When Ice Shoves
It got cold early that year. I remember hearing eerily warping, moaning sounds traveling down the lake during a still and subzero night. The sound fit into the same category with the extraordinary sounds of loons and sandhill cranes. The lake was making ice.
Formation of an extraordinary substance
Ice on our Wisconsin lakes provides us with many things: beauty, recreation and a chance to "walk on water." The trouble comes when the ice leaves the lake and comes on land.
The formation and movement of the ice cover is complicated and many variables affect the quality, thickness and actions of ice.
With the coming of cold air temperatures, water at the top of the lake cools and becomes denser. Warmer water will normally rise and colder, denser water will sink. Here is the amazing thing about water…as it becomes colder from the point of 39.2 degrees Fahrenheit (°F), it becomes less dense and expands. Ice forms when water molecules reach 32 °F. Because it’s now less dense than surrounding liquid water, ice rises. You can see this happen in a pitcher of water as the ice cubes float at the surface. Imagine if ice was less dense and did not float -- lakes would freeze from the bottom up and fish or animals would be unable to live.
By the time water reaches the freezing point, its volume has increased by about 1/8. That’s why pipes burst when they freeze. Water also expands slightly again as it warms, but contracts when it gets very cold. The power of freezing and thawing water is great enough to split granite and is a force that brings down mountains and changes the earth over the eons.
On cold nights when a lake surface is not disturbed, needle-like crystals start to form. The crystals unite and continue to grow. If all goes well, a clear sheet of ice will form. Once the surface is covered by ice, direct heat lost from the water is limited and the ice sheet will gradually thicken.
While ice eventually contracts with an appreciable drop in temperature, a warming temperature causes ice to expand slightly. For example, if the temperature increases from 14 to 32 °F, a lake one mile across can expand laterally about 32 inches, with forces exerted outward as much as 30,000 pounds per square inch. The ice cover on a lake is a floating mass, except at the points where it freezes tightly to anything it touches such as the shore, a bridge abutment or a pier. When ice warms it pushes outward against the things to which it is attached. An ice sheet, however, expands and contracts at differing rates at the top and the bottom of the ice. This internal stress causes cracks. The cracks fill with water which freezes and expands the ice a bit further. The ice sheet will expand again during the next warming spell. So what happens? The ice tends to "ratchet out" during freezing and warming cycles. This process is sometimes called "ice jacking." Under certain conditions, the ice can act like a giant bulldozer that nothing can stop, pushing lawns, soil, docks and whatever else is in front of it into piles (some over 15 feet tall!) right up against a lake home. Under particular shore and bottom conditions the ice may buckle and pile up in huge ridges in the shallow waters just off shore. Winnebago and Shawano Lakes are well-known for their huge ice shoves.
The level of ice shoving can vary greatly from year to year. Certain conditions, such as ice at least five inches thick, little or no snow cover and temperature fluctuations, may increase the likelihood of more aggressive ice shoves.
Can I live with an ice ridge?
If you are considering the purchase of waterfront property, check to see if there are signs of ice shoves (scarred trees, mounds of soil, boulders shoved up, the front of the house is missing, etc.). Just because a property is prone to ice shoves should not stop you from purchasing it. Over years, the earth ridges can strengthen and actually resist further ice shoves. Ice ridges are natural berms that can slow nutrient loading to the lakes and provide habitat. You have a number of options in dealing with ice shoves. You could let nature take its course, or you may decide to enjoy a game of "tug of war" with your lake -- you push the soil back to the lake edge in the spring and the lake returns it to your house in the winter.
In some cases, shoreland property owners may try engineering a solution. Fixes can be very expensive and may or may not have the desired results. In the past, some owners placed sloping concrete walls on their shore or ribs or runners to break up ice. If you do have damage from ice shoving, take pictures of the damage and contact the Wisconsin Department of Natural Resources (WDNR) to provide technical assistance. Depending on what needs to be done to restore the damage, a WDNR permit may be needed. You may also need a permit from your county zoning office.
Ice can be fragile, forming magical shapes that will glisten in the winter sunlight; or ice can be formidable, moving everything in its path. How should we deal with ice?