Grand Falls-Windsor -
It was a perfect circle of ice, nudging against the shoreline on an otherwise ice-free river, and it was unusual enough to prompt at least one person to snap a photograph.
Anyone driving near the Exploits River last week was treated to an extremely rare phenomenon - rare enough that an Environment Canada meteorologist and a Natural Resources researcher specializing in hydrology had the same reaction.
"What did you say? Never heard of ice circles," said Herb Thoms of Environment Canada.
"Why don't you check with someone from the Department of Natural Resources who's involved with ice buildup and flood forecasting on the Exploits River."
That person was Amir Ali Khan, who helped develop technology to monitor the progress of ice on the river in the wake of the Badger flood of 2003.
"I have never heard of these, but I know of someone who would, an ice expert," Khan said.
That expert was Faye Hicks, a professor at the department of civil and environmental engineering at the University of Alberta in Edmonton. One of her specialties is ice formation and its behaviour on rivers.
She said ice circles are rare, but one or two do get reported in Canada every year. Some people even mistakenly believe they only occur in Scandinavia.
"In fact, they can occur on any northern river, if conditions are right," Hicks said.
"I've seen pictures of ice disks from across Canada.
"The ice disk occurs near the outside of a bend in the river. This seems to be typical. 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."
It's the same effect that pushes the body towards the outside of the bend in a road, if you're driving fast, she explained.
"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.
"If the ice is really thin and frail, then the ice cover will break up and be washed away with the flow."
"If the ice cover is thick and strong, then it will withstand this rotational force and nothing will happen," Hicks said.
'Just the right balance'
Sometimes there is "just the right balance" between the rotational drag force created by the curving flow of the water, and the ice thickness and strength, she said.
The result is that ice disks form.
In this case, the ice is weak enough to be fractured, but not weak enough to break up completely.
If the conditions are right, part of the ice rotates and grinds against the remaining ice on the shoreline, thus rounding the edges and creating a perfect circle.
"Incidentally, ice experts in Canada affectionately refer to these ice disks as 'pizza ice,'" Hicks said.
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