AutoCAD Civil 3D :: Sloping Stage / Storage Curve

I see Civil3D has a Stage-Storage curve for determining volumes for incremental elevation increases for a bounded area (e.g. pond). This works well for water where the top surface is horizontal, however, i need to consider a sloping surface to duplicate a slurry discharged into a lagoon. This has a slope of 1% from the point of discharge and how i can determine the volume/height relationship considering this slope without having to do multiple grading and surface creation. The discharge point gradually moves upstream as the filling occurs to prevent the pipe becoming blocked by the slurry.

I thought about creating a 1% surface and incrementally raising it making it a comparison surface, but the 1% slope is not in a planar form, it is 1% from the discharge point in a radial direction.