City planning underwater

Question

In my world, there is a race of fish people and merfolk. For a brief description: They are a mix of the Creature from the Black Lagoon and Luca. For pictures and extra description: What conditions could fish people evolve to be in their current humanoid form?

They have their own underwater kingdoms and civilizations, much like the land-dwellers. However, because of swimming a lot, they do not need as much roads/bridges. For the bigger cities, how would cities and streets be planned for humanoid fish people?

Note: Ironically, the fish people do not settle in the deep ocean. This is due to trading and diplomatic relationships with the land-dwellers, resulting in most cities being built around thousands of feet underwater, lowest major city being at around 1,000 meters. Other considerations include:

• Animals such as whales being used to carry around cargo
• Guiding schools of fish
• Their own aquariums (though their more like zoos to them)
• Eventually as technology progresses, they eventually use submarines like trains or cars

Answer 1

Interesting question - my immediate thought is Underwater Currents. These would form the equivalent of Highways in a modern city. Sure, everyone swims, but to move anything in bulk would absolutely use the Underwater Currents.

Your major city infrastructure would be built around these - things like your mining/aquaculture/primary industries would be arranged around these currents to move cargo/items.

In addition, with the building, these underwater currents would have structures built up around them to guide/smooth/define the natural flow to make it more laminar and predictable.

You would also see a different kind of Verticality to buildings - Tall buildings like Skyscrapers may or may not be possible underwater (the hydraulic pressure on a tall, flat-sided building makes me think they wouldn't be stable) - but because of the buoyancy in water, some of the issues with building 'up' on land is eliminated. So the verticality of building would be drastically different, lots more rounded/blended structures, but could also use tethered buildings (like an Oil Rig, but just below the surface).

The other consideration is Light - Would they need light? would it be a luxury? e.g. a house near the surface is a privilege?

Answer 2

This is a very interesting scenario, there are so many things to consider:

Infrastructure There is hardly any need for streets, bridges etc. Everything and everyone is able to "fly" (swimming but in 3D) around as they wish. @TheDemonLord mentioned ocean currents but this does not seem very efficient since the would have to be created artificially to be really useful and then it is just easier to propel humans and submarines directly instead.

Structures The need for structures is also much lower. There is no weather to protect from and the temperature is always the same (4°C) which is the preferential temperature for your humans anyway since they evolved in it. So the only reason for structurs is maybe the need for privacy and protection from noise. Basically all public events could just take place outside. With the right amount of air containers, structures are effectively weightless so they are a lot less massive than above-water structures since they do not have to support their own weight. They would probably look more like zeppelins anchored to the ground (or not anchored, it is not that difficult to move them around). The dimensions of the bouyancy creating upper part and the lower living part would be inverted (big living part, small bouyancy part). Pressure is not a problem, since the inside pressure is the same as outside.

Traffic Everything will take place in 3D including traffic. The city will be 3-dimensional, going vertically is basically the same as going horizontally, so traffic (swimming humans, later submarines) would probably just move all over the place in the beginning and will have to be restriced to lanes later when speed and density increases. This might look like this:

(Except for the buildings)

Moving around heavy objects will be much easier since everything can be weightless with the right amount of bouyancy created by air tanks. The term submarine for your vehicles might be misleading, they do not need to be massive at all. A lot of the personal transport would probably take place on scooter-like vehicles since there is no weather and hence nothing to protect from except the driving "wind".

Sewage systemy might be challenging. I assume a pressure based system like it is used in space could work. If your looking for more inspiration for details, solutions for living in space might be helpful because a lot of challenges there are similar to your cities.

I cannot stress enough how challenging the 3D part is. All cities we know are essentially 2D regarding most of the planning. Your cities can be entirely 3-dimensional which creates a lot of interesting situations for your planning and stories which may be hard to convey to your readers but very great if it works.