Figure 1: Ice ridge formed along of the shore of Shamineau Lake in Morrison County, Minnesota

Winter Perils in a Northern Climate

DonanEngineering Forensic Engineering

Winter weather can adversely affect properties in cold northern climates in many ways.  One such way is when lakeshore properties are constructed too close to the shoreline.  Ice heaving or ice jacking is a natural phenomenon that occurs in lakes where sheets of ice push on the shoreline soils and cause the formation of ridges.  The Minnesota Department of Natural Resources (DNR) describes the phenomena as follows:

Ice ridges are caused by the pushing action of a lake’s ice sheet against the shore.  Cracks form in the ice because of different contraction rates at the top and bottom of the ice sheet.  This is especially true in years that the ice sheet lacks insulating snow cover.  Ice cracks also develop because the edges of the ice sheet are sometimes firmly attached to the shore.  When water rises in the cracks and freezes, the ice sheet expands slightly.  Rising air temperatures warm the ice, leading to additional expansion, which exerts a tremendous thrust against the shore.  Alternate warming and cooling of the ice sheet leads to additional pushing action, causing the ice to creep shoreward and scrape, gouge, and push soil and rock into mounds (called “ice ridges”, “ice pushes”, or “ramparts”).

Figure 1: Ice ridge formed along of the shore of Shamineau Lake in Morrison County, Minnesota

Figure 1: Ice ridge formed along of the shore of Shamineau Lake in Morrison County, Minnesota

Damage occurs to structures that are supported by or impacted by the ice ridges.  The potential for ice and subsequent soil movement occurs when the ice’s temperature rises from 14 to 32 degrees Fahrenheit on a lake that is one mile across. The ice sheet will expand laterally a total of approximately 32 inches, and the expansion can occur in a matter of hours when there is no snow cover on the ice sheet.

After ice sheets expand and ice ridges form, information regarding remediation and permitting is detailed by the DNR and outlined in the Minnesota Administrative Rules Section 6115.0215, Restoration of Public Waters.  To prevent ice ridges from damaging structures, setbacks established for the specific lake should be followed.  The setbacks are determined for each lake based on the ordinary high water level, as shown in Figure 1 below, and the lake classification established by the governing county.

Figure 2: Shoreline Cross Section

Figure 2: Shoreline Cross Section

Another adverse effect of winter weather is the accumulation of snow on roofs.  Prolonged periods of cold weather combined with heavy snow accumulations and drifting snow can lead to loading conditions on roof structures that exceed many historical design standards, and in some cases, current design standards for a variety of roof systems.

Figure 3: Buckled steel columns at an overhang due to overloading from snow and ice

Figure 3: Buckled steel columns at an overhang due to overloading from snow and ice

Figure 4: Fractured truss members due to overloading from snow and ice

Figure 4: Fractured truss members due to overloading from snow and ice

An analysis using the National Weather Service’s National Operational Hydrologic Remote Sensing Center allows you to determine the snow water equivalent recorded at the selected weather station closest to a given location on any date or date range. From the data, an estimated snow load can be calculated for a given date or the maximum snow load can be analyzed for a given time period.

Figure 5: Snow water equivalent data between February 20, 2104 and February 22, 2014 for a selected weather station

Figure 5: Snow water equivalent data between February 20, 2104 and February 22, 2014 for a selected weather station

Using the modeled snow water equivalent data maximum of approximately 6.3 inches shown in Figure 5 above, an estimated snow load of 33 pounds per square foot (psf) is calculated (6.3 inches / 12 inches per foot x 62.4 pounds per square foot equals 32.8 psf).  Although actual snow loading conditions cannot be known, an estimated loading allows you to make a comparison to historical design criteria or detailed design criteria for new construction.

Ice jacking of lake ice and excessive buildup of snow and ice on roof systems are just two examples of the effects of extreme winter weather.  Warming of the lake ice causes the ice to expand and heave the shoreline soils.  These ice ridges are damaging to structures constructed too close to the shoreline.  Care should be taken to follow local regulations regarding building distances from shorelines to prevent damage from ice ridges.  Extreme snow falls or continued below freezing temperatures lead to excessive buildup of snow and ice on roof systems, which in turn can lead to damages or partial collapse of systems that are not designed to support the actual loading conditions.  Consideration should be given to remove snow buildup on older, more susceptible structures and for structures encountering excessive snow or ice accumulations.

Sources:

http://files.dnr.state.mn.us/publications/waters/shoreline_alterations_ice_ridges.pdf
http://pelicanlakemn.org/Education/Lake_Learning/tremendous_power_of_lake_ice.htm
https://www.revisor.mn.gov/rules/?id=6115.0215
http://www.nohrsc.noaa.gov/nearest/index.html