How could a city build a circular canal?

Question

Within my setting, magic exists as a system of writing that has to be encased within a circle. Usually, these circles aren't any larger than the writer's hand. However, the primary villain of the novel is attempting to create a magic circle large enough to affect a continent, and plans to do so by visiting a specific city that has a circular canal. By using this massive circle that already exists, the villain only needs to complete the magical written instructions at the circle's center to achieve his aims. This large, 'natural' circle would alleviate the villain's need to construct a massive circle himself while also, in my opinion, also make for a cool setting for the city wherein the book takes place. But how would a canal like that be built? Especially considering circles are so hard to construct.

My best idea so far has been a modified oxbow river. The original settlers found the oxbow river and built the city around it. As time went on, they straightened and redirected the river, separating the oxbow itself from the main water supply with a gate. The canal was improved as the city was, changed so it was more circular. It is technically still connected to the river, and the city controls the water level so it can be used for travel and transport throughout the city.

It's worth noting that the city itself is not circular. That question has been answered already. This city has grown naturally around the oxbow canal and the straightened river as needed, expanding out in all directions. In terms of technology, this setting is low-fantasy, being late-medieval (almost renaissance-era) with what tools they'd have. In terms of setting, this circular canal is simply supposed to be an efficient way for traveling and/or touring the city, as well as be a notable tourist destination.

And, of course, there's no way a perfect circle would ever form in nature. This being a fantasy world, I simply hope that people would be willing to suspend their disbelief a little and trust that the city somehow turned a natural phenomenon into a perfect circle.

How could a circular canal be built? Is there another, better method for this?

Clarification: The canal would need to be perfectly circular. However, there's no way to have a perfectly circular river. All my attempts to think of a way to build a self-filling river required magic, and I want to make something that feels natural. That's why I considered a canal.

Also, circles for magic of this size have never been attempted. This is because of two reasons. First, this circle is supposed to be 'natural' insomuch that the main characters don't consider the possibility that the villain could use it, until the villain does. It's supposed to hide in plain sight, as it were, rather than being a massive ring of stone that's obvious from the get-go. Second, the circle must be drawn in blood. The villain, notably, has spent significant time gathering enough blood to dye the entire canal red (which counts for the magic, since the blood forms a circle. Thus why no one but this character has attempted this level of magic. Yes, this is a ridiculous amount of blood. But going into that would deviate from the question).

Lastly, its the circular shape that matters, not the actual shape of the canal. The canal can be deep or shallow, what matters is that the magical instructions of the circle are surrounded by a perfect, geometric circle. Picture the stereotypical summoning circle from systems like D&D. It's perfect insomuch as is possible by the human hand, where a certain benchmark qualifies it (i.e. perfectly circular as distinguishable by the human eye, but not perfectly even on all sides).

Answer 1

One cool way

First start with a lake that was formed from an asteroid impact crater. Then, at the center of the lake, a volcanic island forms due to a consistent lava flow. Both of these forms will naturally form in circular forms just due to physics.

The island grows so large as to nearly fill the crater lake, then the volcano goes dormant. Millions of years later, the natural formation is discovered by a civilization and the defensible nature of the place and its proximity to the waterway makes it ideal to become a city.

Later, the inhabitants develop a lore or religion around the perfectly circular nature of their "River Island Home" and efforts are made to construct circular levees to maintain the circular nature of the land on both sides of the river, as well as add to the amount of dry land they have. This leads the river/lake to be as close to a perfect annulus as they can manage.

Answer 2

Your Villain Was Not the First...

So it wasn't actually your villain who first decided that a city-sized magic circle would be an interesting thing to try. One of the early city fathers, a wealthy man who secretly studied magic in depth, had a plan (now long forgotten) and spent a fortune bankrolling your perfectly circular canal.

That other man died before getting to the whole "filling it with blood" part - or perhaps he had a change of heart at the last minute and spared the city from the carnage required. In any case, it is not at all an accident that the city just happens to have the perfect set-up for for the current villain's plans; it was prepared that way on purpose long ago.

...And who knows what other instructions may already be hidden at the center of the circle.

