How to draw "fall lines" on sloping nurbs surfaces?

Does anybody know of a (free) Rhino or Grasshopper tool, script or plugin that will draw fall lines on sloping and curved nurbs surfaces. i.e., starting from a point on the surface, draw a line that follows the steepest gradient downhill? FYI, I am not designing a ski resort, but want to try an experiment on improving filleting in particularly difficult circumstances.

TIA
Jeremy

I think there should be some GH definitions out thereā€¦ Do a search maybe. I think I did something similar a long time ago by sampling the surface over a predefined grid, then checking the direct neighboring grid points that surround each point and drawing a line from the sample point to the neighbor that was lowest.