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I like to add fantastical things and make them mundane. In this case, instead of building and maintaining a pedestrian bridge in a fantasy world they would create a waterwalking path. In this case, two monoliths on either side of the water give all people between them the ability for waterwalking for a few minutes. That means that if you accidentally step out between them you have plenty of time to make it to the other side or step back between them.

However, unlike "traditional" water walking I would expect the water to simply form a surface to stand on*. Which means that if the surface moves, the person standing on it moves as well. Now a wave won't be too much of a problem. The water particles you stand on move up and down mostly, it is the wave that moves through the water rather than the water moving as a wave. But how does water move "normally"?

The question: "how would water walking be affected by the movement of the water?".

I would like to know for the following conditions:

  • still weather.
  • the effect of the bow wave of a small boat passing by on your ability to walk or stand during still weather.
  • the effect of the bow wave of a large/fast boat on your ability to walk or stand during still weather.
  • slightly choppy waters.
  • choppy waters.
  • high waves.
  • near the pylons of a bridge of a flowing river during still weather.

*the footwear worn will be included in the waterwalking spell to prevent sagging into the water anyway.

Clarification: All basic physics apply, nothing has changed. Everything is the exact same except when a spell adds or alters the physics locally. In this case: water will not yield to a person standing on top.

Clarification 2: there is no need to further explain the physics. As already explained, this question is about what the movement of water would do at the contact points with the foot. Just like you do 't have to explain why a floorboard can carry your weight, I don't have to go into detail as to why you can now stand on water. The water does not alter its properties compared to itself, it can just support the person standing on it.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Jul 20, 2021 at 0:05

8 Answers 8

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It will be WAY harder than it sounds

Not sure if you've ever seen these floating lilly pads at any public pools, but they work more or less as described. The below body of water is completly still, and even with a net above you to hold on to, those things move around like crazy making them impossible to walk on without the overhead net.

The problem you will have is not that the surface will move with the current, but that the area your foot touches will be small and have very little resistance against lateral drift as you put your weight down on it. Unless your "solid water" were to tether all the way down to the ground, the second you go to take a step, your back foot will quickly slide backwards and your front foot, will not be able to create any traction to stop you. It will be like trying to walk on a surface several times more slippery than ice.

enter image description here

That said, the larger and more rigid you make your solid water, the more it will resist lateral slide (at the very least, the area under your feet needs to be interconnected); so, if instead of just making the water just under your feet solid, you were to make a radius around you solid, then you could have a surface you could walk on.

enter image description here

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  • $\begingroup$ Yes, it'll be like trying to stand up on a kayak but much much harder. $\endgroup$ Jul 20, 2021 at 10:03
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Fall over in most cases

Lets get a few assumptions out of the way. I'm assuming it'll be like standing on glass. With the right shoes or bare feet you have purchase on the surface. It'll not be like ice, incredibly slippery. Like glass, you'll not sink into the surface of the water when water walking is activated, nor will your pushing off alter the water trajectory and movement.

With still water, it'll be like moving on strong glass. You can run. You can slide. You can jump. No problems.

With slow running water the problems start. It'll be like a glass treadmill, moving in one direction. You'll likely walk upstream or downstream, and each step slightly sideways. That way it'll be manageable with your balance. Still, the surface can be slippery, making this a lot harder than normal horizontal escalators. Worse still is when there's differences in speed. Water can go slower near the edges, making this very awkward. Worse still is that differences in speed will make eddies. Where first you were going on a slow horizontal glass escalator, now your going on a slow horizontal escalator with varying speeds and areas where it can turn and shift sideways. With a slow river. You're sure to lose your footing regularly, even when very experienced.

The slow river can also have an irregular river bed, making it churn anyway. Even if everything else is the same speed, you'll have weird moving surfaces under your feet that likely make you lose your footing. It'll be near impossible to anticipate the movements. Same for choppy or vavy waters.

With increased speed this gets worse and worse. Inadvisable for anyone.

Boats and the like

Even in still water, boats and waves are very bad. Ever been in an earthquake? Or had people jumping next to you on a bouncy castle? Both make you lose your footing. Keep in mind that this is likely worse, as you'll have the equivalent of a glass surface to get your purchase. Even when jumping over the 'big' wave, you'll encounter tiny waves that are sure to bring you off balance.