Answer 3

The Richat Structure

Also known as the "Eye of Rah", the Richat Structure is a unique geological formation found in the Sahara Desert. It is a 40 km wide circular formation with concentric rings, that is believed to have formed from a combination of volcanic activity and converging subsurface springs. This is the location that the Ancient Romans marked on their maps has having once been the legendary city of Atlantis as described by the Greeks Plato and Critias. Based on geological surveying and these historical accounts, it is plausible that a bit over 10,000 years ago, this place could have existed as the ancient Greeks described it with natural circular water ways filling the low sections and the high sections being dry land occupied by a large city.

Answer 4

"Give Me Two Sticks and a Bit of String and I Shall Compass the World Around"

So said Polygynes of Pycleas. Literally, making a (reasonably) perfect circle is far easier than making even a (reasonably) straight line. You just jam one stick in the ground, loop one end of a string around it, loop the other end of the string around the other stick, tighten it up and drag it around on the ground. Hey presto! Instant circle! (Well, actually, with some chalk dust and the same apparatus, you can make a (reasonably) straight line too!)

This is essentially what dividers and the standard drawing compass do.

All your city's enginothurges need to do is make a large enough compass. They can do this very easily by using standard measuring devices like staves and chains, such as surveyors use.

The enginothurges will simply determine the diameter of the canal and use the staves and chains to mark out the perimeter. Using some geometry they will figure out the curvature of the diameter and create standard forms that the digging crews and stone masons will use to ensure that the diameter of the canal is a perfect circle. The perimeter markings will give the labourers and craftsmen a rough idea of where they're going, the forms, essentially largeish pieces of wood carefully cut and sanded to the appropriate curve, will ensure that the work is finished neatly.

Since they have magic, there's absolutely no reason why they can't just a thaumological chain. This is a specially ensorcelled chain of absolute unthickness. It's so thin it can pass between two quarks without a second thought, yet quite visible to the enginothurges and strong enough to hold its length whilst doing the job. They can go about measuring out the diameter of the circular canal, all without worrying about intervening structures or people. The chain just slices through them all!

Answer 5

Super-Volcano Caldera with Island.

Now, an examples on Earth but a smaller version would be Crater Lake in Oregon, USA (after the eruption of Mount Mazama about 7,700 years ago):

^{Brinley Clark, EyeEm / Getty Images. Treehugger.com, 2024, fair usage.}

Now, of course, the island need not be circular, just as long as the crater is that delineates the lake.

This leads to the possibility of an isthmus that was there when the place was initially settled - which could then sink or get washed away - or the lake can rise obliterating it (the isthmus can well have been placed there by magic to make the island attractive as a prospect, a narrow entrance being easy to defend).

The town would have been well enough established with fishing boats etc. when the isthmus disappeared and so didn't bother moving.

Answer 6

Urban Planners.

Basically someone one day decided that he/she had the wealth and the perfect way to organise a city. That is with circles.

They started off small with some circular paths and buildings in one of the upmarket/newer areas of town. Made some influential people proud that thier city had this circular thing that let them get around their neighbour really quickly.

And then those influential people had an idea. This river is already circularish, If we put it fully in a circle we could solve the grain loading and unloading, make it easier to get from my rich neighbourhood to the opera house, and even better someone thought it would be a great way of getting water to the rich side of town where we could put some sport fish in it.

The poor people bought into the idea because 1 a paycheck, and 2 it let them get access to water.

The merchants loved it because they could build docks and warehouses along the circle more cheaply and do more commerce.

The ruler loved it because more taxes.

The military loved it because defenses.

The people (as in the country/state) loved it because we have this unique thing and justifies why we are better than you while still eating the same basic corn mush.

Answer 7

Prestige

There's two things I would like to take note of. The first is the absolute ingenious of humans, even very early in history. The second is the absolute folly if humans, creating huge projects that are by far not the most efficient in their function, or efficient at all.

For the ingenius projects I'll remark on the Roman aqueducts. Romans have a huge amount of impressive constructions besides them though. They predate your times by a good margin. In these times they were already able to build aqueducts at impressively low inclines. Some boast parts that have an incline of 0,1% or less. Another less steep was still impressive. Pont du Gard lowers 34cm (13") per kilometer. They were already incredibly advanced in exact measurements and getting it all to line up.

