What's necessary for this stratified, flood-prone city to be plausible?

Question

I have a fantasy city where the respectable areas are literally built on top of the slums - essentially, it's on a series of bridges or pillars, with lots of forgotten crawlspaces and hideyholes for my characters to exploit. I know this is, on its own, viable, if somewhat heightened from reality - Edinburgh had wide bridges that buildings were built up to, for instance, and there's lots of examples of large underground buildings being built and then abandoned.

The city has a big wall around it, rising up about to the level of the respectable areas, and a large river outside that. The wall is intended to protect the city from flood; the climax involves the citizens being trapped inside the city as it burns. (A shift in the river after the city was built on stilts made floodwaters rise much higher, necessitating building the wall to stop the foundations from washing away.) There is a large harbour, with a gate to prevent floodwater from entering. Due to its central location, the city has historically been a good hub for trade; it's actually the only viable industry it has left, the rest having become fronts for endemic corruption or organised crime over the last ten or twenty years. Magic is part of the history of the setting, but it's unknown in the time the story is set and if it was used in construction of the city, it would not be obvious or visible.

Is this plausible as a setting?

In particular:

• Is the construction of the city plausible?
• Does the river shifting explanation for the wall make sense?
• Where would the stone for construction come from?
• Where would food come from, and would there be enough to support an obscenely wealthy upper class and a significant underclass?
• The city doesn't have canals. Is this implausible?
• What would need to be true for this to be a plausible setting?

Answer 1

This is a very interesting question!

Is the construction of the city plausible?

Such a city would probably have started as a new, small village near many natural resources - when this village started to trade, it would have caused a huge influx of people to move towards it, due to its newfound wealth. With all the new people arriving simultaneously there would be no way to construct enough accommodation for all of them. Thus, the existing village would be turned into more of a shanty town - a large slum surrounding the resources and the trading port. Of course, the existing wall of the village would no longer be sufficient, so a larger one was constructed before the next flooding season. As time went on, the city was extended, the wall grew, until eventually the wall was so large that it would be impossible to move it any further. The city, no longer able to expand in that direction, would then naturally extend upwards. The original upper houses and streets were the same as below - but eventually, due to proximity to the sun and the higher quality of the new houses, the rich merchants and nobles moved into the higher tier. This would cause the lower levels to house mainly working-class citizens, who would not be able to afford to make the lower levels more hygienic. Voila! The construction of the houses etc. would be via magic, adhering to your question.

One major issue in this is that the upper layers - consisting of more attractive materials, with larger houses etc. - will weigh significantly more than the slums underneath them. Imagine an iceberg, but with the tip of it underneath the water... it wouldn't support itself.

One solution to this would be that the city planners looked ahead into the future when building the upper levels, and constructed huge pillars throughout the city that everything branched off of. This would undermine (excuse the pun) the whole concept of the city being 'built on a slum', however.

Does the river shifting explanation for the wall make sense?

That is one explanation, but I think the idea I have illustrated for the above question makes more sense. The founders of the small village would have seen the wealth of the land - and built a settlement there, with a wall to keep out the water that rises each flood season. The gain far outweighed the risk.

Where would the stone for construction come from?

Plays into my first two answers; it would be one the resources the village was built near.

Where would food come from, and would there be enough to support an obscenely wealthy upper class and a significant underclass?

There are three solutions here. Due to the floods, agriculture would only make sense for short-term crops like leafy vegetables and cereals. When the flood is gone, however, the land will likely be very high quality and multiple of these crops can be produced. Throughout the rest of the year, food will be gained through imports and cattle farms in the city itself.

The city doesn't have canals. Is this implausible?

This actually makes a lot of sense. With the rise of the river, controlling the flow of the canals would pose a major issue, and tunnels through the wall would make it less sturdy.

What would need to be true for this to be a plausible setting?

All the above!

Overall, this is a pretty cool idea - I've been wanting to create a videogame along the same lines ever since I went to Edinburgh myself. Good luck!

Answer 2

If the river transports a significant amount of lime you don't even need a shift. There are several real-world examples of harbour cities currently being at several kilometers from the coast (Ravenna and Ostia come to mind).

You might accelerate the process with higher amounts of lime.

Start with a "normal" harbour city in the delta of a large, muddy, river.

Limestone would be likely to be available somewhere upriver.

In such a setup the wall would be a natural way to cope with rising countryside while sea recesses and port becomes more and more a protected river harbour.

Surrounding swamps would provide protection from foot armies encouraging citizens to stick to the place.

Answer 3

This is almost prosaic. Lots of cities have flood walls. The failure of the flood walls protecting New Orleans during hurricane Katrina was back in the news after other large hurricanes came up through the Gulf of Mexico.

The gate is also very sensible although not as frequently done - probably from the expense of large mobile elements in an ocean environment.
Here is a storm surge barrier from the Netherlands.

The one thing about this city is that the water table might be high as a result of whatever changed with the river to make the flood wall necessary. The lower older sections of the city might be partly or wholly flooded. That reminds me of the 1992 movie Split Second, which remains in my mind only because of the very cool world of future flooded London. Here is a screenshot from the denouement in the Tube.

https://www.youtube.com/watch?v=gbf-hVZORIE

Answer 4

I think the scenario you have outlined is plausible, if not necessary likely. Here is my suggestion for a different way you could implement it:

Perhaps this city was built in a river valley/canyon along a long but narrow corridor. There would be plenty of reasons to want to built a city here. The river provides food and transportation, it can act as a key control point for river traffic up and down the river and the shape of the valley can provide natural defenses.

In this situation, it makes sense that the rich elites would want to build up the walls of the valley. It's further away from the industrial center of the city along the river, it has better access to light and it's physically higher up. It's not much of a jump to go from here to erecting large bridges over the valley, creating more space for the upper class and making stratification more stark.

Town is probably already going to be prone to flooding in the winter and spring as the river swells and wanes naturally, so erecting dykes along the river would make sense, which could gradually transition to walls as events further up river caused it to gradually become larger and as foundations were needed for the upper levels of the city. If it becomes really bad, they might even build a dam to control the waterflow, though this would require at least 1 canal to facilitate river traffic.

Wrapping things up, this being a river valley suggests that it's not too terribly far from the mountains, from where a quarry could supply the building materials. In addition to fish caught from the river and crops grown on the fertile (if flood-prone) delta-lands at the mouth of the river, the rich are well positioned to feast on imported food.