Conclusion

As your purchase to the surface moves, you'll move with it. This is hard to balance. With water it moves in wild and unpredictable ways even when moving slowly. Most tiny movements will make it hard to stay standing, let alone cross a river.

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  • $\begingroup$ This system does open the possibility of rapidly traversing the area between the monoliths by dropping one of them into the water and surfing along the front of the wave it produces. And it would be hard to follow you with the same trick. $\endgroup$
    – Willk
    Jul 18, 2021 at 17:17
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    $\begingroup$ Does it still work the same for people proficient in water walking? People who live in environments full of escalators are capable of adjusting their balance automatically to compensate for the escalator movement. They can maintain normal posture and can move freely, unlike those not used to escalators. $\endgroup$
    – Otkin
    Jul 18, 2021 at 20:37
  • $\begingroup$ Would it really be that hard? After asking the question I had some idea's about it: what about floating birds and water-strider insects? In case of the insects they use the water tension, which keeps the top layer of molecules together. So apparently the surface will be moving less than the water below, which would make water walking more easily. Would that change your answer? $\endgroup$
    – Demigan
    Jul 19, 2021 at 4:22
  • $\begingroup$ @Otkin when experienced for a certain area maybe by avoiding the difficult parts. However, even when experienced I think you'll fall regularly. The problem is that so much isn't predictable. Eddies especially, but waves as well. Likely the most experienced will reduce the amount of contact, pushing off before the water pushes your feet and thus your balance differently. But if you start running on an uneven ground that handles like glass, you'll be have a serious injury in no time by just stepping wrongly. I think too few would be able to handle it in an ok way. $\endgroup$
    – Trioxidane
    Jul 19, 2021 at 9:05
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    $\begingroup$ @Demigan the balance of the water-striders or birds floating is very different. We're tall with our weight distributed in a small area. The rest has is spread widely and with good reason. Also, they purposefully stay in the easy waters. If they stray into slow running water, you can see their legs already moving quite a bit. Thanks to their wide span not a problem, but humans do not have that. Also the water strider has a wholly different purchase of the water than in my answer, making the humans fall flat. $\endgroup$
    – Trioxidane
    Jul 19, 2021 at 9:13
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The answer, as often is the case with magic, is: whatever the spell creator managed to instill in the spell. In other words, what happens is entirely dependent on how the spell works... and there are far too many options there for a complete answer.

Let's start with the naive approach: the target's weight is distributed across the surface of the water widely enough to float as if they were a boat with a high displacement, with only the lowest part of the person's body (usually the soles of the feet) contacting the water itself. Wherever the target stands the water around them would form a depression dependent on the target's mass vs the area of the spell. Since the spell interacts directly with the water the depression will bob and move along with the motion of the water, making walking a rather difficult task. Also the almost total lack of friction makes it functionally impossible to walk anyway, since there's nothing to push off other than a tiny amount of liquid. Great for moon-walking in place, useless for much else.

Next up, we'll make the water freeze in place wherever the target's foot approaches, giving them solid place to both stand and push off from. This is almost workable, as long as the water surface isn't too choppy. The higher the swell the more the target has to work to step up and down on their impromptu stepping stones. Also you might want to add some water repelling shields to the side of the effect to ensure that a wave doesn't wash over and solidify around the target's foot, leaving them stranded mid-stride until the spell expires. While the results may be humorous to observers, the inevitable dunking is unlikely to be appreciated.

Personally for a fixed system like you have described it might be more useful to simply build a bridge, then use magic to push it out of sync with the world. Only people granted some sort of attunement with the out-of-phase bridge will be able to interact with it in any way. Boats, waves, etc. will simply pass through the area as normal, as will untreated individuals. By simply passing through the bridge markers however the pedestrian is temporarily spelled to interact with the bridge itself, enabling them to cross with no more effort than any 'normal' bridge. Not water walking, but it seems to suit the requirements sufficiently.

For a less systematized situation - when a mage wishes to walk across a lake or something - then perhaps a minor modification in which the mage summons a mobile set of stepping stones similarly out of phase would suffice. The spell would ensure that the stepping stones move across the water surface tracking the mage's feet, then fix in place as the foot approaches. The appearance of walking on water is then preserved, and of course appearance is everything.

