They are called ditches and periodically (every 10 years), a road crew will come around and dig them out. What every you plant close to a ditch may be removed or damaged when the ditch digger comes around.

I suspect a willow loosing a few roots won't be too harmed, and every hunk of root yanked out shall probably grow somewhere downstream. LOL. There are various willows suitable for hedging you know. Arctic willow coming to mind. If you have the room, just let them rip without shearing, they'll be tall enough for privacy and quite hardy. I have goat willow planted in a situation much like your's. It's a bog area where one of my springs runs off and very near drainage ditches maintained by the county. It is also about the same proximity from a septic field and I have had no issues whatsoever from it and it's been there years. It's a smallish, but very full tree and the roots are plenty far away from the field at that distance.

Wondering if dabled varigated willow hedge planted roadside will damage new drain culverts.