Spring rains-check river gauges

[Me]

Hydraulic pressures, or the differences in them...a cave's flow will be determined by hydraulic pressure which is controlled in part by the height of the water table. If you raise the river levels so that there is more hydraulic pressure from the water outside the cave, than from the water inside the cave, the cave will stop flowing and reverse. The speed of water flowing in to the cave is similarly controlled by the difference in hydraulic pressure.

I think it's hydrostatic pressure.......

[JahJahwarrior]

You might be right...I honestly don't know the difference. I know Geology of Florida mentioned hydraulic heads in their chapter on hydrogeology, but I have only an amateurish, layman's understanding of this stuff

[Me]

Now you have me curious....I've always heard it hydrostatic and now I'm wondering. I'm sure someone will set it straight shortly.......

[scblade27]

So again, just to make sure I can wrap my head around this. The most high flow times will come after the aquifers have recieved a lot of rain in their recharge zones over a relatively long period of time, not a short period of time when the river waters will dramatically rise as well.

So I assume this is why I always hear of caves in the "old" days having much higher flow. The aquifer was "healthier" so to speak, it had a higher water table relative to the river water. I would also assume the lower flow is caused by water being sucked out much faster than it can recharge back to those historic levels.

[Sludge]

Actually, although we say "hydrostatic" pressure, "hydraulic" is more correct. Hydrostatic pressure is defined as the pressure at a given point in a non-moving fluid.

[JahJahwarrior]

SCBlade: the water isn't "sucked out", it is pushed out by the difference in pressures, which is related to water table levels.

Take a U shaped tube, and cut it, so that one wall of the U isn't so tall. Put a plug in it, and drill a small hole in the plug. Fill the tube with water, and observe how far the water shoots out of the tiny hole. When the water is near the top of the un-cut side, there will be higher pressure. As this water level drops, the water will come out of the tiny hole more slowly, until it stops as the "water table" reaches the same height as the "cave entrance."

Re-do the experiment with a bucket of water handy. Hold the tube so that the "cave entrance" is submerged, but so that ther eis still several inches of "aquifer" sticking up out of the bucket. As the "water table" drops, the same thing as before will happen. Once no more water is moving anywhere (kind of like Peacock), have someone add a gallon of water to your bucket. If you leave the "aquifer system" at the same height in your "river", then water should begin to filll it up. The rate at which it is filled depends on the size of your "river" and how much water was added.

Then, go out and dive, enough fooling around with pipes

Sludge and Me--A quick Google search seemed to suggest what Sludge said, and it makes sense: hydro=water related, static=not moving. I'm just wondering what aulic means...

[Sludge]

I think we're programmed to say "hydrostatic" because we use that word in reference to cylinder tests.

[JahJahwarrior]

A quick sketch:

As water table drops, flow goes down


On the right, you see flow reverse as water levels in bucket rise. It's hard to see but those are arrows pointing up the pipe, to indicate that flow has reversed...

Sorry for the poor sketch...it's what you get when I'm sitting in a science lecture

http://farm4.static.flickr.com/3642/...559a60.jpg?v=0

[Sam2000001]

That sounds like a fun experiment Gibby

[FW]

Hydraulic pressures, or the differences in them...a cave's flow will be determined by hydraulic pressure which is controlled in part by the height of the water table. If you raise the river levels so that there is more hydraulic pressure from the water outside the cave, than from the water inside the cave, the cave will stop flowing and reverse. The speed of water flowing in to the cave is similarly controlled by the difference in hydraulic pressure.


FW-- Sure it is sad from the "I want to dive!" position, but doesn't flow reversal play an important role in recharging the aquifers?

Only if you like the aqufier filled with dirty river water...

Good answer, though!