Copyright © 2018 Don H. Meredith, All Rights Reserved.
Lake ice is a wondrous thing. When thick enough, it allows us to walk, skate or ski on it, or even drive vehicles over it to cross a lake or go to a favorite fishing spot. The ice also insulates the water below from freezing solidly to the lake bottom, allowing fish and other aquatic life to survive the winter. But lake ice can also be quite destructive.
On January 2, 2018, the ice on some lakes in central Alberta expanded laterally and encroached the lakeshore, heaving up the ground and damaging property in the process. Along the north shore of Wabamun Lake (from Seba Beach to just west of the Village of Wabamun), there was considerable damage to some buildings and other structures. Long time residents and cottage owners stated that they had not seen such damage from ice in over 60 years.
When water freezes it expands in volume. That’s why ice floats on water. However, as ice gets colder it contracts (not enough to sink). If lake ice is connected to the shoreline (frozen into the shallow lake bottom there) the force of the contraction exceeds the tensile strength of the ice and it fractures into long and sometimes large cracks. Lake water from below enters the cracks and freezes.
When the temperature of the ice subsequently warms, the ice expands, except this time there is more of it. If the ice is frozen to the lakeshore, the force of the expansion most often causes the ice in the middle of the lake to rupture, buckle and form “pressure ridges” that can be hazardous to travel. However, if the conditions are right and the tensile strength of the ice sheet at a particular moment is greater than the strength of the ice frozen into the shoreline, the expansion shoves the ice into the shore where it buckles and heaves.
So, what conditions are right for shoreline encroachment?
Central Alberta has received much less snow than normal so far this year. The result is the snow depth on lake ice is only a few centimetres thick. Snow insulates lake ice from changes in temperature. The deeper the snow the less heat escapes or enters the ice.
During the last week of December 2017 in central Alberta, low temperatures dipped to near -30ᴼ C (-22ᴼ F) and highs ranged around -20ᴼ C (-4ᴼ F). On the night of January 1, 2018, the temperature rose dramatically from about -28ᴼ C (-18ᴼ F) to above freezing (0ᴼ C, 32ᴼ F) the following day. During that night, the ice expanded causing the damage. The low snow cover allowed the ice to rapidly increase in temperature and expand. Similar conditions have occurred since, increasing the heaving.
What can be done?
Along a natural shoreline, the ridges formed by the expanding ice are a natural occurrence and actually protect that shoreline from future ice expansions (the ice rising up the ridge and falling back under its own weight). The ridges provide a fertile substrate for natural vegetation to grow and stabilize the ridge and help prevent nutrients from entering the lake.
However, if you own a shoreline cottage, such ridges might prevent you from accessing or having an unobstructed view of the lake. In such cases, you might want to level the ridge or otherwise provide access to the lake. In Alberta, such activity likely requires a permit from either your local municipality or Alberta Environment and Parks or both. You don’t want your actions to affect the health of the lake.
To prevent damage from future ice heaves, cottage owners should ensure all personal property (e.g., boats, sheds, etc.) is setback a sufficient distance from the shore. Construction of reinforced barriers is an option but they are expensive and often fail. If you are considering such an option, you should consult a contractor or engineer experienced in this area. Permits will also be required.
As our climate warms and we are subjected to extreme changes in weather, the chances of this kind of event happening again are good. We are all going to have to adapt to the new reality.
References: Some of the best information on lake ice behavior and how to cope with it is found at the Minnesota Department of Natural Resources. I found these documents to be particularly helpful: Ice Power! and Shoreline Alterations: Ice Ridges.
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