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If waterstrider physics are anything to go by, they'll be paddling their way around on their bellies. Also disturbing the water surface too much might be considered murder.

I feel like you saw the concept of water striders and other creatures that move on the water surface thanks to the water properties regarding the tension of the surface, and increased the tension through magic. However, if normal physics apply as normal, then technically all we need to do is observe how it functions to a standing water strider and scale it up to determine what our surface looks like:

1- its malleable. Googling a picture of a water strider standing on the surface easily shows this property: while they are not sinking down since to the force they're applying is inferior to the maximum force the water surface tension can withstand, their legs will be clearly be forming depressions on the water, since that's where they're applying their weight. This alone shows that if physics still apply and all the spell does is make the water surface tension capable of withstanding a human, the surface of the water will probably still be pretty malleable. Instead of walking through glass you'll feel like you're walking on a thick layer of malleable yet resistant membrane that begins sinking below your feet as soon as you use it to sustain your weight.

2- it has very little friction. As we all might know, water is a fluid, the molecules move around all the time, and if you ever saw a leaf floating in a calm lake or puddle, you probably know that things floating in the water can be moved with a fair amount of ease, especially if they're not breaking the water surface tension. Even water strides suffer with it, and it's another reason why they don't move around like a land insect. Water striders normally keep their legs in the water, removing only a single pair which they essentially use as oars to move around and change direction, all while making use of a pretty low center of gravity. Scale that up and you'll see that the human shape isn't all that good for what we want. Mountain climbing shoes won't help you a bit here if you're trying to get some extra traction on each step.

So essentially, if all the spell is doing is making it so people don't sink by making it so that the water surface tension becomes much stronger than normal, this would not only alter some things about how the water itself moves in the area (as in any kind of movement in the water that requires the water surface tension to give in and which is applying a force inferior to what the magic water tension can withstand, then it most likely won't happen as it'd like to). The water surface is now something you won't easily rip through so that you can sink down, but it doesn't change that it has little to no friction on it (water-sliding is extremely dangerous for a car because a thin layer of water forms between the tires and the road and essentially negates traction, making the driver easily loose control of the vehicle. In this scenario we'll have that, except that the water isn't between you and the road, it IS the road).

So essentially, in optimal water-striding conditions (calm water) you'll be better off ripping nature off, laying on your belly and using your arms to try maneuvering your way around, especially since if you try walking on this water like you'd walk on land, you'll eventually end up in that position anyway.

Now, this is assuming calm water or at most water that's undergoing forces weaker than what the new surface tension can withstand,because it's pretty hard to understand the other outcomes of a bigger surface tension being ripped apart by a stronger force. However, if we just do the same thing we did before and assume everything works as it normally would, then being too close to something that's breaking through the water tension probably will result in you sinking. The problem? The water surface should recomposes once it calms down enough, and if it behaves for things below it like it does for things above, should it recompose while you're still below, just like it didn't give in to your weight, it probably won't give in to your attempts to surface for air, and this will hold true until the spell effects end.

So basically, water walking will not make you walk around on the surface like it was glass. Normal physics determine that the water surface behaves like a membrane to anything small and light enough not to breach it, and this will hold true if all the magic is doing is increasing the surface tension. Any person under the effects of the spell will temporarily function like a water strider on water surfaces. Problem is: water striders know that trying to walk on the water surface is not like walking on land, they have all the adaptations and bodyplan necessary to paddle along smoothly and they too can drown if the surface gives in.

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  • $\begingroup$ No I did not realize the idea of water-striders until after the question was posted. Your idea also ignores an option: if you walk like a water-strider, then the surface tension has increased by the spell (which seems the only logical option outside of "the water isn't even affected by you standing on it", both of which require no further explanation @"where is the physics crowd"). So what if the surface tension is simply even higher to make the surface budging neglegible? $\endgroup$
    – Demigan
    Jul 20, 2021 at 4:18
  • $\begingroup$ @Demigan my answer is completely based around assuming the spell is increasesing the water surface tension so that it can hold humans the way it can hold water striders. You also have to take into account that the surface tension effect as physics describe it causes the fluid surface to behave like an elastic membrane, and I'd say even if we increased the maximum tension it can withstand, the water would simply be able to take in more weight, but not necessarily become less elastic, since water only begins to actually behave more like a solid under extreme forces (briefly) and as ice. $\endgroup$ Jul 20, 2021 at 14:30
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It would be like walking during an earthquake

There's rarely such a thing as "still water." It looks still, but it's moving. The wind moves it, the earth moves it, temperature changes move it. Water in a body even as small as a glass is always moving — but the way we experience it means that in small amounts (drinking out of a glass) we don't notice it and in large amounts (tsunamis) we make movies about it. But it's always moving.

