Pond Boss Forums GENERAL QUESTIONS Help Siphon drain

I have a siphon drain that has not be siphoning for several months. Thinking it was a blocked inlet we hired a commercial dive team to come look. Before going into the water they blocked the outlet with a plug. Within 10 minutes of blocking the outlet the siphon kicked in and upon removing the plug siphoned as designed. We left the drain in siphon mode and when I returned the next day the pond was draining normally. After a heavy rain over the weekend the pond is two inches over the vent tube and no siphoning is occurring. Any ideas what might be happening?

stan

I'm not an expert (although I stayed at a Holiday Inn Express last night) but it almost sounds like your system is sucking air through the outlet rather than water through the inlet? I can't picture how this system would have been built to have that as a possibility, but blocking the outlet causing the system to start certainly points that way? Basic Fluid Dynamics principles would indicate that air draws easier than water - but again I don't get where in the system this would come into play, on a standard siphon. A leak higher in the system maybe, but simply the outlet hole? I don't get it.

Do you have a schematic of how the system was built?

More knowledgeable folks than I will certainly chime in soon....

I do not have a drawing, nor do I have photos. But the siphon was working well prior to last summer and I know of no changes to the system during or since that time. I wondered if a piece of the PVC had merely cracked for some reason allowing air to enter between the outlet, where we capped it, and the horizontal run of pipe through the dam. The system is a 6in PVC system with a 90 elbow at the end of the outlet pipe. But again nothing new that would explain the change since last summer that I know about.

There is nothing magical about a siphon pipe. Whenever we had problems it was either plugged up or air was breaking the vacuum.
Please keep us posted.

I am assuming that since the dive team plugged the outlet, and a siphon started after plug was pulled, that the diver did not find any inlet blockage??? That leaves only one other possibility....that air is entering the system. I also assume the "90" on the outlet is pointed upward and that there is a constant pool of water in the bend of the elbow that blocks air from entering the outlet down pipe? If so, I would start electrical taping/regluing every joint first before chasing cracks. The vent pipe is what I would inspect first and most closely as people/critters tend to pull on it. Had one pipe I guessed a deer tried jumping over and hit the vent, cracking the PVC and breaking the siphon.

Place something OTHER THAN YOUR HAND, like a strip of thin cardboard, over the vent tube inlet under the water to see if there is a considerably strong vacuum/suction....if not, there is definitely air entering the pipe. (cereal box cardboard will break down quickly once wet so no extra blockage worries)

Have been doing a little homework on your siphon problem. Not able to discover any thing new that would explain it not working,
Please keep us posted if you find out anything new.

Just to help get it straight in my own mind, you have water running out the pipe as the pond overflows the excess water, but the water is only flowing at a rate commensurate with a standard overflow pipe and not the extra high flow rate that would be flowing when the pipe enters siphon mode flow rate. Is this assumption on my part correct? Does that explain the current situation? You have flow but only standard flow rather than siphon rate flow.

SNRUB,

Sorry to so late responding, but I have been traveling. You are correct, normal drain at present even though water level is 2 inches above the vent pipe. And the outlet 90 does have water standing or as now, overflowing. I have examined every above ground joint and find no leaks. I wonder if the underground pipe may have cracked somehow? I will try the taping of all the above ground joints this weekend and will seal the vent pipe. Will report back next week. This has me baffled.

http://s1383.photobucket.com/user/stant7/library/

Try accessing this for pictures. Do not know if this will work, but thought it might be worth a try.

Taped every above ground joint, end of vent pipe is 2 inches underwater and NO suction. MUST be the main pipe, underground, is all I can figure.

Stant, the vent tube can be underwater, but the water level also has to be above the high point of the main crossover pipe going through the dam also before the siphon begins. Till the pond level rises to force out all the air in the pipe, it will act like a drain. Once a siphon starts, it will continue until the vent tube sucks air.

In short, your vent tube only controls the level the siphon STOPS, while the height of the top of crossover pipe controls when it starts.

