Applying script to a gradient

I've got a grasshopper script that branches out from a starting point, however I want to use it to resemble water flow on my 3d site model and therefore need to apply it to the topography, but have been struggling to achieve this. 

I've attached the grasshopper script and link to the rhino site model that I want to apply it to is below.

https://we.tl/7jdKMJjOim

Any help would be gratefully appreciated!