Which means walking on it as if it were a surface like earth would be identical to walking during an earthquake. Let's listen to what the USGS has to say about that.

The way an earthquake feels depends on where you are, where the earthquake is, and how big the earthquake is:

  • A large earthquake nearby will feel like a sudden large jolt followed quickly by more strong shaking that may last a few seconds or up to a couple of minutes if it's a rare great event. The shaking will feel violent and it will be difficult to stand up. The contents of your house will be a mess.

  • A large earthquake far away will feel like a gentle bump followed several seconds later by stronger rolling shaking that may feel like sharp shaking for a little while.

  • A small earthquake nearby will feel like a small sharp jolt followed by a few stronger sharp shakes that pass quickly.

  • A small earthquake far away will probably not be felt at all, but if you do feel it, it will be a subtle gentle shake or two that is easier to feel if you're still and sitting down.

The type of crustal material the seismic waves travel through on their way to you, and the type of shallow crustal structure that is directly below you will also influence the shaking you feel. Soft thick sediments will amplify the shaking and hard rock will not. If the energy happens to bounce around and get focused on where you are, that will also amplify the shaking. Low-level vibrations that last for more than a few seconds is not indicative of an earthquake, but is more likely a man-made environmental source.

For us, this would be incredibly uncomfortable walking. It would likely give "sea sickness" a whole new name. We'd have a lot of trouble standing up straight (setting aside for this purpose @Trioxidane's incredibly insightful answer, which I upvoted) — for a while. As we grew accustomed to it, it would take larger and larger movement to give us the same trouble.

And this is an important point in your world. People will have lived with this world rule all their lives. They would have learned as very young children to deal with (and not fear) the motion of the water. So when you ask what it would be like, the reality is that for everything other than real waves (say, more than 2"-3" in size), people wouldn't even think twice about the walk. They'd automatically side-step the bounces just like we side-step holes in the sidewalk or small tree roots on a path. And if your world is bureaucratically encumbered as ours is, an assigned official would close the walkway whenever the waves grew too tall to be easily stepped around (just as we close roads for being too unsafe to drive on).

That bureaucracy would also have rules forbidding boats going so close or so fast that they would disrupt the use of the bridge. We see this today in real life with boat speeds limited in marinas and in Venice, Italy. Therefore, the bullet about the bow wave can be, IMO, dismissed.

(And why would you place your waterway near a pylon of a bridge, or a big rock, which would cause the same effect? Your bureaucracy would have rules about where bridges can be established for the sake of public safety. If no such bureaucracy exists, then you'd think the bridge builders would have an ounce of common sense.)

A few things to think about...

Would people's feet remain dry? Is the nature of the magic to always keep them atop the water, no matter how small the particles? How would this be affected by rain? Would your walkers walk "up" a rainstorm so long as the amount of rain being stepped on could bear their weight? If not, how thick must the water channel be to support the weight of all walkers — or is there a weight limit like elevators have? If the motion can be felt by the walkers, does that mean they can be moved by the water? Watching a video of a car on a low-water bridge while it's flooding will demonstrate the problem: the water would simply push people off the bridge. Unless you allow people to get their feet wet, which would mean there's a "top portion" of the water that they're stepping through and thus the waves can't push (at least not as hard). Food for thought.

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It's probably easier to just take a boat.

If the effect is equivalent to increasing surface tension, then in still weather it should be similar to walking on very smooth (and slippery) ice. Although unlike ice, water is slightly compressible, so it would probably have some give where you step on it. Much like with actual ice, inexperienced people would probably slip and fall (Q-is it painful to fall on water?) but those who are used to it can easily "skate" by. They might use ice skates, skis or snowshoes to traverse it. Even more logical would be some kind of sled, pulling on a rope... But that makes the magic seem kind of pointless, because you can already raft across.

