Pond Boss Forums INTRO Questions & Observations Ok, I dont get this siphon stuff

I have a pipe inside my pond with an inlet at the bottom (AKA - the pipe is open on the end). The pipe goes up the inside and thru my dam at the fill line, then across the dam and down at a slight angle, to the backside of the dam where it angles sharply down to the bottom of the backside of the dam and exits there. The exit is lower than the entry point inside the dam.

In this scenario - wouldn’t gravity (vs. siphon) cause the pipe to drain once the water level in the lake hits the bottom of the highest point of the pipe?

Additionally, I have a siphon hole at the top of the pipe where it angles back inside the dam. As far as I can tell, there will never be an actual "siphon" unless I plug the hole and create one myself.

My point is that you could install a simple bottom draw drainpipe in this configuration and forget about all of this siphon stuff (as long as you have some type of vaccum breaker)…right?

Wrong. A siphon is only a siphon when the air vent gets closed off by the high water level in the pond. Until this happens, as you say, the water will just trickle through the pipe when the level is at normal pool. But when the water gets high enough to close the air vent, watch out because a siphon can flow almost 5 times as much water through the pipe as gravity can. Once the siphon lowers the water and uncovers the vent, it goes back to a gravity feed again.

I see what you are saying, its a garvity drain until the water level rises above the shut off (valve or breaker), then it becomes a siphon until the level goes back down. Got it, Thanks.

Jersey, I gotta ask. How is it that you figure a siphon can move water faster than gravity. A siphon is totally run by gravity and its potential can only be the difference in the height of the water from the surface in the pond to the outlet of the syphon. Maybe I don't understand what your system looks like and the placement of this air vent. It is true that an air vent at the highest point of the syphon tube will make a pipe no different from a spillway until the water covers the vent. Is that what you mean? If so, then it is true that the syphon can result in much higher flow because the height difference between inlet and outlet for a spillway may be only a fraction of an inch while the end of your syphon tube could be many feet below the water height in the pond. I'm new to pond management and my pond does not have a dam or overflow need. It would seem that what you describe would be a good way to accomplish overflow that is just a trickle when pond level is relatively close to what you want and then automatically goes into syphon mode so that flow goes up sharply if the level suddenly gets too high. Ingenious, but why do you do this unless you need to have the pipe inlet on the pond side always below the water surface but don't want to drain down below the highest point of the pipe. Is this a common need for ponds?

bz,
You are correct that a siphon is totally run by gravity. When I said that a siphon would flow more water through a given pipe size than gravity could, I was comparing a siphon to a standpipe system. You are again correct that this is because of the added head pressure that a siphon system has, and a standpipe does not.

Why do I think this is better? Well, my yet to be built pond will be on a small spring fed creek. Most of the time my two 12" siphons will be acting like a spillway with the water just trickling through. But when the terd-floater storms come through and the water rises, it will go into siphon mode and hopefully be able to handle all of my 320 acre watershed before it gets up to the emergency spillway, also known as my driveway. I don't like the look and constant noise of a standpipe and all of the trash that collect around it.

More good reasons for a siphon...It draws water from the bottom of the pond, pulling the oxygen-rich water down from the surface. It doesn't require a pipe through the base of the dam where a leak is expensive at best and dangerous at worst. If I ever need to drain the pond, all I have to do is cap off the air vent and wait about a week.

The flow potential of a syphon is determined by diff. in ht. of pond side pipe inlet and how low the pipe outlet is below this behind the dam. it can be several times the flow rate than gravity drain alone. It can suck out the 'bad' water and muck if placed near bottom.

It's amazing how much water a 4" pipe can siphon with only a 2 or 3 foot head. I think it could drain off a 10" rain from my 1.5a pond in 2 days or less. Right now I have to hand start the siphon with a hand bilge pump, but will add a vent with a flapper when and if I ever complete my pond.

Jersey, I think this is a brilliant idea, simple, elegant, and run by water level and gravity. I'm very familiar with syphons but never thought about how a vent in the top could be used to cause it to go into syphon mode when desired. Without the vent it will always be in syphon mode until the water gets too low then you'd have to start it by hand. Burgermeister, I think what you mean is the syphon head is the height difference in the pond level and the syphon outlet, not the difference between inlet level and outlet. The inlet could be at same level as outlet or lower and it will still work. Sometimes it hurts to think about how something even so simple works if I think too much.

