Internal seiches
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ICE COVERED LAKES Lars Bengtsson Department of Water Resources Engineering, Lund University, Lund, Sweden
Ice formation Lakes at high latitudes or high altitudes are ice covered part of the year; typically from November to April and in the very north sometimes from October to early June. Arctic lakes may be ice covered throughout the year. When there is a regular ice cover for several months, the ice thickness reaches more than ½ meter. At mid latitudes occasional ice cover may appear for short periods several times during a winter. Where there is a stable ice cover, ice roads are prepared. Fishing is done also at commercial scale. Ice covered lakes are used for recreation. Ice on a small lake is formed rather quickly after the surface water is cooled down to the freezing point – most often after a cold night with no wind. It takes much longer time for a large lake to freeze-over than a small lake, since relatively warm water is brought to the surface during the more intense mixing in a large lake. Ice on lakes grows in a static way; there is no horizontal transfer of ice but the ice grows in the vertical. Ice is produced as black ice, when heat is lost to the cold atmosphere from the water underneath the ice, or as snow ice, when the ice is suppressed under the water surface by heavy snow and water freezes in the slush layer in the snow on top of the ice. Once ice is formed it grows rather quickly in the first weeks until the ice cover and snow on the ice insulate the water from the atmosphere; the rate of ice growth is slowed down. The heat lost from the water to the atmosphere must be taken from the latent heat released when ice is formed, since the water just below the ice is at the freezing point.
Ice formed from the underside of the ice sheet has crystals with columnar structure; it is possible to see through the ice. Such ice is called black ice. Ice can also be formed in a slush layer between the snow and the top of the ice. When the weight of the snow is more than the lifting force from the ice, the ice cover is forced under the water surface and water enters into the snow which becomes saturated with water. When this slush layer freezes, snow ice or white ice is formed. The crystals in this kind of ice are randomly distributed. This ice is not transparent but looks somewhat like milk. An example of ice growth is shown in Figure 1 from a bay in the Luleå archipelago (almost fresh water). The ice on a lake has ecological consequences. Since there is no exchange with the atmosphere, the oxygen content of the water decreases and the bottom layers may be completely depleted of oxygen. Man’s impact on lake ice is mainly from release of oxygen consumption substances, which increases the rate at which the lake water is depleted in oxygen. All releases from industries and waste water treatment plants and also the increased winter flows in regulated rivers have local effects on the ice cover.
Thermal regime The thermal regime of ice covered lakes considers the heat fluxes to and from a water body without direct
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