Ditch grade is too flat. Reset the pipe match the invert to the channel bottom. Regrade ditch to maintain correct flow. Fix pipe ends. Install maker posts or guardrails.
Water flowing through culvert is acidic.
Install a PVC sleeve or replace with a non-corrosive material. Piping around the culvert. Pipe is incorrectly installed. Reinstall pipe with proper bedding and compaction. Install a head wall.