Without seeing a layout the minimum would be 36inch. Also design rock wall or conrete wall around pipe inlet to prevent errossion of incoming water removing soil from around pipe and design the inlet sump to channel water directly into pipe. If you have the room design inlet sump to retain water if there is a backup so it does not flow over the road.
Good Luck


Scott Trava
Catskill Pond
http://catskillpond.com
[email protected]
Returning Catskill Waters To A Simpler Time
EST. 1923