The way to build such a canal is to get some skilled people, measure, measure, measure until it's absolutely perfect. Build it and correct during as you keep measuring.

The second is the absolute folly of men, of which we can find many modern examples. If we stick with water, I'll remark on the United Arab Emirates for their palm islands. They aren't sustainable, require constant maintenance to not flood over time, they destroyed the ecosystem, and in general people mention it is horribly impractical to live there. China has build islands in the sea to claim territory. These were insanely expensive and many are already threatening to sink back into the ocean. In history cities have been burned for palaces, 'unbeatable' galleons have been created that sunk the moment they left the dry dock because of the weight of the cannons. We've made the tallest structures in the world that do the job a building the tenth it's size could fulfill. The list goes on.

It is then not a question if a perfect circular canal is possible. It is more a question if we don't already have some, or why some wealthy royal or influential artist didn't push for it in real life. Some fued of engineering can easily spark such a folly. Who has the best skills and wealth to create such a thing? In your story, you might even find two or more such cities close together as a result. In the real world I couldn't find examples (yet), though many cities have shown pretty close to circular designs in city walls and canals.

Answer 8

The question doesn't specify the size of the city. But for centuries and millennia cities, whether tiny, medium, or large, have had irregular plans or else were designed to have various geometric layouts.

And most cities up to about 200 years ago or something had defensive surrounding walls. And usually there was a ditch on the outside of the city walls, sometimes a dry ditch and sometimes a moat full of water from rivers and other sources.

And most cities built to planned designs had rectangular shapes for their walls and moats. But some had circular shapes.

[added 12-12-2024. examples of round or oval cities, some having surrounding ditches or moats, include the description of Ecbatana by Herodotus, one of the cities of Ctesiphon, Hatra, the Round City at Baghdad, Gor, Karlsruhe, and Annapolis. And of course Plato's fictional description of the capital of Atlantis.

https://en.wikipedia.org/wiki/List_of_circular_cities end of addition]

Hadrian's Villa near Tivoli Italy has many structures and gardens. One is called the Maritime Theatre.

One structure in the villa is the so-called "Maritime Theatre". It consists of a round portico with a barrel vault supported by pillars. Inside the portico was a ring-shaped pool with a central island. The large circular enclosure 40 metres (130 ft) in diameter has an entrance to the north. Inside the outer wall and surrounding the moat are a ring of unfluted Ionic columns. The Maritime Theater includes a lounge, a library, heated baths, three suites with heated floors, washbasin, an art gallery, and a large fountain.2 During the ancient times, the island was connected to the portico by two wooden drawbridges. On the island sits a small domus, complete with an atrium, a library, a triclinium, and small baths. The area was probably used by the emperor as a retreat from the busy life at the court.[citation needed]

https://en.wikipedia.org/wiki/Hadrian%27s_Villa#Structure_and_architecture

I guess that the outer diameter of the ring shaped pool would be less than 100 feet in diameter and the inner diameter less. From the pictures I have seen the ring shaped pool or canal seems to be quite precisely shaped.

You don't need the structures except for the ring shaped pool. And I guess that you would want to make it larger, maybe 10 times the diameter (c. 800 feet?) to 100 times the diameter (c. 8,000 feet?). Since the city is supposed to spread out beyond the circular canal the circular canal doesn't have to be large enough to contain the largest possible medieval or ancient city.

A cothon is an artificial inner harbor in antiquity.

The cothon at Carthage was divided into a rectangular merchant harbour followed by an inner protected harbour reserved for military use only. This inner harbour was circular and surrounded by an outer ring of structures divided into a series of docking bays for ship maintenance, along with an island structure at its centre that also housed navy ships. Each docking bay featured a raised slipway. Above the raised docking bays was a second level consisting of warehouses where oars and rigging were kept along with supplies such as wood and canvas.

On the island structure, there existed a raised 'cabin' where the admiral in command could observe the whole harbour along with the surrounding sea. Altogether the inner docking complex could house up to 220 ships. The entire harbour was protected by an outer wall and the main entrance could be closed off with iron chains.4 Most records of Carthage were destroyed when the city was razed by the victorious Romans in the Third Punic War.

https://en.wikipedia.org/wiki/Cothon#The_harbours_of_Carthage

I think that sheds wide enough to hold a war galley would have to be separated by at least 20 to 40 feet, so the total circumference for 220 galleys would be about 4,400 to 8,800 feet. Dividing that by about half since some ships were docked on the island, and dividing that by pi, the diameter should have been about 700 to 1,400 feet. I don't know the size as revealed by archaeology.

