etx, sorry about your spillway damage.
Looking at your pics, it is clear that the erosive force of your water flow is clearly too much for that size of rip-rap.
I am not a pond builder, but I am a geologist, so I have some experience with erosive forces.
I am missing something important from your posts. When the three waterways converge (at the current location of your pond), there must have been a larger waterway developed in the draw that heads NW. That draw should have been capable of handling the existing water flows and come to some sort of natural equilibrium. However, when I look at your pics, I do not appear to be observing a waterway that used to be capable of conveying a heavy rainfall prior to your modifications of your spillway. What am I failing to grasp?
Normally, a pond should actually reduce the erosive force of the water flow since it delays the peak discharge as the freeboard of the pond fills up, while a significant amount of water is be transmitted through the "normal" outlet. Your pond outlet is essentially all "emergency" spillway. That actually maximizes the peak discharge rate. It gets even worse if erosion cuts out some depth from the top of the dam or the top of the spillway. In that case, you have the water flows from the three waterways PLUS the impounded water in the pond that is now released due to down cutting of your containment.
One option would be to install a relatively large siphon system to serve as your normal outlet. That could be constructed with only a pipe through the freeboard of the dam, so as to not reduce your dam integrity. I would also build it at the opposite side from your emergency spillway so any erosion in that spillway does not cut our your siphon system. A properly constructed siphon can convey a LOT of water!
Were you ever on location when your spillway was at peak flow? If so, can you estimate the peak discharge? For example, 15' wide channel with an average depth of 1.5' means any type of drop structure would have to have an area 22.5 sq. feet to pass the flow.
If you add some more info, I can add some (possibly harebrained) ideas for you to assess for your situation.
