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What I don't understand is how that works from a conservation of mass perspective. The water can't be literally flowing down at every point and keep a stable su

What I don't understand is how that works from a conservation of mass perspective. The water can't be literally flowing down at every point and keep a stable surface. I assume it's something more complicated, right.

steve_adams_86 4y ago
Part of the difficulty of turbulent water is that it can really whip you and smash you around. It doesn’t necessarily look that violent from outside of the water, but you could be getting broken limbs and brain trauma in that kind of turbulence. The force of the water pressing you into walls and boulders is tremendous. Loads of fatalities in turbulent water occur because the victim rapidly becomes unconscious in the water.
throwaway09223 4y ago
It can. It's an underground tunnel with a small exposed strip at the top. the water can be circulating around the tunnel, flowing back and forth under the rocks. It can be pulling sideways or down at the exposed strip with the water circulating back up in the underground areas out at the sides.
- andrewflnr 4y ago
  Looking at the underwater footage, I think this is roughly it. You could see bubbles running up the walls.
jakeinspace 4y ago
Of course the flux of water into any volume is equal to the outward flux. But I would imagine the situation is that there are circulating turbulent branching streams of current pulling you down. If you were incredibly lucky, and not killed by impacting the walls, you might pop back up by catching a ride on another section of current.
carabiner 4y ago
I bet it's flowing horizontally but faster the deeper you go. You fall in, the higher speed flow at your feet apparently "sucks" you under due to its higher speed (higher inertia), like being grabbed by your ankles and yanked down.
rz2k 4y ago
In addition to the explanations of the turbulent water smashing victims against the rock walls, presumably a very turbulent flow could also turn into a mixture of air and water, with a corresponding decrease in density such that it would be more difficult or even impossible for a person to maintain buoyancy.
amluto 4y ago
Sure it can. In an incompressible flow (e.g. water under anything resembling normal circumstances), there is no net flow into or out of any given volume, i.e. a 3-D box or other shape. But the surface is 2-D! So you can look at a little box with its top at the surface and its bottom slightly below the surface. If there is downward flow out the bottom (and zero flow out the top, of course), there must be net inward flow in the sides. Nothing wrong with that. This could happen if the river has constant speed and constant cross-sectional area everywhere but gets progressively narrower and deeper. The water moves inward because it has to get away from the encroaching banks and moves down to fill the deepening channel. If this is abrupt enough even in the absence of turbulence, the downward current a foot or so below the surface will be substantial.

steve_adams_86 4y ago

Part of the difficulty of turbulent water is that it can really whip you and smash you around. It doesn’t necessarily look that violent from outside of the water, but you could be getting broken limbs and brain trauma in that kind of turbulence. The force of the water pressing you into walls and boulders is tremendous. Loads of fatalities in turbulent water occur because the victim rapidly becomes unconscious in the water.

throwaway09223 4y ago

It can. It's an underground tunnel with a small exposed strip at the top. the water can be circulating around the tunnel, flowing back and forth under the rocks. It can be pulling sideways or down at the exposed strip with the water circulating back up in the underground areas out at the sides.

andrewflnr 4y ago
Looking at the underwater footage, I think this is roughly it. You could see bubbles running up the walls.

jakeinspace 4y ago

Of course the flux of water into any volume is equal to the outward flux. But I would imagine the situation is that there are circulating turbulent branching streams of current pulling you down. If you were incredibly lucky, and not killed by impacting the walls, you might pop back up by catching a ride on another section of current.

carabiner 4y ago

I bet it's flowing horizontally but faster the deeper you go. You fall in, the higher speed flow at your feet apparently "sucks" you under due to its higher speed (higher inertia), like being grabbed by your ankles and yanked down.

rz2k 4y ago

In addition to the explanations of the turbulent water smashing victims against the rock walls, presumably a very turbulent flow could also turn into a mixture of air and water, with a corresponding decrease in density such that it would be more difficult or even impossible for a person to maintain buoyancy.

amluto 4y ago

Sure it can. In an incompressible flow (e.g. water under anything resembling normal circumstances), there is no net flow into or out of any given volume, i.e. a 3-D box or other shape. But the surface is 2-D! So you can look at a little box with its top at the surface and its bottom slightly below the surface. If there is downward flow out the bottom (and zero flow out the top, of course), there must be net inward flow in the sides. Nothing wrong with that. This could happen if the river has constant speed and constant cross-sectional area everywhere but gets progressively narrower and deeper. The water moves inward because it has to get away from the encroaching banks and moves down to fill the deepening channel. If this is abrupt enough even in the absence of turbulence, the downward current a foot or so below the surface will be substantial.