Emperor Claudius constructed Portus, an artificial harbor and port city for Rome.

In AD 103, Trajan constructed another harbour farther inland — a hexagonal basin enclosing an area of 39 hectares (97 acres).

https://en.wikipedia.org/wiki/Portus#Trajanic_phase

Measuring on the map the hexagonal basin seems to be about 2,000 feet in diameter. And the Roman could have dug a circular basin about 2,000 feet in diameter as easily as a hexagonal one. And ring shaped basin with a center island would have been even easier to excavate.

[Added 12-12-2024]

People have been digging earthworks for many millennia for practical, religious, or prestige, purposes.

In prehistoric Europe there were many rings of standing stones. And many were parts of complexes with circular ditches which would sometimes fill with water after rain.

Circleville, Ohio, is mostly known for its Hitler family.

Circleville is named after its original layout created in 1810, which was based upon the circular Hopewell tradition earthwork within which the city was built. This earthwork measured 1,100 ft (340 m) in diameter, and was constructed in the early centuries of the Common Era. The county courthouse was built in the center of the innermost circle.

In the late 1830s, for various reasons, residents requested authorisation from the state legislature to change Circleville's layout to a standard grid format. This was accomplished by the mid-1850s. All traces of the Hopewell earthwork were destroyed, although hundreds of other monuments of its kind still remain in the Ohio Valley.

https://en.wikipedia.org/wiki/Circleville,_Ohio

The Newark earthworks in Ohio contain the Great circle and the Octagon.

The 1,200-foot (370 m)-wide Newark Earthworks Great Circle (located in Heath, OH) is one of the largest circular earthworks in the Americas, at least in construction effort. A 5-foot (1.5 m) deep moat is encompassed by walls that are 8 feet (2.4 m) high; at the entrance, the dimensions are even more grand.6

The Octagon Earthworks consists of an Observatory Mound (connected at the southwestern edge of Observatory Circle), Observatory Circle (20 acres), and the connected Octagon (50 acres). The Octagon has eight 550-foot (170 m)-long walls, from 5 feet (1.5 m) to 6 feet (1.8 m) high. The Octagon is joined by parallel walls to Observatory Circle .6

https://en.wikipedia.org/wiki/Newark_Earthworks

Obviously Hopewell culture societies had the manpower necessary to dig circular canals with diameters over 1,000 feet and the skill to create geometric earthworks including circular ones.

One form of earthwork is a "dyke", consisting of a ditch dug out of the ground and a mound of dirt from the ditch made into a wall. A defensive dike would have the ditch on the outside of the wall so enemies would have to climb down into the ditch and then on the inner side climb up the side of the ditch and then the side of the wall and often find a wooden palisade on top of the earth wall.

Obviously the ditch of a dyke would have at least a little water after it rained, and in some cases would always have water at the bottom and thus be a type of canal.

Many dykes were built as more or less straight lines on the border between two realms to serve as practical defenses and/or as obvious, impossible to miss, symbolic markers of the border.

Many thousands of such linear dykes were built in Europe over thousands of years from prehistory to the Middle Ages. And some of them were very long.

The Danevirke or Danework5 (modern Danish spelling: Dannevirke; in Old Norse: Danavirki, in German: Danewerk, literally meaning earthwork of the Danes6) is a system of Danish fortifications in Schleswig-Holstein, Germany. This historically important linear defensive earthwork across the neck of the Cimbrian peninsula was initiated by the Danes in the Nordic Iron Age about AD 650. It was later expanded multiple times during Denmark's Viking Age and High Middle Ages. The Danevirke was last used for military purposes in 1864 during the Second War of Schleswig.

