Meetings at the North Branch Nature Center frequently get interrupted by natural phenomena. Usually it’s a bird sighting or a butterfly, certainly something alive. But what had us pulling on boots and hats recently was something more geometrical in nature. Larry Clarfeld, environmental educator at NBNC, had found what’s called an ice circle/disc or “pizza ice”. When he first came upon the ice circle along a bend of the North Branch of the Winooski River, it was slowly revolving in place. This particular stretch of river has a sharp bend near a spot we call “otter rock”. The river here features a deep hole and an eddy that may have helped the circle to form.
Expert on ice formation, Faye Hicks, a professor at the department of civil and environmental engineering at the University of Alberta in Edmonton, says these rarely-seen discs form near the outside of a bend in the river. When flow enters a river bend, flow velocities increase due to the effects of centrifugal acceleration; also, the bulk of the flow is pushed towards the outer bank."
"This high-velocity flow curves around the outer bank of the bend and, if there is an ice cover in the bend, the curving flow creates a curved drag force on the underside of the ice," Hicks said.
The rotating ice scrapes along the bank or nearby ice, eventually smoothing off the rough edges and creating a nearly perfect circle.
By the time we all got down to the river the ice circle had frozen in place, but there were actually two frozen ice disks nearly in contact with each other. One wheel, the recently rotating one, measured an estimated 25 feet in diameter, while the smaller one was about 15 feet across. I imagine one wheel formed first, eventually floated a bit downstream and froze into place, while another formed in its place.
If you’d like to learn more about how ice circles form, you can visit the Burlington Free Press's coverage on ice circles last year and even watch a video of it rotating in place (somewhat akin to watching paint dry).
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