and all that keeps them up, is continuous rebuilding and repairing. Something that might not be viable in a more constricting economic environment. So to make the question more precise - how to adapt a existing city, located in the danger zones, to these new occasional floods?

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    $\begingroup$ You mention "calderas". Is that relevant? Is it somehow based around extinct volcanoes? Could you include that information. $\endgroup$ Jul 25, 2023 at 13:05
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    $\begingroup$ Fair warning, we embrace questions looking for a finite list of things but your question is dancing a bit closer to the off-topic infinite list of things. What you should be expecting for answers are generalizations (e.g., you might want to look up arroyos and how they're used intentionally in the U.S. Southwest, such as the Los Angeles storm drain system). But you shouldn't be expecting specific details about everything you've asked about. $\endgroup$
    – JBH
    Jul 25, 2023 at 13:06
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    $\begingroup$ The reason for this is the one-and-only-one question rule (VTC:Needs More Focus). Questions asking for a finite list of things regularly ask just one question that happens to need multiple perspectives to answer. Off-topic infinite lists of things questions regularly violate the one-and-only-one question rule (which you do). Finally, (a) what research have you done in regards to storm mitigation in places like Asia where monsoons drop huge amounts of water? and (b) what does the geography of your world look like? People will usually take advantage of existing natural solutions. $\endgroup$
    – JBH
    Jul 25, 2023 at 13:11
  • $\begingroup$ Caldera was a error, i ment valley. Im not a native speaker. Corrected. $\endgroup$
    – Pica
    Jul 25, 2023 at 13:55
  • $\begingroup$ @JBH: In asia there are classical storm drains, but more often that not, its just accepted as a fact of life that all things are destroyed by the monsun at some point. The destruction and reconstruction is seen as a sort of tax on the living, especially in river deltas. I have a hard time imagine that on a regular scale working with a industrialized society though. Thus the question. China has gigantic problems with flooding in new cities with thousands of deaths. $\endgroup$
    – Pica
    Jul 25, 2023 at 13:58

7 Answers 7

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Put your towns and cities on hills.

Towns grew next to roads and rivers. These went along valleys. This meant towns often grew over good farmland. New housing estates are often put on flood plains, as if the name 'flood plain' didn't contain obvious advice.

On the hills, you get good views and clean air. Not all the roads would go up and down. The hills of San Fransisco ought to be a good example, but I remember seeing a subway completely full of water after freak rains up on the hills because the drains had been blocked with waste paper.

There will still be ports and airports which cannot easily move to the hills. You could look at what they do in Holland. Roads and railways raised on dykes. That would not make the roads safe from flash floods, but it ought to give you more of a chance to get to higher ground on good time.

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    $\begingroup$ Hills, if made of easy swept away material like sand of clay are not really safe. Especially in tsunami like environments, were the water contains abrassive materials like scrap from houses, people, etc. The hills would need to be cased with concrete. $\endgroup$
    – Pica
    Jul 25, 2023 at 13:54
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    $\begingroup$ I don't think there's a moving problem--I think he's envisioning a society that built for it. $\endgroup$ Jul 26, 2023 at 1:16
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    $\begingroup$ @Pica In a world with frequent tsunamis, you won't have any hills made of sand or clay. They wouldn't have long enough to build up before the next one washes them away. $\endgroup$
    – Graham
    Jul 26, 2023 at 7:14
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    $\begingroup$ And even if you did have hills of sand, just don't build on those specific hills! There are no doubt a few other interesting things that will happen (runoff directions taken far more seriously, underpasses avoided like the plague, etc.), but the fundamental answer to not having your city destroyed is building it on higher ground. Note that while many people don't listen, or perhaps their situation provides no better choice, the real-world practical, historical response to tsunami-prone areas has always been "don't build there." $\endgroup$ Jul 26, 2023 at 17:51
  • $\begingroup$ The towns in tyrol are by definitions in the "hills" but the bigger clouds crash into that and come down the mountain, like an avalanch coming from a fountain. So be in the hills, even in the hills. $\endgroup$
    – Pica
    Aug 1, 2023 at 7:12
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Take some design cues from Florida Man!

