Wow, a 4' diameter culvert. That is a big outlet.
Is there a small creek that runs into your pond? Or do you have a gigantic watershed (drainage area) for the pond?
Why is the state involved? If your rotten culvert collapses, there is a very high probability that the dam would wash out and send all of the pond water volume downstream. Are there any humans or habitations at risk if that happens?
Sorry for the bad (apparent) news.
I am NOT a dam expert, but I believe that it would be very expensive to replace a culvert style outlet of that size.
If you had a 10" diameter outlet, you might be able to block the downstream end, fill it all with cement to remove the collapse risk, and install a siphon-style water outlet to handle your heavy rain events. That would be prohibitively expensive at the apparent sizes that your pond dam requires.
How large is your pond in surface acres? What is the average depth? What is the depth at the deepest point? Is there an emergency spillway?
If you add that information to your post, then hopefully some of the experts will chime in and give you some helpful advice.
P.S. You should probably go back and delete your other post, so all of the responses are consolidated under this "active" post.
Good luck getting your pond back into good shape!
