I am not completely picturing what you have but I think I see it, is there a way to reconfigure the whole inlet, like say maybe removing the vortex cap and redoing the the top of the (I'm assuming you meant 13") vertical pipe? we had a similar problem with a CMP coming in thru the dam at an angle for the overflow and it was cut off too close to the dam and the beavers would pile up mud and sticks and plug the end off, we would come by semi regular and dip it out with a mini excavator which was a pita. after several near catastrophes from water getting too high, we extended the over flow on out another 10" into the pond which resulted in the end being way up off the bottom, too high for them to build it up to it, or at least it hasn't occurred to them yet, then we cut the end of the pipe back on a 45% on the bottom side of the pipe where they cant lay any thing on it, knock on wood, they haven't touched it in several yrs yet. Good Luck!
