How ice affects your pier.
There are two major ways in which ice would affect a pier if left in over the winter. 1. Ice expansion/contraction, and 2. Ice flow.
- Ice expansion/contraction
Anyone who has spent time on the ice up nort is intimately familiar with the unique groans and moans of larger ice sheets. Sometimes it sounds like whalesong, sometimes like loud bangs, and sometimes like distant thunder. This is due to the ice cracking, which in turn is caused by expansion and contraction. Most people know that water expands when it freezes – that’s why it floats. Less known is that, once frozen, ice expands and contracts depending on its temperature, just like any other solid. In the simplest case where there is no snow cover and the thickness of the ice is uniform, the whole ice sheet expands when it warms up and contracts when it cools down. But ice thickness is never uniform due to local variables like wind exposure, underwater springs, sun exposure, water movement, chemistry, rain, and numerous other factors. Nor does ice warm up or cool down uniformly, and that’s because of variabilty in its reflectivity, its local thickness, its chemistry, its structure, its air content, and, again, numerous other factors, particularly snow cover.
Snow complicates ice formation because it acts like an insulator and therefore reduces the rate of temperature change of the ice beneath it. This, of course, changes both ice’s growth rate in the long term and its expansion/contraction rate daily. If this were not complicated enough, snow cover is never uniform in thickness, density, or insulative value. With patches of snow on an ice sheet, the strain caused by different rates of expansion and contraction is amplified, and during times of large temperature fluctuations frequent cracks are the method of choice in relieving those strains. Yet a further complication is that the temperature of the bottom of the ice is always the same – 32 degrees. So the vertical temperature disparity plays a big role in the overall stresses that build up.
When the ice cracks, water often seeps into it and freezes. Now the ice sheet is a little bit bigger. New stresses build up and new cracks form. The process goes on all winter. The general expansion of the ice sheet is complicated and seemingly unpredictable. But one can easily see that the outer edges of a large ice sheet can creep significantly over the course of a season, and that usually means it will creep shoreward, especially in the early spring when the general trend is toward a warmer (expanding) ice sheet. Pier legs that are encased in the ice sheet will go with the creep. If that creep is toward shore, that might be bad for any shore mounting schemes. Otherwise, this ice phenomenon is not the most destructive form of ice behavior.
An incidental phenomenon that many snowmobilers and cross country skiers are familiar with is slush on top of the ice. If there was no snow on an ice sheet and a crack developed and filled with water, that water wouldn’t rise above the surface of the ice because the ice has to be higher than the water just to float. But when a bunch of snow (weight) stacks up on the ice, at some point the ice sinks. After all, it’s not all that buoyant. So the first crack that develops allows water to rise above the top of the ice where it soaks into the snow. There, it’s insulated and can remain liquid for long periods depending on the time of year and the amount of overlying snow cover (insulation). This process can continue all winter long, making it seem like the lake is just having a hard time freezing.
- Ice flow
During the spring thaw, the ice sheet starts melting from the shore first and proceeds toward the center. For varying periods of time each spring, the entire ice sheet is a big, pancake-shaped free-floating iceberg, literally. It’s at this point that the ice is dangerous to any structures in or near the shoreline. Wind blowing across the lake will push this iceberg against the downwind shore. The stronger the wind, the more forcefully the ice sheet will push against shore. If the wind is strong enough, the ice will break up at the point of contact and pile up on shore. The process will continue until a broad enough area of contact develops to hold back the ice flow. With a strong enough wind or a large enough ice sheet, the ice may just continue flowing regardless – until too little ice sheet is left to catch the wind or the wind subsides. Obviously the larger the lake, the more dangerous the flow. With the right conditions, the forces on the shore or its structures can be enough to move car-sized boulders, let alone piers. The rule of thumb for pier owners is: sooner or later you will get a strong wind during those critical few weeks of spring and if you leave your pier in over the winter, sooner or later it will be destroyed or taken out to sea. The larger your lake, the worse your chances. For a pond you needn’t worry. For Lake Superior you’ll lose your pier, period.
In the late stages of the thaw, the ice becomes vertically honeycombed and loses its destructive power. Ice piling up on shore during this stage is particularly special to watch and hear. To the sound of a thousand clinking chimes the ice piles millions of crystals up on shore almost imperceptibly.