Pond Boss Forums GENERAL QUESTIONS Help Siphon drain

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.