Feature Line Flow Labels

Q: How do you make Civil 3D display drainage flow arrows based on feature line slopes to assist in design?
A: Make a general line label with a direction arrow that knows which way to point.

Two expressions located in the general line label section of the settings pallete accomplish this.
You can past the following into expressions.  Make sure if you change the names that the second expression references the name of the first exactly.

#1 "negslopecheck"
{General Segment Grade}>0

#2 "Flip 180"
IF(negslopecheck,3.14,0)

enter the "Flip 180" expression into the Direction Arrow's rotation value.

Here are two feature lines.  You can see on the top that the feature line has two slopes.  The arrows point downhill. Please note that the label needs to be added per section.  The slopes and elevations shown on the screen shot are just part of a general line label.