AutoCad 3D :: Create 3D Model Of Segment Of Jogging Path / Trail

I need to create a 3D model of a segment of a jogging path/trail that has a drainage feature called a knick. I pasted a freehand drawing of what a knick looks like below this post. It's a semi-circular area that slopes to one side at a small angle on a trail that is otherwise flat. I need a model of a trail segment with two of these sloping indentations that is just a smooth, gray object- nothing fancy. I managed to create a simple 3D model for a second trail design that has a slope to one side throughout the trail- made a box with a tapered edge and rendered it gray.

Since to my eye a knick looks like a lemon wedge indented into the face of a box- that's what I've been working towards. Made a box, made a cylinder, moved the cylinder so half of its base sits on top of the box face on which I want the indentation, and then used pushpull and subtract to take out a shallow semi-circular cut off the top of the box (did two of these about 20 units apart on the same face).

Here is where I ran into my own limitations. I don't know how to taper the semicircular cut so that it actually slopes off the face to the edge and doesn't look like a clean cut depression. I need it to slope to the edge at a 2% angle. I've tried tapering many times, but I don't think I completely understand the concept of the tapering axis (or rather I know that I completely misunderstand it) because all I end up doing is tapering the various faces of the box and the knick cut stays the same. I also tried out all the tools on the ribbon that look like they can be used for filling in or smoothing edges, but without any luck. Or should I be approaching the process of making the knick from a completely different set of commands?