Hydraulic Lift Tubes

Hydraulic lift-tubes in caves:

Hydraulic lift tubes are passages where water flows 'uphill' from lower passages to higher passages, due to hydrostatic pressure. In caves, these passages frequently have a fan of well sorted sediments radiating up and away from a constriction. (picture here at some point) If water velocities are high enough, sediments in a cave passage can be suspended and transported. The classic lift-tube fan is formed as high velocity water exits a small passage and enters a larger passage. At this point, the water velocity drops to the point where some sediments can no longer remain in suspension. The size of the sediments dropped out at this point is related to the water velocity. For example, as velocities increase, the size of sediments that remain in suspension increases. In caves, there often are flow-metering points or other factors that apear to have some control over the velocities at many lift-tubes. These controls limit the velocity and allow similar sized sediments (often small gravel) to remain in place and effectively be tumbled and sorted during every flood event. In some lift-tube fans, the gravel is composed of highly polished pieces of chert, or other relatively insoluble rock. This is a good indicator that the maximum flow velocities have not changed in quite some time. If they had, the gravel would not be well polished.

Ok, now that I've rambled on about what a lift-tube is, I should explain how and why I became interested in them.

Some lift tubes have carried much more than small gravel through them. An impressive example of this is found in Aqua Cave in Highland County, VA. The cave is the downstream end of a major karst conduit network draining a significant portion of the Burnsville Cove and the entrance is a large spring or resurgence. There are two sumps in the cave: one at the upstream extent of the active stream and another which is normally a static pool known as French Lake. The passage leading back to the sump at French Lake is a large lift-tube that only overflows during large flood events. Several years ago, I noticed that there are huge (up to 4 feet in diameter) sandstone boulders that have been transported up this passage and deposited in a room at the top of the lift-tube. Realizing that this is a bit unusual, I wondered if an approximation of past flood events could be obtained by combining calculations of flow required to move these boulders, water-level/stream-gaging of the spring and sump during flood events, and rainfall data.

At this point, the only thing I can say for sure is that the flood required to move these boulders is many times larger than the largest floods ever recorded for this region. The largest recorded floods have not come close to producing the flow volume (and velocities) required to move the boulders. Some preliminary calculations on what would be required to move them can soon be found here. (Thanks to Dr. Art Palmer for his constructive criticism and comments on these.)

Data is currently being collected that will reveal much more about how the cave stream and the lift-tube sump react to flood events. More on this later.