"I'd also recommend you get the black plastic culvert that's smooth on the inside, yet corrogated on the outside. They don't clog as bad as the single layer corrogated metal ones"
Me too. It is called Corrugated Polyethene Pipe or CPEP. A very common brand is ADS it is their N-12 pipe. The CPEP is light, UV tolerant, easy to cut, easy to splice together with cheap connectors, and pretty much the cheapest out there. In my area, 12" CPEP is 8$ per foot. 36" is 27$ per foot but it is a petroleum based product like all plastics so the price might be volatile.
I had a deep ditch cutting off a corner of my property. I scrounged up a 36" corrugated metal pipe (aka wrinkle tin, CMP) and a 24" CPEP pipe and layed them side by side in the ditch. The pipes were extra, throw aways, and I was happy to take them. I can now cross and get work done. These ditches can be a big problem if you plug them. Upstream there could be a little old lady in her creekfront house that floods and kills her because your culvert was too small or plugged. It is very wise to overbuild the crossing.
This picture is when they were pretty new. The crossing was soft at first and has since really firmed up to carry excavators, log skidders, etc.