Small ripples would probably be similar to walking on large rocks, that are being shaken and bounced around. Uncomfortable and likely to twist your ankle. Even on a sled it would be extremely bumpy. Any significant wave would be like walking on a very large sheet, and someone start kicking the sheet under you. Cats really enjoy this game -- but humans are less agile and can't grab the surface with claws. So basically any kind of wave or ripple more than an inch or so would probably make the passage unusable. It would also be dangerous near the shore or pylons, where you can get thrown against a hard surface. Ironically, in slight to moderate waves people could still just take a boat across, so you would have people bypassing the water walkway because of inconvenience.

Usually, water walking is described as not sinking at all. But you could imagine a magic that simply increases your buoyancy so that submerging only your feet is enough to make you float. Which is basically what happens with mercury - there are some videos of people trying to walk on that and it seems kind of hard but doable. I imagine that if you had to actually do it you would get something like a walker to help with balance, which is incidentally what new skaters use. Or even better, you would just use a boat... Anyways, with high buoyancy, waves would be worse. Because your center of mass is above the surface, and the submerged parts of you don't create much drag on the liquid, you don't have much stability and would fall over easily. Normal ships benefit from a keel, so you would need one for your water walkway vessel, at which point, once again, just use a boat.

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As others have said, it depends on exactly how the spell is making this possible.

For example, websearch the various videos where a shallow pool was filled with a shear-thickening fluid (usually cornstarch in water, since it's cheap and nontoxic) and people tried to cross it. It is mostly possible to run across that stuff -- assuming the mixture is right -- since the impact causes it to locally solidify for a moment. If you don't go fast enough, it doesn't thicken enough and you sink in.

Whether the spell behaves at all like that, gods and the gamemaster only know.

(If you haven't done so, I do recommend that you try mixing up a bowl of this stuff at some point just to play with. It is distinctly odd to have a material that turns into a thick paste or solid when you poke it hard or grasp it firmly, but reverts to being a liquid the moment you relax the forces on it. It does strange things when forces are applied in other ways too.)

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    $\begingroup$ Since I use the idea of walking and standing, running across a corn-starch like substance is not the type of water-walking I am looking for. But since you bring it up, what kind of methods could create water-walking effects within the limitations I already mentioned? Because I see pretty much one: the water surface carries the person without budging. $\endgroup$
    – Demigan
    Jul 20, 2021 at 4:04
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Like others, I thought of something that would float in the water that you could walk on, like ice. Perhaps at one specific place in the water the temperature somehow rapidly decreased to form an ice bridge.

Another thought I had is perhaps the water somehow became very dense in a specific location. If the density of water was more than that of the person walking on it, then the person would float rather than sink. Only thing is that if you were to pressurise water to such an extent it would solidify and then be ice anyhow. Maybe if the density could be increased but remain amorphous then it would be glassy solid water rather than crystal ice.

Disolving material, like salt, in water can make it denser and people can float in salt water but they tend to be lying so the pressure is spread over a larger area. There is a limit to the amount of material that can disolve at certain conditions of temperature and pressure. But perhaps the water is supersaturated so forms a solid deposit. This could be achieved by disolving material in heated water then cooling the water reducing the solubility so deposits form. A "seed" can be used as a point of nucleation where a crystal can grow. Quite a common technique to grow crystals.

Another idea is that perhaps water could be replaced by a more viscous fluid perhaps like tar.

Another thought I had is that some insects do float on water due to surface tension but have small surface area. Perhaps it is not the water that changes, but somehow the person could be different to walk on water, e.g. have very thin legs and less weight.

Remember that walking on water is a biblical phenomonon and some biblical scolars have discussed it so there might be some inspiration there.

Of course if it is being done by magic, I guess you have license to bend the science or perhaps this place follows different laws of physics. I quite like cartoon physics where people don't fall until after a delay of a few seconds to contemplate what is about to happen.

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    $\begingroup$ Welcome to the Worldbuilding SE! I'd just like to quickly point out this post doesn't really answer the question asked: the OP wants to know the effects of water-walking, while your answer focuses instead on how to create the phenomenon. You could improve this answer by adding your interpretation of how the water moving would affect the act of walking over it. Good luck! $\endgroup$ Jul 20, 2021 at 11:24

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