Ice dam
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The expression ice dam can refer to various phenomena:
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[edit] Caused by a glacier
Sometimes a glacier flows down a valley to a confluence where the other branch carries an unfrozen river. The glacier blocks the river, which backs up into a lake, which eventually overflows or undermines the ice dam, suddenly releasing the impounded water.
In modern times, the Hubbard Glacier regularly blocks the mouth of Russell Fjord at 60° north on the coast of Alaska. See William S. Reeburgh, D. L. Nebert, "The birth and death of Russell Lake", Alaska Science Forum 832 (3 August 1987) at http://www.gi.alaska.edu/ScienceForum/ASF8/832.html : an image on that page shows the record of the tide gauge behind the dam superimposed on an image of the lake, showing the buildup and then the release of all impounded water within 24 hours.
About 13,000 years ago, the Cordilleran ice sheet crept southward into the Idaho Panhandle, forming a large ice dam that blocked the mouth of the Clark Fork River, creating a massive lake 2000 feet deep and containing more than 500 cubic miles (2,000 km³) of water. Finally this Glacial Lake Missoula burst through the ice dam and exploded downstream, flowing at a rate 10 times the combined flow of all the rivers of the world. Because such ice dams can re-form, such Columbia River floods happened at least 59 times, carving Dry Falls below Grand Coulee.
[edit] Caused by a river being blocked by ice formed by the river freezing
If a river flows away from the equator, and freezes over in winter; in spring the upstream part thaws first, and the ice gets carried downstream into the still-frozen part, where it causes a blockage. The water behind the ice dam then rises and often to such a height that it breaks the levees that contain the flow of the river and creates disastrous floods. While this usually occurs in spring, it can happen as winter sets in when the downstream part becomes frozen first. The blockage sometimes has to be cleared by airstrike. Ice dams sometimes form on the Hwang Ho, and in rivers in Siberia and the Canadian Arctic.
[edit] On roofs of buildings
An ice dam, on a smaller scale, is a problem of house and building maintenance in cold climates. An ice dam can occur when snow accumulates on the slanted roof of a house with inadequate insulation and ventilation in the attic. Warmth coming up through the roof melts the snow. Meltwater flows down the roof, under the blanket of snow, onto the eave and into the gutter, where colder conditions on the overhang cause it to freeze. Eventually, ice accumulates along the eave and in the gutter. Snow that melts later cannot drain properly through the ice on the eave and in the gutter. This can result in:
- Leaking roof (height of leak depends on extent of ice dam).
- Wet, ineffective insulation.
- Stained or cracked plaster or drywall.
- Rotting timber.
- Stained, blistered or peeling paint.
Under extreme conditions, with heavy snow and severe cold, almost any house can have an ice dam, whereas a house that is poorly insulated and ventilated will have ice dams during normal winter weather. Giant icicles hanging from the eave are one indication of a poorly insulated, poorly ventilated attic.
How to prevent ice dams:
- Keep the roof cold by keeping the attic cold.
- Install more insulation in the attic floor.
- Seal bypasses into the attic.
- Draw more outside air into attic through gable and eave vents.
- Make sure vents aren't blocked.
- Keep gutters clean.
- Install slippery metal roofing along the eaves so that snow and ice slide off more easily. Install real metal roofing such as standing-seam metal panels (either integrated with asphalt roofing, or a 100% metal roof). Do not install a roll of aluminum flashing or inexpensive sheet metal, because the nails and nail holes will be exposed.
- Before installing any shingles, install an adhesive, waterproof, rubberized ice-shield membrane on the sheathing to seal around nail holes and protect the sheathing from backed-up water. These membranes are usually made of rubberized asphalt, polyethylene, or bituthane.
- Switch to a warm-attic system. Make your attic part of the conditioned space of your house. This requires that you insulate the attic roof from both sides, insulate the exterior walls of the attic, and install energy-efficient attic windows.
To fight an ice dam after it has already formed:
- Remove excess snow with a roof rake.
- Hose off the roof with tap water on a warm day. Note that water run-off could damage shrubbery.
- Steam the roof.
Things not to do:
- Avoid electric heat cables - all they do is move the ice dam higher up, and they use enormous amounts of electricity.
- Do not chop ice off the shingles. You will damage the shingles.
- Do not use a blowtorch. You could burn your house down.
[edit] References
Allen, John Eliot; Burns, Majorie; and Sargent, Sam C. (1986). Cataclysms on the Columbia. Portland: Timber Press. ISBN 0-88192-215-3.