The Danevirke consists of several walls, trenches and the Schlei Barrier. The walls stretch for 30 km, from the former Viking trade centre of Hedeby near Schleswig on the Baltic Sea coast in the east to the extensive marshlands in the west of the peninsula. One of the walls (named Østervolden), between the Schlei and Eckernförde inlets, defended the Schwansen peninsula.

https://en.wikipedia.org/wiki/Danevirke

The Great Fence of Thrace was a Medieval Bulgarian earthwork on the border with the Roman Empire, about 142 kilometers or 88 miles long.

https://bg.wikipedia.org/wiki/%D0%95%D1%80%D0%BA%D0%B5%D1%81%D0%B8%D1%8F

Hadrian's Wall in northern England is 73 miles or 117 kilometers long, and built of stone.

Just south of the wall there is a ten-foot (three-metre) deep, ditch-like construction with two parallel mounds running north and south of it, known as the Vallum.[26] The Vallum and the wall run more or less in parallel for almost the entire length of the wall, except between the forts of Newcastle and Wallsend at the east end, where the Vallum may have been considered superfluous as a barrier on account of the close proximity of the River Tyne.

https://en.wikipedia.org/wiki/Hadrian%27s_Wall

So the Vallum is a double dyke almost 73 miles or 117 kilometers long.

Offa's Dyke (Welsh: Clawdd Offa) is a large linear earthwork that roughly follows the border between England and Wales. The structure is named after Offa, the Anglo-Saxon king of Mercia from AD 757 until 796, who is traditionally believed to have ordered its construction. Although its precise original purpose is debated, it delineated the border between Anglian Mercia and the Welsh kingdom of Powys.5

https://en.wikipedia.org/wiki/Offa%27s_Dyke

Offa's Dyke Traditionally stretched for about 150 miles or 240 kilometers, but some archaeologists think that it only stretched 64 miles or 103 kilometers, and that other dykes north and south of it might or might not have been connected to Offa's Dyke.

So if a linear Earthwork including ditches could be that long, a fort could be surrounded by a dyke with a ditch with a long long total length. And in face many forts and towns were surrounded by dykes in ancient and medieval times.

Sometimes they had irregular plans based on the landscape, and sometimes they were roughly rectangular, and sometimes circular.

So I guess that some circular dykes were built around some prehistoric, ancient, and medieval, sites. And sometimes there would be some water in the bottom of the ditches, especially after it rained.

Ringforts or ring forts are small circular fortified settlements built during the Bronze Age, Iron Age and early Middle Ages up to about the year 1000 AD. They are found in Northern Europe, especially in Ireland. There are also many in South Wales and in Cornwall, where they are called rounds.3 Ringforts come in many sizes and may be made of stone or earth. Earthen ringforts would have been marked by a circular rampart (a bank and ditch), often with a stakewall. Both stone and earthen ringforts would generally have had at least one building inside.

https://en.wikipedia.org/wiki/Ringfort

And obviously the circular ditch of a earthen ringfort might usually be dry, but must sometimes have some water in it after it rained, and thus could be called a circular canal.

The Pannonian Avars (/ˈævɑːrz/ AV-arz) were an alliance of several groups of Eurasian nomads of various origins.2 The peoples were also known as the Obri in chronicles of Rus, the Abaroi or Varchonitai13 (Greek: Βαρχονῖται, romanized: Varchonitai), or Pseudo-Avars14 in Byzantine sources, and the Apar (Old Turkic: 𐰯𐰺) to the Göktürks.14 They established the Avar Khaganate, which spanned the Pannonian Basin and considerable areas of Central and Eastern Europe from the late 6th to the early 9th century.14

https://en.wikipedia.org/wiki/Pannonian_Avars

In what is now Hungary, the Avars built one or more circular ring forts, which have become legendary. In one medieval story their entire homeland was enclosed by a series of concentric circular walls, the outermost ones being hundreds of kilometer or miles in diameter. The Avar Ring, or Ring of the Avars, was their capital, filled with their loot, until captured by Charlemagne's armies. When I first heard of a book called The Lord of the Rings, I imagined that lord ruled a bunch of ringforts like the Avar Ring.

I don't know what the diameter of the real Avar Ring was, but if the Avars could construct a wall with a diameter of at least one kilometer or mile, they could have dug a circular ditch with a diameter of at least one kilometer or mile, and possibly with less effort than building a wall of that size.

