AutoCAD Civil 3D :: Model Existing Drainage Channel?

how one might model an existing drainage channel.

Here’s the scenario – field crews are going to collect data for a 500 ft long concrete (trapezoidal) drainage channel. The bottom width varies from 7 ft to 19 ft wide with vertical walls varying from 4 ft to 7’ high. The channel will be filled and capped.

I considered using one of the channel subassemblies with Civil 3d and establish targets along the existing channel top and bottom of walls. However, I don’t see that the stock subassemblies are intended for this and don’t really lend themselves to perform this. I suppose the other option is to use linkwidthandslope subassemblies and use multiple targets for inside bottoms, inside tops and outside tops of walls.