I’m wondering if they had been filling it for a while, and it had built up a plug of gravel sitting over a void of water (hour glass shape), and that last bucket of gravel was enough to break through the pinch point and release all the gravel into the water void.
Could this be a natural spring that they are tapping into. Before the gravel goes in the water is just gradually coming up the sides of the pipe and seeping into the surrounding earth, but when the gravel is put down it blocks the spring water coming up anywhere except the pipe?
Basically yes. It's a airlift pump, but acting in reverse. Instead of air pushing water up, it's gravel pushing water down. The water gets pushed down the outside of the pipe, deposits the gravel, and then the gravel-free water flows up the interior of the pipe.
Those pumps give you quite a high flow rate so long as there's not much head pressure. So I'm not surprised at the amount of water we saw.
I’m wondering if they had been filling it for a while, and it had built up a plug of gravel sitting over a void of water (hour glass shape), and that last bucket of gravel was enough to break through the pinch point and release all the gravel into the water void.
Could this be a natural spring that they are tapping into. Before the gravel goes in the water is just gradually coming up the sides of the pipe and seeping into the surrounding earth, but when the gravel is put down it blocks the spring water coming up anywhere except the pipe?
Is it water that is given velocity by the friction of the gravel that is falling through it?
Pressure at the bottom increases due to the water and gravel wanting to travel down. Then the path of least resistance is the interior of the tube.
Basically yes. It's a airlift pump, but acting in reverse. Instead of air pushing water up, it's gravel pushing water down. The water gets pushed down the outside of the pipe, deposits the gravel, and then the gravel-free water flows up the interior of the pipe.
Those pumps give you quite a high flow rate so long as there's not much head pressure. So I'm not surprised at the amount of water we saw.