That's why I only have spillway only with 2 24" culverts and no drain. Sound like it collapsed in the horizontal run maybe. Plumbers have cameras that they run in plumbing lines underground sewer lines to ck for problems
I havn't tried going to the other end to check there. I wonder if there is a long flexible pipe that can be pushed from one end or the other. Sounds like a job for somebody that knows what they are doing, but my lake guy had no solution.
I would run a straight nozzle power washer up from the outlet. The end that the hose attaches to the gun can come apart. Take the end off the gun that changes out the tips and put it directly on the line. Add the straight nozzle or a roto nozzle and feed it in. With the line pressurized I should not turn around on you. The roto nozzle will cut up wood easy.
I think it likely that a piece of wood has mostly plugged the pipe, then caught a bunch of leaves and sticks to complete the plugging. The pressure washer is a good idea, or you could dry assemble joints of PVC pipe and push them in as you assemble them. That won't do a lot of good, though, if the plug is at the ell. Try ramming something in from the top. If you cannot unstop it, an external siphon could be installed to drop the pond level.
I think if the pipe was collapsed it would still be flowing some.
I had this same issue last year. I went and found the outflow and stuck a very long(30ft) pvc pipe and pulled out a mass of dried clumped up vegetation.......problem solved. Then, I put a wire mesh cap over the inflow so nothing of any size can get into the pipe to screw it up again.
Another vote for the PVC pipe. I used 3/4" pushed up the outflow and successfully dislodged something. I never did figure out what it was as an 8" pipe running full bore made me and the clog move in a hurry.
I had the same issue with the drain pipes for both of the ponds on the property I recently bought. I was able to unclog both of them by connecting sections of 3/4" pvc pipe ~ 80' total. At the head end of the pipe I drilled a 5/32" inch hole through both walls of the pipe and inserted a length of 5/32" tig wire. I bent the wire to parallel the pipe and then bent 2" "barbs" into the wire. I also attached some small rope just behind the wire and added 3 half hitches and secured the rope and the wire with duct tape. The rope was used to pull the pipe from the first joint in case it got stuck (which it did). I didn't want the pipe to come apart inside the drain pipe and make things worse. I then pushed the pipe in as far as it would go and gave it a twisting and pulling motion. I had them both cleared after about 5 minutes of work. This worked great for me, hopefully it can help in your situation.
This reminds me I need to get guards ordered for my overflow pipes.
In my sediment pond I have an overflow pipe running into my main pond. It clogged during a large rain event. I reached down to see what was clogging it (I would never do this with a large pipe sloping downward - could be dangerous. This pipe lies mostly flat). Felt like a rock or something.
Got a rod and pried around and finally a turtle popped up. Threw that turtle on the bank and it promptly crawled to the big pond. That helped but there was still a larger clog. Finally pried out a larger turtle. Looked dead, head hanging to the side. Threw it up on the bank. Couple minutes later it came to and also crawled to the big pond.
Had I not pried them out, they surely would have eventually drowned. I need to get guards. This pipe plugging is not really a big deal, because the emergency overflow is made to handle any really large rain as the pipe is not big enough by design (designed so that ordinary rains drain slowly to give time for sediment to settle out in the sediment pond).
But I would hate for my main pond to clog up like that and rely on the emergency overflow.
Had turtle block a drain from one of our wetlands. Got it out with a PVC pipe from the outlet side (it was dead). I added a closed cylinder of 2x4" wire fencing to the upper end and it seems to have worked to prevent more clogs. Did the same thing on a 12" culvert in a ditch crossing and captured a large carp coming upstream during a flood (headed towards my neighbor's pond). Unfortunately, his pond has already been invaded by carp.
This particular culvert is elevated above the ditch bottom to trap water upstream. The whole area floods during heavy rains due to many field tiles in the watershed of a nearby creek. I suspect the water could have been running at less than half the pipe capacity in a very flat pipe, but I was not there at the time the fish was trapped. The water could even have been running backwards though since this is very possible during flooding. The pond above is a groundwater pond with a small watershed. The critters made fast work of the dead carp, but the spine is still in the pipe months later.
I have goldfish in my pond, and as soon as there is a blockage, they are covered with spots because the filter can't work. Thank God, my friend is a plumber, and he helps me with a clogged drain pipe every time. I don't trust other plumbers, once one of them poured all the dirty water that was stagnant in the pipe back into the pond and some of the fish died... How do you deal with such problem? Also, tell me, if it is possible to settle koi carp in a small pond with goldfish? Will they live in peace, koi is still bigger than gold?