Florida is famous for many oddities among them are Hurricanes, its flat nature, and a high ground water table. This means that in Florida, almost nobody owns a basement and if they do, they cleverly hide the fact (The Magic Kingdom in Disney World, has a famous tunnel system underneath it for cast members to perform their jobs, with access cleverly hidden in the park's theming. It's not built underground, but rather the landscaping hides from the guests that they are actually on the second floor of the park when walking on a foot path.). Typically, Floridian's don't use their garage for storage of a vehicle but rather to serve the function of a basement. In most places, the ground water table will be hit after a few feet of digging, and on avarage, Florida is about 100 feet above sea level, it's highest point being 300 feet. And if you do not have a basement, you store things outside, in your front yard, in heaps and piles like a good Florida Man. Basements simply do not exist.

While traditionally Florida homes were single story ground level units (a cost saving measure as being low to the ground reduces risk of damage from high winds and is easier to cool as heat rises) but with the advent of AC to beat the heat, second story homes could exist and some Floridians, being the people they are, take the basement/garage to a new extreme. Enter the Stilt House. Which is what it says... It's a house on stilts. Essentially, a flat deck is built that can support the weight of a full single-story home on top of it. The drive way will go under the deck, giving the owner a car port for their vehicle (not a garage but an attached over hang cover.) and in some cases, a "basement" of sorts is formed by building walls under the deck and is used for the interior storage that the garage would fill for a Floridian or a basement for other areas. The stilt house has the added benefit of lifting the primary living areas off the ground, which makes them very popular in swampy and coastal areas of Florida (which is pretty much the entire state) which are prone to frequent flooding by Hurricanes or even heavy rains. That said, it's more for the Storm Surge protection than it is for the Swamp, which is much rarer to see (Mostly because most Floridians live near the coast. Aside from Orlando, there are no major land locked communities, though there is plenty of agricultural development. It's rare to see a stilt home built on swamp land because it's rare to find someone who wants to live there.).

Now, if you are asking what about the car and the stuff if a storm surge comes in, well, the question is a matter of simplicity... typically storm surges that will enter residential areas are powerful enough to prompt an evacuation, in which case the car is not going to be at the house when the storm surge is at it's worst, and the point of the stilt home is to keep the important living area like the beds, clothes, food and dishes out of the rising water. After all, basements flood frequently in areas that can have basements without storm surges. And Hurricanes typically have enough time that a home owner can move it to the upper levels (which they aren't actually going to live in while the storm is raging, so the whole living area becomes storage).

Stilt homes are not just a Floridian thing and can be found in many parts of the world and in many sub-tropical or tropical areas, they give a nice appearance (sort of a tree house look) that fits the region well. One of the most famous media depictions of a Stilt House was the home of the titular characters in Lilo and Stich, and was located in a part of the island that was up hill from the shore in various depictions (in the film, the fire trucks responding to a fire reported by Lilo are seen turning towards the mountain in the center of the island from their present location.).

With this in mind, transit can also be compensated for by relying on elevated tracks rather than tunneled lines. The whole point of using underground tunnels is grade seperation of the tracks which allows the trains to get into urban cores while not interfering with surface traffic. Florida again does not have the option to build tunnels, so they tend to have grade seperation in the form of elevated tracks (Such as the Disney Monorail and TTA People Mover attraction or the Miami Metrorail system.

It should be pointed out that Hurricanes and other similar cyclonic storms are displacing the water, not leaving it permanently in new once dry places. These storms are fueled by warm ocean water and rapidly lose strength over land, so the surge, however devastating, will recede and allow for the repair.

