I have a town that I'm designing (in a D&D campaign, think Neverwinter in terms of technology) that I want to be a town on a river/lake/ocean/whatever. I would like for this town to be unique from other towns in the ways that they have figured out how to wield the power of water. I know that we have things like the Hoover dam that provide power for a lot of people, but what would the logistics of such a town be? Could they survive? Could they thrive? Would there be a population limit, or any limits at all?
Since you state that this is a D&D campaign, I would have to imagine that magic would be available in the construction of a dam, which I would see as the most logical way of getting power from water. Hoover Dam, which you mention, generates a tremendous amount of electricity every year (wikipedia page lists it as 4.2 billion kWh per year). Putting this in perspective, that is enough to supply (if my math is correct) the entire state of Colorado with electricity. I'm using the Wikipedia page on the Hoover Dam and the information at eia.gov for Colorado electricity usage and their sources. What I think would be more interesting is the use of the dam for more than just energy production. The structure could be used as the city itself, which would lend to defense as long as the dam is made from similar materials as Hoover Dam is.
I would also see a large part of the food for the city to come from aquaculture. You have a giant lake so you might as well use it. Something to consider is the amount of area now submerged that would have been above water originally. Hoover Dam flooded enough area that small earthquakes were detected as the area filled with water.
Depending on the surrounding area, your dam city could end up being targeted for conquest simply because it has a large reservoir of water available, especially if it is freshwater. I have no idea how salt water would or could be handled differently.
Now, time for the elephant in this room. You mention Neverwinter as a source of technological inspiration. I've never spent much time in the realm of Forgotten Realms, but I don't think Neverwinter had electrical generation. If it did, I would assume it would be more along the lines of a coal plant or similar. Could someone create hydroelectric power? Sure, as this snippet from the Department of Energy covers:
During the 1700s and 1800s, water turbine development continued. In 1880, a brush arc light dynamo driven by a water turbine was used to provide theatre and storefront lighting in Grand Rapids, Michigan; and in 1881, a brush dynamo connected to a turbine in a flour mill provided street lighting at Niagara Falls, New York. These two projects used direct-current technology.
So to get to the specific questions raised:
- Logistics - The logistics would be extremely complicated, even by today's standards. Could it be done? Yes, but you'd better have a real genius running things. Maintenance would also be a potential nightmare.
- Survivability - Unless there is a catastrophic failure, it would be quite survivable. If you were to have a catastrophic failure, I'd imagine something along the lines of the Yellow River flood of 1887.
- Thrivability(is this a word?) - Don't see why not. Fresh water is usually the hard thing to come by for civilizations. As long as an Evil Bad Person didn't contaminate the reservoir, I don't see much that would impact it.
- Population Limits - As I said above, the state of Colorado's electricity needs match what Hoover Dam produces. Colorado has a population of around 5.3 million according to Google, so I think a fantasy medieval setting would hit other limits such as just how much food is available.
I hope this provides some food for thought.
If the water has enough pressure (large dam, coming from great height, or naturally coming at high pressure out of earth (possibly due to volcanic activities) and you can maintain that pressure over a larger pipe system, you can use the water for power almost like you can do electricity. Instead of electric motors, you'd use water turbines (a town that has water as main energy source would almost certainly optimized the water wheel into a water turbine), or for limited movements, use the water pressure directly in the manner of hydraulics. Instead of electric switches, you'd use valves.
Modern plumbing (running water in every room, water toilets, central heating — even though the actual heating might still happen with wood) would be known and used in that town. On the other hand, houses may also be cooled in the summer by simply having some water running over their roofs/along their walls. And probably there would also be sophisticated fire-extinguishing systems using water.
Water, if available in sufficient quantity, can also effectively be used for defense, beyond the usual moat. For example, have big valves besides your town gates, which allow to simply sweep away the enemy. If cannons are already known in your world, a directed water jet can make a cannon temporarily unusable (wet powder isn't useful for shooting).
And of course the town would be full of water-based pleasure installations like fountains. Swimming pools would be common, both public and private ones. Possibly some houses would even have lifts, which could basically be cabins swimming on a lock like those used for ships. A major attraction could be a water-powered pipe organ which, unlike other pipe organs, needs not be powered by humans, and thanks to the power of the water has the loudest sound of all organs known in your world. That organ might also be used for practical purposes, for example for everything church bells have traditionally been used.
