What can plausibly explain many of my very long and low-tech bridges?

Question

I have a very large developed (relatively rational) fantasy game world which mostly has medieval technology with some Renaissance technology, and some low-powered magic.

In it, when I was much younger, I mapped a great lake with several islands connected by seven very long bridges. The map style is rough and large-scale, so their lengths are not necessarily as long as they appear on the map. On the large scale map, the bridges are drawn as if they were 10 - 25 kilometers long.

I soon realized these were excessively long (longer than modern bridges), but not before my players learned about them.

Rationalizations I invoked to explain / retcon them included:

• They may really only be bridging at most 15 kilometers and usually less, due to the real details at smaller scale.

• Each bridge shown might actually be a series of many small islands not shown and the bridges could be a series of little islands connected by bridges.

• Some or all of them might be wooden floating bridges.

• The lake might be quite shallow where the bridges are.

• Parts of them may be piled rocks in the shallow water that make a path above the surface.

• It may have taken hundreds of years to build them.

• The great lake might be a massive caldera from an ancient eruption (it's sort of the right shape) and there could still be a hot spot below. So maybe these bridges are along ridges and/or archipelagos formed along cracks where magma rose up connecting the central island to the edges along three winding but roughly radial paths.

There is at least a reason for them to have been built: there is a very industrious and clever nation whose capital is on the main central island, and their main source of wealth involves mining in mountains on the islands. The lake provides great defensive and transport benefits. (In hindsight though, they could probably do all right with just ships and ferries.)

The bridges do need to be able to bear the weight of horse-drawn wagons, and the marching of armies.

Some magic could be used, but the less, the better.

Some "ancient artifact" high-tech might be a last-ditch sort of explanation.

So my question is, whether anyone can see a plausible reasonable way to explain how these bridges were built and continue to be maintained, using the least excessive, least magical, least post-Renaissance technology feasible?

If not, how much should I revise the map details to make the bridge paths be chains of islands linked by smaller bridges and/or ferry routes?

Edit: FYI, as requested, below is a very crude map. (I haven't posted the actual map because a major element of the game is about discovering the game world's details.) The numbers by the bridges indicate the rough distances in kilometers that need to be bridged somehow, though possibly by inventing chains of small islands along them.

Edit 2: Changed my mis-use of the word "span" to "bridge" - they just need to cross the distance, often through shallow water, not to engineer impossible single spans through the air.

Answer 1

These aren't long-span bridges, but causeways

The current record for a single span is just under 2km long, and any single vehicular span over oh, 100 to 200m long is going to require modern truss or suspension bridge technology. However, it has been possible to span far longer distances over water for much longer periods than that, using only pre-Roman era techniques.

How? Simple: you mix short spans, achievable using basic masonry arch or timber girder/beam technology, with earthen embankments built out of the lakebed, forming a causeway across the lake. For 15km+ stretches, you would have a regular pattern of embanked sections with interspersed spans to provide for waterflow only, using a masonry arch, or both waterflow and navigation, using a single-leaf timber bascule to allow small-ish sailing vessels as well as rowed pleasure craft through.

Construction would be gradual, dumping and trampling earth to progressively form the embankment from one shore to the other. Ox-drawn or horse-drawn carts could be used for earth-moving, with the beasts of burden also used to trample and compact the loose earth into an embankment. Large riprap-type boulders and local grasses would be used for slope stabilization along the sides of the embankment in order to keep it from eroding into the lake, while the road atop the embankment could be left as bare earth or paved with cobbles, depending on how sophisticated you want this to be.

Answer 2

Have a look at Rama's Bridge here. It is 48k long and runs between the southern tip of Tamil Naidu and one of the northern islands of Sri Lanka

Investigations suggest that a chain of atolls was connected and brought above the water by manual labour. The entire length was above water until the 15th century.

Given the right geomorphology and backstory I see no reason your guys could not do the same.

The transport economics also works.

Barges for bulk low value ores in. Wagons for high value low volume products out. Roads for fast high worth individual transport.

Also tolls. If I were running that empire I'd be charging all the market would bear.

