How to create gravity pipe network from multiple polylines with specific depth

Civil 3D
1

create gravity pipe network from multiple polylines with specific depth

2

What have you currently tried and where are you having issues? Also i would suggest you read the following - How to get help on the Dynamo forums - FAQ - Dynamo

My thought is as follows:
You create or get the polylines
Change the Z value of all associated Points
Regen PolyLine
Create pipe network
Create Pipes from Polyline(EG Curve)