And if you want something really advanced, you can even make a water-driven computer. It would of course not be as powerful as our computers, but probably would we on parity with the earliest computers in our history.
It depends on what kind of water source they have. If it is a large slow delta like the end of the Amazon or Mississippi, then there isn't a whole lot you can do with it other than large water wheels, otherwise you are doing an awful lot of work to get the water up high enough to do any work. Of course a very large water wheel might be able to make that happen.
However, a better solution would be to have the town near mountains or cliffs, where the water is already high above the town and the aqueducts would then be able to do their work more easily.
At that point many things could work, indoor plumbing, many kinds of physical labor, including all the mills that used to use it. A black smith could have a valve to run his bellows, the town gates could be opened with water power etc.
If they're using it to create electrical power, then the town and tech can be largely similar to any earth-based city. It all depends on how much water is available, and how high of tech you want. For example, in the Northwest United States, up to 80% of electricy is supplied by hydropower. So really the sky's the limit, you can have lights, electric vehicles, etc.
Alternatively, since you want closer to medieval tech, you could run the power throughout your town (in pipes and aqueducts) and use it as direct mechanical power. I think this would be more efficient, because you're not losing power in the water -> electricity conversion, but the downside is that it's much less flexible and needs to be fixed installations. You can't have a hydro car, for example, but you could have a hydro-powered public transport system. You can also use it to run industry, like large steel mills, or manufacturing.
Another use would be as weaponry/defense. You could trap water and heat it to power catapults or cannon, or hit them with large clouds of steam. Or have rapid-flood moats that aren't up most of the time, but you can activate in case of invasion.
Note that I'm assuming pre-electricity technology in this answer.
For getting power from water, there's not a lot you can do with lakes or oceans. Having the town located on a river is your best bet. Also, since the flow of water in a river is caused by gravity, you could get more power from a faster-flowing section of river. If you want to have water power widely available, you'll need to distribute it across your town, which means being able to get the water from a higher elevation than the bulk of the settlement. This means that a hilly or mountainous region makes the best location.
The availability of water power would make your town an industrial center. Mining, metalworking, masonry, lumber, and textile industries all benefit from watermills. This, again, makes placing your town in the hills or mountains make sense—you'll want easy access to a mine or quarry.
The complexity of the logistics for the town depends on how large and spread-out it is. In general, there should be a dam and resevoir somewhere upriver of the town. Some larger factories or mills might be built next to the dam, to utilize a greater volume of water. After that, you will need an aqueduct network servicing your town. Exactly how this is laid out is pretty much up to you. You could have lots of small channels connected to one primary canal, or a branching tree-like network. It is important for you to have sluice gates to control water flow, so maintenance on the channels is possible.
Beyond this basic setup, what you can do with the water power is pretty varied. You could use canals and water bridges for transport around the town, or you could have water-powered cable cars. Water power could be used in the operation of city gates or drawbridges. Also, related to logistics, the city's firefighting and sanitation systems should be exceptional (as long as you can tell the difference between the fresh and waste water lines). Just beware that if you get too inventive with the water power, your setting will seem a bit steampunkish (especially if you use steam!).
The potential for population growth in the city depends mainly on the amount of food that they can get. This is the main drawback to a mountain location—most of the food will need to be imported. So the limit would be based on how easily travel is in the region, and the relationship between the water-town and the surrounding towns and villages. There are already some questions on this site to help with that.
So, yes, such a town could exist and function. Depending on the quality of agriculture and transportation in your world, it could even become a substantial city and trade center.
I haven't seen an answer involving steam yet so here goes:
Steam could be a possible water (and of course heat) based system for generating mechanical or electrical power.
Where do we get the steam from? Well the geography of your city will have most of the impact here:
Iceland for example generates about 30% of its power from geothermal sources Wikipedia and of course this comes from volcanic type activity and many many geysers.
For something like small lakes and rivers you would likely need to find a source of energy for heat. Coal, wood, or other burnable materials.
