FNC, I agree with you. I think the failure was due to water following along the outside of the pipe, and relatively quickly eating away the dirt. It's hard to determine exactly what happened, but I'd guess that the soil wasn't compacted properly around the pipe, water followed the outside of the pipe thru the dam and the water flow kept increasing.
Vortex or no vortex, the water should have stayed in the pond until it reached the emergency overflow and exited that way.