It would still work, but the greater the inlet and outlet height difference,, the more suction. There is a formula, but dont have time now. Will post it later.

It has to do with acceleration.

bz is right again. The pressure is determined by the difference between the pond water level and the siphon outlet. It would work just the same if the inlet was just below the surface. But you put it at the bottom for obvious reasons.

Look here -->

SIPHON SYSTEM

Jersey, in your example you are correct because that outlet end is not much lower that inlet level. This is due to the pressure of the volume of water is helping with the outflow. BUT, I will guarantee you that if you lower the outlet hole significantly, the flow will increase dramatically because thare will be much more water inside the inclosed system.
I my work, I have syphoned many ship gyro containers filled with liqued to do an annual overhaul and the higher I can get the container above the end of the syphon tube the quicker it empties.
I think we all are correct. the pressure of the water in the pond is relevent, also the height difference of inlet and outlet.

OK, you're right. My 2 reference heights were incorrect. they should have been the height of pond level and height of drain outlet.
Suffice it to say, since we can't manually manipulate the height of the pond, if we get the outlet lower, we can get the pond drained faster to avoid disaster during a flood.

Yep, I will put some effort into getting the outlet of my siphon as low as possible since I will only have 14' of water at the dam.

I think I am going to add another 12' section to my drain outlet, then turn it up and see how far I can "shoot the water" when the siphon kicks in.

Actually I've seen recommendations of turning the outlet 90 deg. vertical to help prevent air from gurgling back up the pipe & stoping the siphon.

My only complaint of a siphon is when used to drain a pond for some reason (which happened to me at least twice) the siphon stops it is very difficult to get started again with the water level down in the pond.

I have one of the syphon systems like shown at http://ponddampiping.com/syphon1.html

On three occasions, the air vent got clogged with trash and with a heavy rainfall my pond has had an accidental drawdown.

I put a screen around the thing to trap trash.

Frank

re: extending drain pipe, just remember that 33 ft is the maximum from crest of system to drain point for water, or else you will lose the siphon.
This is because the weight of water in pipe will exceed the atmospheric pressure on the pond.

If I understand siphons correctly, the 30 to 33 ft limit (literally, one atmosphere of pressure in feet of water) is how high you can lift water on the upward portion of the siphon (entry point to crest). Is there really a maximum limit on how far the water can drop going from the crest down to the exit (minimum limit would be that the drop has to exceed the climb)?

Theo, you have uncovered more of my fuzzy math. The crest to inlet ht. must be less then 1 atmosphere. Maybe I should stick to 1s and 0s also. Or at least to electronics. Anyone have an RF question about their pond?

Hey, at least I am perpetuating an interesting discussion. This ws a tough weekend, mentally and physically.

Ditto tough weekend here. The well/pump that feeds our house is acting up - pump won't deliver over about 55 psi without (somehow - pressure safety switch using wrong value?) shutting itself off (fortunately horse barn and water are a separate well; cattle drinking pond water from hydrant below the dam). Then it turns back on when it feels like it. We put in a new P-switch and have (probably) ruled out electric supply problems; I currently have things running in a reduced pressure range (42-50 psi vs usual 40-60 psi) until we can assemble all possible spare items we could use and then pull/replace the pump. All in about 400% humidity

Theo and Burgermeister,

- So the longer the outlet = the greater the siphon. right?

The bigger the drop from crest to outlet, the more force/pressure/siphon it will discharge with.

Of course, I have no experience with pond siphons, just a little with gasoline and a lot with diesel fuel (Ptuuey!).

No, the bigger the drop from pond water level to outlet, the stronger the siphon. The crest height does not matter as long as it is no more than 33' above the water level.

I would guess that at some point with a very long drop pipe , the resistance of the pipe itself would come into play. But I can't see this situation in a pond siphon system. Just like I can't envision a dam that is more than 33' above the pond.

Does anyone have any pictures of their siphon during construction?

Pardon my imprecise use of nomenclature. I was implying that it's impossible to make a bigger drop from the water level to the outlet without lowering the outlet wrt the crest. But it is of course the net difference between the inlet and outlet elevations that drives the siphon strength.

I don't think anyone but Governments can afford enough dirt moving to make the top of the dam 33' above the desired water level.