I've had a similar (but MUCH smaller) vortex on my big pond. Last spring we had 100 year rains, and my 18" culvert was completely covered, plus around maybe 10" over the top. It drew one heck of a vortex for probably a day or so. I did not feel comfortable until after the water was back below the top of the culvert and it was draining normally.

Dale

[Linked Image from i90.photobucket.com]

"When tempted to fight fire with fire, remember that the Fire Department usually uses water." - anonymous