The surface area only drains 2-3 acres plus a 4in tile. Have watched in storms. That is why I am thinking 12in line will handle the vast majority. This has been 50?+ years of nothing done. It was created when the Feds took land for a lake and cut this spot for a new road since the old path would require some really good snorkels. We installed some diversion ditches years ago. This area has nowhere else to go.