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  • $\begingroup$ Avoid at all costs and if you cant, dont invest to heavily. Adapt to what can be used.. $\endgroup$
    – Pica
    Aug 3, 2023 at 13:23
  • $\begingroup$ @Pica: I beg pardon? $\endgroup$
    – hszmv
    Aug 3, 2023 at 15:14
  • $\begingroup$ Trying to formulate a general problem solution strategy. It currently looks like - do nothing, till catastrophe strikes, then rearrange depending on the situation. Maybe pre-arrange for vital services to not be in the most dangerous zones. $\endgroup$
    – Pica
    Aug 7, 2023 at 11:18
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Let's take a page from the local map. While we never see anything at the level you envision we do on occasion get it coming down awfully fast.

In the areas that take flood engineering seriously houses are set above their streets (while I have not measured I would say our house is at least a foot above our street) and for the most part those streets are above the big streets.

Our street is sloped with the land and never gets any appreciable water level. The streets that follow the contours can get a fair amount of water but they're arranged to drain out to the main streets--they don't collect enough water to threaten the houses.

The main streets following the land are another matter--they become torrents. I have seen them completely impassable due to water. It doesn't last long, the water is soon gone and the road crews come out and clean up the rocks (I've seen it bad enough I would hesitate to take a normal passenger car over them) that get left where the side streets join the main streets.

Woe to the old areas built before they got serious about flood control, though. With 4" of annual rainfall people don't realize the issue and occasionally it kills some homeless people who used stormwater passages for shelter.

As others have said, don't build on the valley floor.

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To the Skyscrapers!

Modern skyscrapers are hundreds of feet high. City planners take note of the typical flood level, add a healthy buffer, and set the lowest residential levels at that height. Add a decade for civil engineering to progress, and new purpose-built residential, commercial, and industrial skyscrapers would be on the rise (ha). And since you don't have to worry about the streets below, these could be less like individual buildings and more like a network of massive arched bridges that have skyscrapers as hubs.

Transportation

Automated flying cars/taxis are already a thing, and would replace cars for those that can afford them. Cable gondolas would replace busses and subways to move large numbers of people between megastructures, then people would navigate between floors with enlarged e

Won't the foundations wash away?

However big you think skyscraper foundations must be, they're bigger, and some connect to bedrock itself. Even if the floodwaters below virtually never subside, modern construction technology is at a point where that's not really a problem. Hydraulic-setting cement has been around since Roman times, and many Roman structures have spent nearly 2,000 years underwater perfectly intact. Current foundations would last long enough to be reinforced and for new construction to take place. And if we can make steel oil rigs that survive literal tsunamis, our engineers are up to the task

But realistically...

This would be a massively expensive endeavor, and all expenses are eventually paid for by taxpayers and customers. People live in cities because the convenience is worth the cost, which would no longer be the case. I expect people would migrate to mountainous areas that are already geographically suited to divert that much rainfall, and those areas would organically develop into new population centers.

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    $\begingroup$ So, if it happens ocassionally, slowly, people will demand upgrades. If it destroys the town completely, it will be rebuilt elsewhere? $\endgroup$
    – Pica
    Jul 26, 2023 at 11:00
  • $\begingroup$ Probably a combination of both. I could see a lot of people moving out of cities if the costs rose too high for them, and a lot staying. It depends on how big a town or city you are talking about, and how often these floods come. $\endgroup$
    – automaton
    Jul 27, 2023 at 14:36
  • $\begingroup$ Looking at that graphic for the depth of foundations, I wonder if I'm the only one who had the opposite reaction. To me they feel surprisingly shallow. $\endgroup$
    – Chuu
    Jul 27, 2023 at 15:13
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Simple; build a city on a plate:

enter image description here

This is a render of Midgar, the central and largest city in the Final Fantasy VII metaseries. While it was done for different reasons, the modern city was constructed in 8 radial sectors around a central pillar, with street level being about 50 meters (165 feet) above true ground level IIRC. In the game, the ground level is also populated, but mainly a mix of slums and disposal yards, becoming ever more polluted by the urban activity above.