Tens of thousands of castles were built in medieval Europe. And the early castles were built of earth and wood.

A motte-and-bailey castle is a European fortification with a wooden or stone keep situated on a raised area of ground called a motte, accompanied by a walled courtyard, or bailey, surrounded by a protective ditch and palisade.

https://en.wikipedia.org/wiki/Motte-and-bailey_castle

The bailey and its ditch could be irregular or have a geometrical shape, including a circular one, while the mound of the Motte was usually oval or circular, as was the ditch at the bottom of the mound.

So I guess that a country which had a lot of rather small circular ditches around mottes that at least sometimes had water in them could have built a single large circular ditch that at least sometimes had water in it with the same amount of effort.

Kenilworth Castle, Kenilworth, Warwickshire, England, is famous for its water defenses, being surrounded by water moats and large lakes. Kenilworth is an example of a water castle.

A water castle, sometimes water-castle,[a] is a castle where natural or artificial water is part of its defences.2 It can be entirely surrounded by water-filled moats (moated castle) or natural waterbodies such as island castles in a river or offshore. The term comes from European castle studies, mainly German Burgenkunde.49 When stately homes were built in such a location, or a Wasserburg was later rebuilt as a residential manor, the German term becomes Wasserschloss, lit. "water palace/manor".

https://en.wikipedia.org/wiki/Water_castle

And many water castles are surrounded by water much wider than the typical moat, and the lakes often have straight shorelines. So if people could built rectangular castes surrounded by rectangular lakes they could build circular castles surrounded by circular lakes.

[end of 12-12-2024 addition]

So if people wanted to dig a ring shaped canal for navigation, recreation, defense, prestige, or something, they could have dug one which was a few hundred to a few thousand feet in diameter.

Or maybe a mighty ruler planned to built a vast circular city tens of kilometers or miles in diameter surrounded by a great circular wall and a great circular moat. And first he built a giant outdoor model of his city at about 1/100 scale so the 100 foot tall wall was only 1 foot tall tall and the 300 foot wide moat was only three feet wide and it was easy to jump over the model moat and then step over the model wall.

And maybe the king never got to built his real city, but the model city remained and was preserved as a wonder of the world and a tourist attraction.

Or maybe a real giant circular city surrounded by a circular wall and moat was actually built and inhabited, and one time a wizard king captured that city and shrank it - like Brainic in the comic books did - and magically transported it back to his city in triumph and set it down in the royal gardens along with other enemy cities he had captured.

And so your villain comes to the comparatively tiny city with its circular moat which is probably still the widest circular canal in the world with a diameter of possibly hundreds or thousands of feet or meters. And he pours buckets of blood and barrels of blood into the tiny canal and then shrinks himself to the scale of the shrunken city.

And because of magical double talk the circular moat around the shrunken circular city is both hundreds of feet wide and deep and miles in diameter and maybe a foot or two wide and deep and a few hundred feet in diameter. To the people living in the city and the villain once he shrinks himself and enters the city the moat is still hundreds of feet wide and deep and miles in diameter. So perhaps through magical double talk the moat will be as magically effective as one miles in diameter would be.

Answer 9

Brugges

A city like this already exists, Brugges in Belgium. There is an approximately circular canal system surrounding the city that were started in the 12th century, are shown below in a map from the 1500s and still exist in the modern day. I don't think it would be a huge stretch to have dug this in a more exact circular pattern if you paid the workers a little more and added some mysticism.

Answer 10

Venice/Technochtitlan

Venice notably just built the city on top of a lagoon creating canals from the negative space. Pre-colonial Mexico city did something similar on a shallow lake. Their notable addition is the Chinampa, highly fertile farmland build by structurally land-filling chunks of the lake. All it would take is a beltway canal and you've pretty much got what you're looking for.

Answer 11

Its a geologic feature.. as in its a milling stone - disc, thats also afloat naturally. As soon as there is water in the pond, the stream makes it rotate, grinding away the edges of the mill- and the canal. Something similar occurs with ice https://www.youtube.com/watch?v=-1CEisOsIgE - and there is rock that floats https://en.wikipedia.org/wiki/Pumice_raft

Now building a city on top of that phenomena- thats strange. Why would your citizens do that? I have no idea.