View Full Version : Hydraulic engineering question

J. Dillon
04-23-2007, 08:23 AM
During a recent visit to the Sacred valley near Cusco Peru the guide demonstrated a hydraulics engineering event. ( sorry no Pix, lost due to a screw up)

Picture a pool of water about 6' across and three feet deep cut into solid stone and constantly fed by water from the near by Andes mountains. From this pool at the top is a spout of stone about 3 to 4 inches across and 3" high projecting out from the pool with water constantly cascading out from the spout and into another pool beneath it of similar size. The velocity of the water is constant and I guess about a gallon in 20 seconds.

The guide drags her index finger across the mouth of the spout of the upper pool and immediately the water flow diminishes by about 50 % and in about 40 seconds or so resumes to full flow .....Why ?:confused:


Rick Starr
04-23-2007, 09:13 AM
Is that Tipon? If so I've read a little about it and it sounds fascinating!

Welcome back--your trip sounds wonderful!

Uncle Duke
04-23-2007, 09:17 AM
My guess, and just a guess.... there is a second outlet which is a "pending siphon". When you slow the primary flow even a little it backs up the level just enough that the secondary siphon outlet starts. That has higher flow and quickly lowers the upper level just enough that the primary now has less to deal with. At some point (30 seconds in?) the secondary 'inlet' (to the siphon) is exposed, the siphon stops, the level rises, and the primary outlet resumes its normal flow.
Just a guess though....

J. Dillon
04-23-2007, 09:59 AM
Mat , I guess my description is lacking. :o You're right the walls of the spout were 3" high but the flow height in the spout was about 1/4" to 3/8". But the delivery rate of the water was about a gallon in 20 seconds. Still a reasonable flow rate.

Uncle Duke, You're theory sound plausible maybe some kind of siphon effect, sounds good.:)

Is surface tension a consideration in this ?:confused:


capt jake
04-23-2007, 10:11 AM
Is surface tension a consideration in this ?

That was my thought also.

Uncle Duke
04-23-2007, 11:14 AM
I'm trying to play with some numbers here, but all I'm doing is confusing myself. I'm having a tough time, though, seeing how surface tension could make a difference, unless you were temporarily lowering the surface tension of a concave meniscus or raising the surface tension of a convex meniscus - and I can't see how altering an already existing flow would do that.
Smarter people than me better log in here soon...:D

George Roberts
04-23-2007, 11:30 AM
I like the siphon idea.

In any case I suspect it is a trick.


There is that traditional equator/drain/water trick.

What is life without tricks and magic?

04-23-2007, 11:38 AM
because you are amazed doesn't mean i am

Uncle Duke
04-23-2007, 12:03 PM
Matt - I understand that the numbers are not huge (11.5 cu in per second) and that a flow like that can be disturbed. I think that the key mystery here is why the effect lasts so long after the finger is past the spout.

04-23-2007, 12:13 PM
Her finger is really a sponge.

04-23-2007, 01:54 PM
I recall having it explained at one time, although I sorta remember that it was created to control water flow for the constant irrigation of the terraces. It seems to me that the pools were a series of shallow pools a couple/three meters across and a meter and a half front to back....with stones somehow creating a siphon. It was something truly amazing to see. and there are mini machu pichu's around the area also with the same agricultural terraces.