what are usfs specs regarding culvert fall? How much drop per ft? 2" in 12"?

Answer 1

You will have to find the appropriate design standards for the forest service. But, 16.7% is way too much slope for a minimum. I would think at least 2% would be fine for most applications, but it all depends on the size and type of culvert, entrance and exit conditions and expected peak flow.

Answer 2

Culvert width=1.2(channel width)+2 ft
Culvert slope<1.25(channel slope)