Well said

To add to what Rainman said:

There are two separate purposes to your system that you possibly could be getting confused about.

The purpose of having the intake of your system towards the bottom of the pond is to draw water from the bottom of the pond instead of the top as a regular overflow pipe would.

The purpose of a pipe going into siphon mode is to increase the carrying capacity of a given size of pipe.

At first glance both of the above statements seem obvious. But where the confusion can come in is if the two statements are considered to be mutual. They are not.

First lets just consider a regular overflow pipe that does not draw from the bottom of the pond. For example, an ordinary overflow pipe that is angled downward through the dam can and will go into siphon mode once the water covers the entrance to the pipe and all air is expelled from the pipe. What causes a siphon is the pipe being angled downward and the effects of gravity. Gravity causes the water to increase in velocity. This increase in velocity causes the pipe to flow more water than what it would at a lower velocity. Therefore, the greater angle the overflow pipe is angled downward, the greater the capacity the pipe will have when it enters siphon mode. Think of sucking a drink on a straw from a glass. The harder you suck, the more liquid will flow. Only in the case of an overflow pipe gravity is doing the sucking. I have an ordinary overflow pipe (not bottom draw) in my pond. The NRCS guy figured the angle of the pipe and figured its flow rate at regular flow (when the pipe is nearly covered but can still suck air) and siphon mode (when the pipe becomes completely covered and is not sucking air). The flow of my pipe in siphon mode is considerably more than in regular mode.

Now lets consider a bottom draw system. It works exactly the same except where the potential to draw air comes from. In the case of a bottom draw system, the only place air can enter is from the vent instead of the pipe inlet itself as in a regular system. This means that as long as the pond is over full by less than the amount where water would cover the vent, the pipe will overflow in regular mode only. You may be expecting the system to operate in siphon mode any time water is exiting the overflow pipe. This is not the case. The ONLY time your system will go into siphon is when there is a big enough rain event to completely cover the highest point of the overflow pipe as well as the vent. Then it will go into siphon mode. As soon as water level drops to where the vent is uncovered by water, the system will suck air, go out of siphon mode, and enter regular drain mode.

The water will always draw from where ever the intake pipe is at. If it is a regular drain pipe, from the top of the pond. If a bottom inlet, from the bottom of the pond. Does not matter if the pipe is operating in siphon mode or regular mode. The water will come from where the intake is placed.

You really do not want the system going into siphon mode. If it does it means that there has been a rain event big enough that the overflow pipe size will not handle normal water flow rates and the system is essentially going into what could be called "turbo" mode, so to speak, to handle the extra water flow. Siphon mode could be considered to be stage one of emergency flow with the emergency overflow section of your dam being the second stage of emergency overflow. If your overflow pipe is regularly going into siphon mode it likely means either you are having unusually large rain events for your particular locale or worse, that the overflow system is inadequately sized for your pond and watershed. The pipe should rarely go into siphon mode. It should normally operate in regular overflow mode.

During normal rainfall events, your overflow should NOT go into siphon mode.

Hope I did not oversimplify this explanation. I just got the feeling there might be some confusion as to how a siphon system operates. And some confusion about siphon mode of an overflow pipe and where the water is drawn from. Either type of overflow pipe (surface or bottom draw) can go into siphon mode. Neither will do so until water levels exceed a height enough to keep air from entering the pipe.

Hope this helps. If I'm wrong, feel free to correct me. This is my current understanding of how a siphon works but I have been known to be wrong. I'm no expert.

Thank you, your response certainly makes sense. My concern was/is that even with significant rain events I am not observing the syphon flow that we experienced on previous occasions. In that we are not seeing overflow out the spillway, or any indication that the water level is changing by an appreciable amount I guess it is best to leave well enough alone.

Your explanation also makes sense in that when we capped the outflow the syphon DID kick in and work as designed.

We are scheduled to electro-fish this Friday if the rain lets up so not worrying about the drain is a real relief. I do appreciate everyone's help and advice. Thank again.