I posted here a couple months ago about how our community 3-acre pond drained when the 13-year-old metal control gate at the bottom of the riser failed. This board helped me discuss the siphon option to replace it. I am just a home owner, not a contractor or expert in this like you folks.

My question now: We have a metal 42" riser going into the 42" culvert at the bottom of the dam running under the dam with a road on top and emptying into a stream. Can we insert something like a 30 inch PVC riser pipe with a trash catcher on top that ends in an elbow running inside the existing 42" metal culvert at the bottom of the pond. The overflow water (We get about 20 gallons per minute from several natural springs flowing into the now empty pond)would then flow into the top of the PVC instead of the control gate at the bottom. Our PVC fix would be entirely inside the existing pipes. Does this would like it would work? I've got a drawing I've made, but I don't see how to attach the drawing in this forum.

Check with NRCS to see if the 30" pipe will handle the water that will enter the pond in a 50-100 year rain event. Then and only then if they say it will, would I take that idea further.

30" dia PVC isn't light. How would you put sections together, shove under the road to the other end, and then life the 30" vertical section to put in the elbow? You talking about gluing it together or using rubber gaskets and belled end pipe?

I think this is a bigger job than your usual DIY project.....

Once the PVC is installed in the existing pipe, I'd look at filling the 6" void around it with concrete to stabilize the whole thing.

x2. This is a job for someone with heavy equipment to handle the pipe. At a minimum, a standard loader/backhoe to set and ram together the horizontal and a small crane to set the vertical. The 30 inch PVC is usually used for main water or sewer lines and comes in 20 foot pieces with bells and gaskets. Why don't you put 36" PVC inside the 42? I don't think you could even get a 30 inch ell inside a 42 inch pipe. The radius would be too great. I think you will need to remove the riser part and sleeve through the horizontal part, and pack it with concrete around the new pipe.

Get dimensions of the volume you're dealing with for doing the 90 degree turn. Those are often done with a precast concrete structure with holes 90 degrees apart. And hdpe or concrete piping would be more viable than 30 inch pvc.

But likely you'd be better off with the entire riser being concrete, either a manhole or a catch basin. Both can be fitted with trash racks easily. How tall would it be?

Do you know your watershed or other inlet characteristics? A 42 inch outlet pipe is _huge_ and has me thinking this is somehow a lot more serious than most things discussed on this forum.