...Seriously, I'd be very hesitant to partially block that drain. It would strain the surrounding soils holding it in place.
That was my thought as well. Moving water is consistent, relentless, and predictable. The two erosion areas near the 5' pipe inlet would concern me, and I'd look at their impact on a partially blocked pipe that raises the water level immediately around it. Looks like eddy's may form there and create another potential problem.
Keep us in the loop, interesting scenario.