Answer 3

Those bridges are built by connecting together floating rafts or pontoons.

They are easy to build and maintain: just chop trees and craft the trunks in the proper shape. Assemble them together, place some anchors along the path.

A similar technology was used in Japan to build wooden bridges which could withstand typhoons: during a flood the wooden parts would float away and were kept together by ropes, so that the river was flow was not blocked and the resulting damage to the bridge was minor.

Answer 4

A deposit of pumice, a light-weight volcanic rock, near the bridges could help the construction a lot. It's not necessary the volcano exists anymore, the rock could have formed thousands of years ago.

Pumice floats on water for a time, making it easy to move even large rocks into place. It will slowly sink when water seeps into its pores, and it could be weighted down with normal rocks.

Pumice is also quite easy to cut even with simple hand tools, such as chisels. With medieval level technology, it should be easy to cut out large blocks, float them into place and have horse carts bring paving stones to make a road on top.

Answer 5

Consider an ancient civilization.
In the middle Ages, some of the biggest architectures (aqueducts and bridges) were Roman-built, like the Alcantara Bridge. So, in the past history of your world, a big and organized empire left some magnificent relics of its past power, like these bridges. Maybe they were built with the aid of some magic, or just following some very tiny islands, like connecting the dots.

To justify their maintenance, together with the extremely sophisticated and long-forgotten techniques of building, there could be something like an alliance between the most important states to keep them working (something like corvees) or even an independent entity that almost religiously dedicates to their preservation

Answer 6

This lake on which your bridges are built on may have only recently become a lake. What I mean by this is that what if several decades/centuries ago, this lake was only a canyon/crater (and has only fairly recently flooded) and these long bridges, as well as the islands, are the only parts of the canyon which were above sea level?

These long bridges could have been originally made to cross the canyon and were supported by shorter rises in the canyon's surface, however, they are now submerged, and the 'islands', as well as the 7 old bridges, being the only things that were above the lip of the canyon, are the only things that the water didn't manage to reach.

This can also provide a story element to your map, as you can hypothesise what exactly caused this flood.

Hope this helps (or at least makes sense)!

Answer 7

It being a lake makes the problem somewhat easier from my opinion - any structure in the water would erode much slower then it would in the river or the sea, due to the slower currents.

I've looked at the list of longest bridges and dams of the ancient world, and to my surprise, there were actually structures of 2 kilometers in length in both categories. (The longest bridge - https://en.m.wikipedia.org/wiki/Constantine%27s_Bridge_%28Danube%29?wprov=sfla1, the longest dam is on the https://en.m.wikipedia.org/wiki/Lake_of_Homs).

One was built in the second, another in the third century the Romans. So, it means that the construction on this scale is actually possible for a developed pre-industrial civilisation - having the structures only twice as long doesn't stretch the limits of credibility much.

What makes I it a problem for me, is a concentration of several huge architectural projects in one place - it should be terribly important for your people, and they should never stop the maintenance.

It shouldn't necessarily be bridges. I do not see any rivers entering or leaving your lake, but if we assume the inflow on the south, both southern constructions could be dams, not bridges. Maybe all the southern part of the lake is artificial, flooded for agricultural or military reasons. Gravity dams are surprisingly sturdy, and even if the segments fail, they could be built back.

The northern construction could be a bridge - a dam doesn't seem to make sense in that place. And it could be newer then the dams. Shorter segments could be done as Constantine's bridge I've linked above:

...a construction with masonry piers and wooden arch bridge and with wooden superstructure.

While a longer part may be a pontoon. The bridge is a point of failure, and needs a lot of maintenance. I'm also afraid that one it's destroyed (in a war, for example), your people would find it hard to rebuild it back.