A third option which I view as viable with large bodies of water or sufficient flow rates from aquifers would be to have the sun power your steam operations. Parabolic mirrors (or giant perfectly cut gems?) could provide the heat for your system.
Lastly you always have magic as an option to generate steam and then the water source can be a bit more varied. Fire elemental cores, Rifts to the elemental plane of fire, demonic chili peppers, etc.
Water has two incredible properties - it does not compress, and it retains heat well.
As long as your water source is up high enough and you have constant feeds, you could create intense pressures in a pipeline and release it in short bursts, like water jets and water rockets. We already use water based cutting jets to cut through steel over 2 inches thick.
Steam would be another great resource for weapons - steam based cannons and even rifles of some kind could be created. Steam itself can be piped at temperatures over 700 degrees Fahrenheit and intensely burn attackers. Use steam like a cloud cover in winter, create snow hillsides, ice walls, and even avalanches from your artificially created snow!
In terms of supporting a town, many sawmills converted to electric generators simply to keep the tools performing well (easier to buy an electric saw these days.) however, the mill generates almost the same amount of electricity to run the saws as the mechanical power the wheel generates on its own. Remember, saw mills and fabric mills were the first real factories. If you need any mechanical energy, water can provide it.
You can easily support 100,000 people for power, depending upon the size of your water resource.
Just make sure to have more than one "pipeline" or aqueduct to support each side of your fortress. I'd make a ring of storage tanks and interconnect them to share their water pressure. If one is lost, cut it from the loop immediately or you lose pressure everywhere.
How about a town build on top of a lazy flowing river. The houses are build on rocky formations in the river having water wheels supplying the power for metal workers and other craftsmen. Water is pumped by the wheels up to the surrounding fields for irrigation and for running water in the town. This is all easily done even when very low tech.
Materials are transported through the town on bridges, with cranes powered by the water wheels or by boat.
To add complications plant life in the river meeses with the water wheels, those annoying beavers upstream simply will not stop building their dams, the enemies of the city notices the annoyance of the beavers and try to build their own dams, and will attack anyone trying to remove the dams.
And every spring the water rises in the river wrecking havoc.
EDIT: I forgot to mention that a tidal river could be a challenge in my scenario. The water flowing 'backwards' at times is perhaps not good for the water wheels.
I'm unsure of the "magic" available in NeverWinter, but so far all the answers don't really include magic in their answers, except for the construction phase for building.. a normal real-world construction.
If magic can be sustained in a way that doesn't require effort, for instance, if you can have a group of mages cast an enchantment once in a particular spot and they never have to come back unless someone else dispels it, you could essentially create even more efficient constructions.
Dams create "power", (whatever you're using it for), by the force of water moving downhill. The reason you block the water is either so you have a standing lake for other uses, or to help create pressure to generate more power.
- What if we could create "anti-gravity" areas, where the water flowed back up as well? (alternatively, magic "heated" areas to turn the water to steam)
You could essentially create a perpetual motion machine.
Take flowing water, put a wheel in it, we have a mini water-mill! Now, at the end of the wheel, use magic to make the water flow "up", or, use magic to turn the water to steam. Provide a trough or something above the wheel, to guide the water back to the beginning of the wheel. Then, allow natural gravity to take over or cool the steam back to water. You can now cut off the water source if you wish.
There will naturally still be some loss of water, so being able to "magically" create more will assist with that, or, if they are advanced enough, they could try to "seal" the contraption together so the water won't ever escape. Being able to seal it would mean you could increase the pressure as well.
With canals, I imagine you could design some way to control the flow of water easily using magic. Put rafts in there and you have a system of conveyor belts which could be useful.
As a slight aside, no one has mentioned that such an arrangement has existed in the real world.
The "power" was just the pressure of water applied at the point required. Using a dam or just a large hydraulic accumulator filled by some other means would do the job of providing pressure. But you would need a technology for decent high pressure pipework.
Actually, if you think about it, a town could in reality be constructed by some of the worlds most efficient designers and workers. The water could be electrically generated through a sort of canal. All of this water could power up certain things. The town wouldn't be perfect or anything near perfection but it could be a livable place