Looking at the agridrain site, I envisioned it working in the reverse direction so that water would flow out of the spillway, not into it. Besides being more prone to clogging with debris, is there a good reason people don't do it this way?
Best thing to do would be to call the manufacturer to get their opinion. With it on the back you will have more erosion with water spilling over the top and falling down. That could could be fixed with mounting the riser on a concrete base and placing rock all around the base so that the water doesnt fall on to dirt.