Answer 8

• There is a good reason to keep the city on an island in the lake.
  The reason may be irrational but still good. "This is were the kings or government have been for centuries, it would put their legitimacy into question to move elsewhere" or "this is where the main temple has been since the founding of the state, the god would be angry if we move it."
  Consider Saint Petersburg, Russian capital from the early 18th century to 1918, or Brasilia, captial of Brasil since 1960. Moving the government may work, or it may not. In your case, it won't work. The traditionalists win. Could have been a close victory, or a clear one.
• Some important people want bridges, not boats.
  Perhaps winds almost always blow in one direction, making sailing difficult. Rowboats or galleys are too slow, or the rowers are smelly ruffians, or in the rest of the country a proper noble simply does travel on horseback to set him apart from the proles.
• Bridges are feasible.
  The lake is not too deep. The ground is suitable for stone piers. There is no rapid flow of water to cause erosion. No ice in winter, either.

Answer 9

I'm offering this more as storytelling advice than bridge-realism advice, but you can always just leave the origin of these massive bridges a mystery. Tell your players that nobody in the world knows who built the bridges or why. Let them have their theories but never confirm anything either way. Our real world, especially in the time period you mention, was full of mysteries and things that nobody knew, including these in your world will make it feel more realistic than anything you could throw in about bridge construction methods.

Answer 10

The lake is shallow -- almost a swamp. Three real-world examples of such lakes are the Tulare Lake basin, the Everglades, and the state of Iowa during flood years. Tulare Lake is named for the reed-like plants that grew in much of its area. If San Francisco Bay, the Baltic Sea, and the Persian Gulf were freshwater, they might also be similar lakes.

In the eighteenth century, any sensible Englishman could see that it would make sense to build roads across the lake, as requested in the original post. They were even willing to pay taxes to do so. But no sensible Englishman could figure out a way to do it.

Fortunately, England had plenty of rascals. One of them was Blind Jack of Knaresborough, who literally couldn't see why it was impossible. He figured out that the answer was to "float the road" on a base of bundles of either straw or heather and gorse. He would fit right in to a fantasy game world; you could have legends about him. (For that matter, our world has a man who was made a saint for daring to build a bridge.)

Answer 11

There are places in the real world where roots of rubber fig trees have been trained across rivers and woven together to grow sturdy footbridges.

Wikipedia: [Living Root Bridges][1]

I imagine to extend something like this to the island would be a lot easier if support pillars are allowable, but with enough time, and a plant whose roots do well underwater, you could grow those too. Perhaps an ancient culture made the supports by floating large tree trunks to the front of the bridge, and weighting one end of them down. Then your roots could be trained around the tree trunks and into the lake bed. Over time perhaps either roots or fallen trees might become petrified under certain conditions.

Wikipedia: [Petrified wood][1]

Even if not petrified, the roots could plausibly last a long time, giving you a lot of freedom regarding when the culture that built them thrived.

Answer 12

The Lake Pontchartrain Causeway is 38 km long. Interstate and train bridges have also be built in a similar fashion.

An Interstate highway and train tracks running over the same shallow lake.

And another bridge ("only" seven miles) built in the same manner.

Answer 13

To add to the excellent answers already given: your "lake" is marshy and swampy and quite seasonally variable, to the point where barges are not economically viable for year round transport. Your roads and bridges are engineered to remain passible in flood stage, drought stage, and everything in between.

As an alternative, you can hamper boats with something like water hyacinth. https://fishbio.com/field-notes/the-fish-report/green-tunnels-of-invasive-weeds

The key in my mind is not in making such long bridges technologically possible, but in crippling water traffic until road traffic is economically superior.

Answer 14

The ancient empire who lived there believed that water was the source of all life, and water is the after life.

Their despots built huge tombs over the water in order to experience the best after life.

The further from shore, the more impressive the tomb. Over generations, the despots built chains of tombs, connected by bridges. Each pillar the resting place of yet another great leader.

When this bridge of tombs crossed a span of water, its surface was turned into a transportation link. And a new chain of tombs where started elsewhere.

Eventually the island civilization linked to the mainland, and thus started the golden age. It spread and conquered and brought back slaves and riches. More tomb-bridges where built, fanning out from the central island. Some petered out in the water; others reached another island, and continued bridging to the land.

But all things come to an end. This empire fell, its core finally torn out by invading hordes who used the very bridges that built the empire to destroy it.

