Call Mike Otto (940)665-2258.
He's repaired many corrugated pipes, some of which he installed 25-30 years ago.
There are several approaches he takes.
One is to pump the rotten pipe full of concrete and improve the emergency spillway and leave at that.
Another is to run pvc pipe into the corrugated pipe, pump concrete around it and the pvc pipe becomes the new primary spillway.
Another is to dig up and replace the rotten pipe.
Another is to dig into the emergency spillway, put a pipe there, along the entire length of the emergency spillway, add anti-seep collars, seal around it and bury it...and you have both the overflow pipe and emergency spillway.
Another is to build a siphon pipe, install it at water level through the dam and concrete the rotten pipe.
What you CAN'T do is leave it as it is.
I'd call Otto...he's "been there, done that".
