re: Update- Pond Outlet Clogged... Construction pics added on page 3

That’s honestly a terrible design


My fish ponds I use a 10 in pipe with a screen that stands up inside the pond and on the discharge side an alfalfa valve on bottom and a riser pipe that’s built to maintain a specific water level

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You should not have to remove all of the slats all the way down from the bottom, just enough to get far enough below the current water level

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This, pressure is your friend, and rent a drain auger from Home Depot and put in down the intake side, it’s most likely organic and this will break apart anything in there. That cut into the side of the intake pipe also gives you a good angle, so you may be able to just try and push the clog through with with a long pvc pipe or some rebar.

My comment above about using pressure by removing several of the boards assumes that the obstruction has allowed enough water to pass such that the drainage side of the obstruction is filled with water.

If so, you need to get rid of that water, and just wait.

If not, then that's one hell of an obstruction

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You are absolutely going to have to get those slats out and put some head pressure on the drain.

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This. Then I'd take a long piece of rebar to locate the interior of the elbow, slide a long enough piece of pvc pipe over the rebar into the elbow which will allow you to guide and run a rotor rooter into and through the elbow. Pull out the rebar to give the rotor more room to cut. After that elbow and remaining pipe is cleared I would run the correct size pvc with a pvc coupling straight down the smaller pvc pipe (using the smaller pipe as a guide) and just press it into place over the elbow. Cut this larger pipe to a length that is necessary to maintain the pond depth. Then just pull out the smaller pvc pipe leaving the larger pipe in place over the elbow. Put a screen over the top of the larger pvc to keep debris from accumulating around the new intake. I'm betting that pond sediment and leaves are the main culprit. A GoPro underwater camera or one of those newfangled bass boat fish cameras to see what you are dealing with and doing will be worth it's weight in gold. Just my opinion. Good luck! Following this thread.

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seams in the corrugated pipe leaked air and killed the siphon

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Probably what's happened to us.

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You are absolutely going to have to get those slats out and put some head pressure on the drain. Then I would get in a small boat and use some rebar or metal conduit to poke around and find the intake and maybe, if you're real lucky, you can get some water moving. If it ever starts flowing it will probably clean itself out with head pressure on it

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You need to understand the fluid dynamics here. For so long as the level of the slats is at the level of the water in the dam there is no pressure available to force the clog to move. Removing the slats changes the head pressure on the clog and as the slats are removed the pressure against the clog will increase. When there is sufficient elevation difference between the level of the water in the dam and the top of the slats, the clog will likely clear itself.
For each foot of elevation difference between the water surface in the dam and the top of the slats at the outlet, the water pressure on the clog will increase by 62.4# per square foot of inlet size. Removing the slats down to five feet below the water level in the pond (for example) would increase the pressure on the clog to 62.4 x 5 = 312# per square foot. Removing 10 feet would provide 624# per square foot or 43.3 psi which is probably similar to your household hose bib pressure.
It may still be necessary to do a little work on the clog with a rebar if it includes sunken tree branches etc. but the pressure alone should do the job.

Wonder if a diesel air compressor and a hose attached to rebar or conduit fished down that vertical pipe could blow it out.