The bridges fell under neglect. Over time, pieces collapsed. Others where cleared out to make navigational passages.

It was 200 years ago that the new civilization started the rebuilding project. Built on the ruins of old, a huge project finished a span all the way to the mainland. From the wealth that flowed over that bridge, the others where repaired and made more mighty than before.

These bridges are wonders of the world. People travel from the far and wide to see these structures. On the Day of the Dead, huge festivals of colour and flame occur on them, as remnants of the ancient religion that caused them to be built echo through time.

A famous poet is known to have said that they can finally die now that she has done the dance to death on the bridges of the lake people. It sounds better in the original language.

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TL;DR - the original bridges where ridiculous projects of manual labour by an ancient, wealthy, despotic civilization. Thousands upon thousands of dead slaves built the bones; a mixture of causeways and bridges.

The original "bones" took many centuries to built -- maybe even a millennia.

The current bridges are built on these ancient bones, and are wonders of the world. Both the original "bones" and the current bridges are independently awe inspiring.

The "bones" themselves will have many myths about how and why they where constructed. In at least one of them, something like the giant's causeway between Scotland and Ireland may be visible (an ancient lava flow was submerged by the land falling); visible by huge hexagon-shaped rock pillars on the surface, and isthmus on both sides going out into the water. Make that isthmus a bit longer, and you'll have the seeds of the claim that the current bridges are the 3rd generation of them.

First, the giants built the bridges. Then it was struck down.

Then, the ancients built on top of the ruins of the giant bridge.

Today, the modern people built on top of the ancient's bridges.

Answer 15

Simple answer: objects on the map are not to scale. It's rather an artist impression of the world than an navigator's map. Lord of the rings featured such maps. You can hand-wave these details away in this manner and not have to add intricate details to the history of that world that may entangle you in a web of side-effects of the new explanations.

Answer 16

If you make your central island the old capital of a long gone empire, then your island sits at the centre of an important crossroad. The empire is long gone, but it influenced settlement patterns and the roads that radiate out from your island are important trade routes between major towns. This is reason enough to keep such routes open.

Consider the founders of the empire settled the upland region where the island now is for safety. There was fertile land on high ground surrounded by marshes cut with streams for clean water. As the civilisation rose to power they drained the marshes by damming the sources (no rivers on your map but there must be some due to the mountains and perhaps also underground water pushing up from somewhere). This provided more farmland and also created deeper areas of water where fish stocks could be managed and water generated power for mills, industry etc. They'd already been putting down bridges to link the raised areas to create military roads which also pleased the merchants as the road toll was cheaper than taking a barge.

Towards the end of this civilisation money was poured into keeping the roads functioning which further bankrupted them but as the old dams gave way the raised roads were kept going. By this time the ability to repair let along build dams had been lost but something simple like putting down piles and keeping the road above the encroaching water was endlessly copied over the centuries until the present day.

Parts of the old capital suffered plague or somesuch and people abandoned those areas, that's why the roads through them were allowed to be submerged by the lake creating islands barely inhabited and why the coasts of those regions are more heavily forested. It may even be that the last major invasion of the area came from that direction so it was allowed to fall into ruin and form a more difficult path for future invaders from that direction.

Answer 17

Consider Venice... it was built in a (shallow) lagoon by driving wooden piles (tree trunks) into the mud of the lagoon and infilling with 'stuff' that have lasted many centuries. Those were done pre-industrial. Consider the sort of walkways people still build out into the shallow waters of lakes to boat docks... drive wooden piles into the mud of the lake shore and fix plank or log 'lintels' across the tops of them... providing perfectly safe walkways that span many metres across the water.

Make a hybrid construction... run walkways (they could be wide enough to take a horse and cart...) to 'islands' built at convenient places... use the islands to build ramps from the close to water walkways up to a height that allows water craft to pass under them and run a simple lintel between two such islands to make a high-level bridge. Repeat as necessary... to cover whatever distance you want to cover. You can easily get a 5m long tree trunk that can span supported at both ends... so you could have passages 4m wide and several meters high for shipping to go under. Choose the right wood it will last decades if not centuries.