SAFETY ON FLOATING ICE SHEETS
For many of us in northern climates, working or playing on the frozen
surface of a river or lake is part of winter. Knowing how to do so
safely can be a matter of life or death. This handout presents general,
common-sense precautionary measures that should be followed when you
plan to be on a floating freshwater ice cover. Since it cannot cover
every ice condition you may encounter, your judgement is critical.
Remember: Only you are responsible for your own safety!
PREPARATION Physical condition Clothing Equipment Procedures
WHAT YOU NEED TO KNOW ABOUT THE ICE How thick is the ice? On rivers the ice thickness and quality can change a lot in a short distance; be particularly alert to variations in ice thickness due to bends, riffles or shallows, junctions with tributaries, etc. For both rivers and lakes, warm inflows from springs can create areas of thinner ice. The ice near shores can either be thinner (due to warm groundwater inflow or the insulating effect of drifted snow) or thicker (due to the candle-dipping effect of variable water levels). Measure the snow cover thickness on the ice cover; significant variations in thickness may mean highly variable ice thicknesses. How thick does it need to be? A simple formula to estimate the minimum ice thickness required to support a load is
where h is the ice thickness in inches and P is the load, or gross weight, in tons. You can also use the graph or table to determine the minimum thickness. Remember that the load is the total load in tons (not a vehicle's load capacity).
The equation, graph, and table are valid when the load (such as a person on foot, or a wheeled or tracked vehicle) is distributed over a reasonable area of a continuous ice sheet. The larger the load, the greater the area it should cover for the calculation to remain valid. Neither large loads that are concentrated in relatively smaller areas, nor loads that are at or near the edge of a large opening in the ice, are safely described by the equation, graph, or table.In such cases, seek more advice. The equation, graph and table assume clear, sound ice. If white, bubble-filled ice makes up part or all of the ice thickness, count it as only half as much clear ice. Any recent large snowstorm creates a new load on the ice. If the new snow is heavy enough, the ice sheet will sag and its top surface will be submerged below the water level. Then water will flood the top of the ice sheet through cracks, saturating the lower layers of the snow. Until this slush is completely frozen, stay offthe ice sheet. When the saturated snow becomes frozen, it is an added thickness of white ice. Contrary to what you would expect, a rapid, large air temperature drop makes an ice sheet brittle, and the ice may not be safe to use for 24 hours or more. If the air temperature has been abovefreezing for at least 6 of the past 24 hours, multiply the load by 1.3 before you use the equation (or use the lower dashed line on the graph), obtaining a larger minimum ice thickness to account for any possible weakening. If the air temperature stays above freezing for 24 hours or more, the ice starts losing strength, and the equation, figure, and table no longer represent safe conditions. Stay off the ice! You are likely to encounter cracks in the ice. Cracks are either wet or dry. If they are dry, they do not penetrate the ice sheet and are not a concern. If they are wet, multiply the load by 2, as shown on the graph, before you use the equation to obtain the required minimum ice thickness. If you plan to leave a load on the ice for extended periods, usually more than two hours, multiply the load by 2 (as shown by the upper dashed line in the graph) before you use the equation to find the required minimum ice thickness. SAFE OPERATIONS ON THE ICE COVER If you drive across wet cracks, your path should be as close to perpendicular to them as possible, instead of parallel to them. A load deflects the ice slightly into a bowl shape. When you drive on floating ice, this moving bowl generates waves in the water. If the speed of the waves equals the vehicle speed, the ice-sheet deflection is increased and the ice is much more likely to break. The problem is more serious for thin ice and shallow water. In general you avoid this danger by driving below 15 mph. When there are two loads on the ice, the safe distance between them is about 100 times the ice thickness at the required minimum thickness. This is shown in the third column of the table. When the two loads are different, choose the spacing shown for the larger load. At ice thicknesses greater than the required minimum, this spacing can be reduced. A loaded ice sheet will creep, or deform, over a long period of time, without any additional load. If an ice sheet has to be loaded for a long period, drill a hole near the load. If the water begins to flood the ice through the hole, move the load immediately. Remember this if your vehicle ever becomes disabled: if left for a few days, it may break through the ice as a result of long-term creep. IN CONCLUSION...
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