Winnipeg Free Press - PRINT EDITION
Anatomy of an ice shove: An uncommon phenomenon
Development of an 'ice tsunami' a matter of probability: researcher
It sounds like the stuff of speculative fiction: A wall of ice materializes on a lakeshore within the space of minutes, toppling trees and smashing structures in its path.
But scientists say the ice that devastated Ochre Beach on Dauphin Lake last week was merely an uncommon phenomenon, capable of occurring on the shore of almost any large temperate body of water when sustained high winds send free-floating pieces of ice in motion.
The phenomenon that damaged or destroyed 27 properties on the southern shore of Dauphin Lake on Friday is formally known as an ice shove. Engineers and geographers describe these events as unusual but hardly unknown.
Over the past two months, ice shoves have also destroyed or threatened property in Lake Mille Lacs, Minn., Lake Winnebago, Wis., Alberta Beach, Alta., and Lake Champlain, N.Y.
"This is a fairly normal thing to happen. It's just a matter of timing," said Klaus Hochheim, a research associate at the University of Manitoba's Centre for Earth Observation Science. "It's just getting the right combination of factors. It's all probability."
-- Klaus Hochheim, research associate at the U of M's Centre for Earth Observation Science
The first factor is the presence of ice floating on a large body of water that's in the process of thawing. While solid sheets won't move, any form of free-floating ice -- large blocks or smaller shards -- can be pushed around by wind.
When the wind is strong enough and blows long enough to push that ice in one direction, it can become an ice shove when it hits the shore. The leading edge of this mass of ice will stop, forcing the trailing ice to pile up on top of it.
Given enough momentum, the resulting pile can surge onto the shore.
"It's like a freight train, and you have all these cars at the back end with nowhere to go," said Jay Doering, a University of Manitoba civil engineering professor. "Some have even referred to it as an ice tsunami."
The reason ice shoves may be unfamiliar to Manitobans is this phenomenon likely occurs most often in unpopulated areas and therefore goes unreported. The most wind-exposed sections of Manitoba's largest lakes are largely undeveloped areas such as the beach ridges at Delta Marsh on Lake Manitoba and Netley Marsh on Lake Winnipeg, both of which bear scars from ice shoves.
"We know from previous shoreline damage that ice piling happens regularly," said Minnesota State geologist Harvey Thorleifson, a former Manitoban.
What was unusual about the event at Ochre Beach was the extreme amount of property damage. The final damage tally -- as well as the number of homes and cottages that will wind up being completely lost -- will not be known until the cleanup is completed, said Doug McNeil, deputy minister of Manitoba Infrastructure and Transportation.
The height of the ice pile on Dauphin Lake was also extreme, although ice piles as high as 12 metres have been documented. The low gradient on the shore of all of Manitoba's Great Lakes presents little resistance to the forward progress of ice piles.
While bridge piers are designed to withstand the pressure of moving ice, homes and cottages cannot.
"A wooden house is nothing," Doering said.
In theory, it could be possible to forecast ice shoves by placing transmitters on ice floes, similar to the ones U of M scientists use to monitor the movements of Arctic sea ice.
In practice, however, this would be difficult, Doering surmised.
"You'd need real-time information about the ice and then have to notice someone turned on the wind," he said.
Ice shoves also are not dependent on high water levels. Dauphin Lake stood at 856.1 feet above sea level over the weekend, which is 0.7 feet above the upper target level for the lake and almost five feet below the crest of the 2011 spring flood.
Republished from the Winnipeg Free Press print edition May 14, 2013 A4
Updated on Tuesday, May 14, 2013 at 6:19 AM CDT: replaces photos, adds video, adds slideshow
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