Vehicle Accidents on Lake Ice
On Thanksgiving weekend two vehicles, a van and a snowmachine, fell through the ice on Harding Lake in interior Alaska. No human lives were lost, but a family pet went down with the van. The ice thickness measured at several places was 10 inches and should have been sufficient to support a loaded 3/4-ton truck. What probably happened?
Several possibilities immediately come to mind. Ice can be weakened by the action of upwelling springs eroding at the bottom surface of the ice. This possibility can be ruled out because this is a shallow-water phenomenon, and the two vehicles now lie in approximately 100 feet of water. Another possibility is that the heavier van was traveling at the critical speed where it rides the crest of the wave created in the ice sheet by the vehicle's weight. Motion at this critical speed can cause the effective weight of the vehicle to increase and the ice to break if not thick enough. It is also potentially a problem when vehicles follow each other since one may be traveling a wave length ahead and both may be riding wave crests. This second possibility can be ruled out in the Harding Lake case since the much lighter snowmachine fell through while stationary and at a different spot 20 feet from the hole caused by the van.
The likeliest cause therefore is thin ice associated with a pressure ridge, as speculated by the driver of the van and as actually observed by his son the next day following the accident. These pressure ridges are a rarity in interior Alaska because they are caused by an infrequent weather pattern. After the ice sheet is initially formed in the fall, the air temperature usually stays below freezing in this area. However, if the temperature goes above freezing, as it did on November 10th and 11th this year, the ice will expand like any other solid when warmed. Friction around the shoreline tends to resist the outward movement of the expanding ice, so it heaves up and cracks near the center of the lake. The resulting plates of ice may then slide past each other to rest one atop the other, leaving open water. Subsequently, drifting snow may insulate any open spots that are formed. This phenomenon is called expansion cracking and is quite common in the continental United States.
When I was in graduate school in Iowa, a fellow student and I put a truck through one of these cracks formed in 33 inches of ice. He was a New Englander with no ice experience and I was only familiar with contraction cracks from my experience working on lakes in Interior Alaska. It was unfortunately too late when the Midwesterners told us their "rules of thumb" for driving on lakes: (1) never drive over a snowdrift on otherwise wind-swept ice, (2) do not form lines of vehicles driving along one path, (3) drive slowly--less than 10 mph, and (4) keep the windows open for a quick escape. One person even told us he drives with the doors ajar because if they are quickly opened they may catch on more solid ice.
Other lakes than Harding may have dangerous expansion cracks this year, and with relatively heavy snow for insulation, they may not mend at all this winter to the point of allowing safe vehicle passage. The thick drifting snow also makes it impossible to see these cracks. Follow the Troopers' advice and stay off the Interior lakes with your vehicles this winter.