Build the structural supports strong enough, and 50m would be well above the storm surge of most hurricanes, as well as most tsunamis including the one that caused the Fukushima nuclear disaster, enabling the elevated city to survive such events simply by letting the water rise and recede beneath it. There have been tidal waves over 50m - the Lituya Bay tsunami was more than ten times taller than that at cresting height - but they are very rare and localized, largely because the event involves an intense geologic event in an otherwise-sheltered area that contains the energy (case in point, the Lituya Bay tsunami was triggered by an earthquake literally dumping half a mountain into the relatively small and almost fully-enclosed bay, giving the water nowhere to go).

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I agree that you should research what coastal cities have been doing to protect against the risk of flooding due to climate change. Here is a link to NYC's coastal flood initiative. Similar to this, here is a link to NYC coastal waterfront plan.

Here is a good article that tells about what Japan did after the big Tsunami in 2011. The Green Forest Wall project is one strategy. Similar to this initiative, more recently here is link to what Japan is doing now to protect against Tsunamis.

Underground dams is another strategy worth looking into that Tokyo employs.

enter image description here

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For tsunamis: a gigantic sea wall

If you really want to get around the "rebuilding" problem after a tsunami for already existing cities in the problem zone, a good way is to build a gigantic wall around the city.

From the outside it should feature star like structures like on old castles facing the ocean side to help breaking the waves and give additional structure.
To the land side the wall can be a bit smaller but should still be present so that the retreating water does not get trapped after the tsunami.

On the inside the wall should be supported by earth and concrete that falls of in 45° to give the wall the most strength you can get against the tsunami. So the wall would need to be a bit outside the city.

Now we handle the problems introduced by the wall

Transportation into and out of the city would ofcourse start to be problematic. For individual people I imagine helicopter taxis that bring people to and from an big parking lot. For goods and commuters we would need a big deep underground railway and maybe a harbour ontop the seawall.

The underground railway should be sealed as good as possible to prevent leakage and also needs a good drainage for water that might pour in from somewhere. It would need to be underground for at least a few dozen kilometers, until we are out of the problem zone. On the other end there would also quickly develop another city as with all trading routes.

The harbour would consist of a lot of cranes to load containers on and off with multi line railway leading from the harbour down to the city.

Also the city should feature a sewage that can be closed with big and tight doors within minutes. This is to ensure that rainwater can be drained while not giving the tsunami a way into the city.

One problem remains

This will be hella expensive but possible. After all even small local rulers could get castles build within a few years using medieval technology. And here we have a quite similar situation just scaled up (and the enemy is "just" water). However this would either need a strongly government dominated society (like medieval times) with huge taxes, or a community driven effort where every big and small company does its part.

However in the western societies this would not be feasible since repairing your buildings every few years is much cheaper in the short run and thats what investors are interested in.
A seawall that would only break even and start to save money dozens of years in the future would even be considered by a investment oriented society. If it becomes to expensive to rebuild the companies would just move out.

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  • $\begingroup$ Not sure you are addressing the correct thing. OP asks for "tsunami-like flash-floods" caused by heavy rainfall. Your proposal would make that a lot worse. $\endgroup$
    – Burki
    Jul 26, 2023 at 12:54
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    $\begingroup$ @Burki but heavy rainfall induced flash floods are nothing like tsunamis. I already have put a comment under the question asking OP to clarify what of these two his cities are facing but I thought my answer would at least help those coming to this question that come here for the title, so I decided on posting it regardless. But true, for heavy rainfall this is not the way. $\endgroup$
    – datacube
    Jul 26, 2023 at 12:58
  • $\begingroup$ Actually small seawalls and aqueducts to redirect the problem might be parts of a solution, were some parts of town are abandoned as a sort of "ghetto"/"park" to redirect torrents of water towards. $\endgroup$
    – Pica
    Jul 26, 2023 at 15:16

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