I read with interest the recent conversations initiated by Bob Jordan about

his (potential) siphon in New Zealand. I read this, of course, because I

have a siphon problem of my own. Unfortunately, the previous thread's

discourse didn't solve my problem, so I'm hoping "you" can give me a little

direction. My problem is considerably simpler than Bob's, but I'm still

stumped.

My project is discharge from a reservoir. Way back when the dam was

constructed and a creek cut off, the owners installed a siphon pipe to

provide continuing flow in the downstream creek. It consists solely of

6-inch galvanized irrigation pipe laid over the top of the dam and three

gate valves for 'charging' the system. Total head differential is less than

30 feet at an elevation of around 500 feet. It's worked fine for the last

50 years, but that didn't stop a regulatory agency from requiring its

upgrade. That's where I came in. I've never designed a siphon before, but

I thought, Heck, it's

___just___Bernoulli's equation, right? (Well, to be

honest, I thought, Hmmm, I remember something about Bernoulli; I can dig out

my undergrad hydraulics book.but I digress.)

In all the years of operation, the siphon has never been lost and no one has

complained about the quantity of water, so they want essentially the same

system re-installed with the engineering to back it up. My particular

wrinkle is that the pipe outlet is not submerged.

I need to calculate the flow rate they are

***actually***providing (to prove

they are meeting the minimum under all operating reservoir levels) and I

need to calculate the maximum negative pressure inside the pipe to verify

the pipe class selection.

All my textbooks, handbooks, and online literature research assume either

the pipe end is submerged and/or that the pipe is flowing full. Since my

outlet is on a slope (as opposed to pointing straight down) with a free

outfall (the pipe invert is at least 6 inches above the downstream water

surface), my flow area is not the full pipe diameter.

So.I can calculate outlet velocity from Bernoulli's, but I don't know the

area to find Q. Should I use a culvert or orifice equation? Is that

appropriate since I have a negative pressure upstream inside the pipe? How

do I apply it?

And speaking of that negative pressure, it seems like I have too many

unknowns. Velocity and area are not constant through the pipe, but I can't

see how to calculate them while also not knowing the pressure.

Am I missing something just totally obvious here? Would a few more details

help (like elevations, lengths, equations, etc)?

Let me know, and my thanks for any and all assistance!

Selene

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...make IT go away...to email me, that is