As regards your emergency overflow spillway: Most emergency spillways are routed around a corner of the dam, or at least away from the primary spillway. The main reason for this is in the event of serious erosion in the emergency spillway, the damage will NOT cut through your valuable structures or your dam core. Repairs are then much simpler after the damaging flood event subsides.
If you are still planning to create an emergency spillway low spot, I would design it to slightly be offset from your existing culvert, but still easily connected to your improved drainage ditch.
Good luck on your project!
