Ice dam
The expression ice dam can refer to various phenomena:-
Contents |
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 about 13,000 years ago 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.
Caused by a river being blocked by ice formed by the river freezing
If a river flows north in the northern hemisphere, or south in the southern hemisphere, from a warmer area to a colder area, and it 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.
On roofs of buildings
An ice dam on a smaller scale is a problem of home and building maintenance in cold climates. When snow accumulates on a slanted roof, warmth coming up through the roof can melt it. The meltwater flows down the roof to the gutters, where it refreezes to form the ice dam, filling the gutter. Snow that melts later cannot drain properly through the ice-filled gutter, and may instead leak through the roof into the building.
Alternate description:
Houses with inadequate insulation and ventilation lose enough heat to melt snow on the roof in the winter. The resulting water flows down the roof, under the blanket of snow, and onto the cold overhang, where colder conditions along the eave cause it to freeze, forming an ice dam. The ice dam backs up water, creating giant icicles and forcing the water under shingles. This can cause:
- Leaking roof.
- Wet (ineffective) insulation.
- Stained or cracked plaster or drywall.
- Rotting timber.
- Stained, blistered or peeling paint.
How to prevent ice dams:
- Keep the roof cold by keeping the attic cold.
- Seal bypasses
- Draw more outside air into attic through gable and eave vents.
- Make sure vents aren't blocked.
- Install more insulation in the attic floor.
- Keep gutters clean.
- Install slippery metal roofing along the eaves so that snow and ice slide off more easily. Do not install a roll of aluminum flashing. Install real metal roofing (either integrated with asphalt roofing, or a 100% metal roof). Encourage snow and ice to slide off the edge of a roof by installing standing-seam metal panels along the eaves only. Using inexpensive sheet metal is not recommended, because the nails are exposed.
- Install an adhesive, waterproof, rubberized ice-shield membrane on the sheathing, before installing shingles. The membrane seals around nail holes and protects the sheathing from backed-up water. They are usually made of rubberized asphalt and polyethylene.
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.
References
Allen, John Eliot; Burns, Majorie; and Sargent, Sam C. (1986). Cataclysms on the Columbia, Portland: Timber Press. ISBN 0-88192-215-3.
