The first pic is of the siphon tube. At this point, the end is still fully submerged. Note I now have a screen over the end to keep the turtles out. Right next to the siphon tube is the drain tube; not sure but looks like 8". It runs up through the dam and down the embankment to the receiving stream. I see no indication of any access to either of these pipes anywhere between the edge of the pond and the receiving stream.
The second pic is of the drain releasing water into the receiving stream. Note there is a small amount of water coming out; I estimate 1/2 deep stream of water. Am I wrong in assuming the flow should be full-on as long as the end of the siphon tube is fully submerged? I am wondering if the entry end of the 8" drain pipe has clogged on the bottom of the pond? Is there a way to tell? To clean?
Yes, you are correct, with the vent pipe under water, your siphon should be absolutely gushing water at the discharge. I suspect you either have a blockage, OR there is a crack somewhere in the siphon pipe or a joint that is allowing air to enter (and break) your siphon.
If it is blocked, then the best way to test is to cap or plug the discharge end and do the same to the vent pipe inlet. You then need a water source to fill your siphon pipe from the top. If it starts overflowing, then you almost certainly have a blockage on the inlet side. (If you only have a partial blockage, then you may need a high rate water source to supply water at a faster rate than the partial blockage allows water to go out of the inlet side.)
If you can't fill the siphon pipe, you need to then take off the outlet cap while your water is still running. You should get a massive gush of water as the inclined portion of the siphon pipe releases the held water, but then the flow should go down to the exact rate of your incoming water supply. If it is partially blocked, then you should see a slower response.
I have poked and prodded the top of the dam thinking there may be an access valve at the highest point to clean out the drain pipe and to use to fill the pipe to initiate a draining?
Look at the tee where the vent pipe is connected to the siphon pipe. Usually there is a cap or plug on the side of the tee that points straight upwards. That should be removable and provide you access to try the steps I described above. If not, then you will need to re-pipe your vent piping to provide access. 2" PVC pipe is still cheap and easy to get perfect seals on your glued connections. Do NOT do anything that involves changes to the 8" pipe unless you absolutely have to in order to fix your siphon system.
If you do find (or suspect) a blockage, then the best thing to do may be to cap your vent pipe and let the pond drain down to the level of your siphon inlet. You can then wade in and try to remove any blockages. If your inlet is really deep, then you probably need to drain the pond as much as you are comfortable doing to make your blockage clearing job easier while still protecting your fish. DO NOT GET YOUR HANDS OR BODY NEAR A FLOWING INLET PIPE! If you create a seal, you are not strong enough to free yourself when there are even a few feet of water over the pipe.
The 3rd pic is of a "mystery" box I found about 10' from the end of the drain pipe, and only after finally getting the dam clear of years of growth.......trees up to 10" diameter......yikes. My dog actually found the metal lid under years of leaves. Lo and behold, there is plumbing inside. Not for sure, but I think it is all one line. The open pipe with a "flag" welded on appears to be a valve. Quite rusty, so a little afraid to try to turn. Down from that (an inch away) is a boiler drain. Again, appears to be closed but I am a bit hesitant to try to turn. Finally, 6" down from that is a spigot. This will turn, and is presently closed. I have no idea what any of this is. Can someone help me understand what all of this is, and how to operate it to make it do whatever its intended purpose is? I sort of wondered if this valve closes the main drain pipe to make it easier to fill the pipe in order to start a siphon?
IMO, that piping arrangement is too small to be any type of drain. It looks more like a water supply line? Is there any kind of tree planting, garden area, etc. in the vicinity where someone could have attached a hose to that piping for watering purposes?
I have what is to me a critical situation here. Every overflow is causing serious erosion. I need to make sure everything is working as designed.
thank you in advance for the advice! David in SC
Yes, water going over the dam is probably the worst thing possible for the well-being of a pond! Keep working to fix your existing siphon system!
I don't know if your siphon system was large enough to protect your pond from overflowing during Helene. However, it should have been designed to pass a 3" rain without the dam being topped! If it is an older pond, and the dam did not previously get washed out by multiple overtoppings, then I think your siphon system has been partially disabled fairly recently.
One emergency solution would be to buy some 6" flexible corrugated pipe at your local big box store. (It is cheapest at the Menard's near me.) You can create a siphon by capping the discharge with a painter's plastic drop cloth and duct tape. Use a water source to fill the inlet. When it is full, carefully place it in the pond, and have a buddy immediately slash the discharge end plastic with a knife. It should start a roaring siphon. You can either do that before forecast rains and draw down your pond a little. Or if you get quick about it, you can do it when your pond level is rising towards the top of your dam.
Good luck on saving your pond!
