Frazil ice
From Wikipedia, the free encyclopedia
Frazil ice is a collection of loose, randomly oriented needle-shaped ice crystals in water. It resembles slush and has the appearance of being slightly oily when seen on the surface of water. It sporadically forms in open, turbulent, supercooled water, which means that it usually forms in rivers, lakes and oceans, on clear nights when the weather is colder, and air temperature reaches –6°C or lower. Frazil ice is the first stage in the formation of sea ice.
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[edit] Formation
When the water surface begins to lose heat at a very quick rate, the water becomes supercooled. Turbulence, caused by strong winds or flow from a river, will mix the supercooled water throughout its entire depth. The supercooled water will already be encouraging the formation of small ice crystals (frazil ice) and the crystals get taken to the bottom of the water body. Ice generally floats, but due to frazil ice’s ineffective buoyancy, it can be carried to the bottom very easily.
Through a process called secondary nucleation (see Crystallization), the crystals quickly increase in number, and because of its supercooled surrounding, the crystals will continue to grow. Sometimes, the concentration is estimated to reach one million crystals per cubic meter.
As the crystals grow in number and size, the frazil ice will begin to adhere to objects in the water, especially if the objects themselves are at a temperature below water’s freezing point. The accumulation of frazil ice often causes flooding and/or damage to various water objects, such as trash racks. And since frazil ice is found below the surface of water, it makes it very difficult for humans to detect its formation.
Usually what happens is the frazil ice accumulates on the upstream side of objects and sticks to them. The frazil ice accumulates as more gets deposited. The growth will extend upstream and increase in width until the point where the frazil ice accumulations bridge together and block the water. As more and more water flows against this block, the pressure on the upstream side increases and causes a differential pressure (difference in pressure from the upstream side and the downstream side). This will cause the growth of the bridge to extend downstream. Once this happens, flooding and damage is likely unless otherwise prevented and/or controlled.
Frazil ice has also been demonstrated to form beneath temperate (or "warm-based") glaciers as water flows quickly uphill and supercools due to a rapid loss of pressure. This "glaciohydraulic supercooling" process forms an open network of platy ice crystals that can effectively trap silt from the sediment-laden water that flows beneath glaciers and ice sheets. Subsequent freezing and recrystallization can result in a layer of sediment-rich ice at the base of the glacier which, upon melting at the terminus, can result in significant accumulation of sediment in moraines. This phenomenon has been verified by elevated concentrations of bomb-produced tritium in the basal ice of several glaciers (signifying young ice) and the observation of rapid growth of ice crystals around water discharge vents at glacier termini.
[edit] Control
There are several ways to control frazil ice build up. They include suppression, mechanical control, thermal control, vibration, prior thought and damage mitigation.
[edit] Suppression
Frazil ice forms in supercooled water which occurs because the surface water loses heat to cooler air above. Suppression is the idea of ‘insulating’ the surface water with an intact, stable ice cover. The ice cover will prevent heat loss and warm the supercooled water that might have already formed. Sufficient area needs to be covered in order for this method to work, but it is still unknown what is meant by sufficient.
[edit] Mechanical Control
These methods include stabilizing freeze without restricting water flow, such as implementing weirs and ice boom, installing water jets to break up any accumulation that might occur, and using manual labour to rake away the accumulation. This final method is often not preferred because of high labour costs, cold, wet and late night working conditions. Back flushing is another technology that uses the idea of cancelling out the differential pressure caused by the frazil ice accumulation. This technology creates a high pressure on the downstream side of objects to reverse the differential pressure.
[edit] Thermal Control
These methods either heat the structures in the water to prevent frazil ice adhesion or heat the water to prevent frazil ice from forming in the first place. When heating the structure, they must be heated to a temperature above freezing temperature. Resistance heaters have been found to work well. Installing hollow bars in the structures where steam and/or warm water is pumped through also works, but this method has been seen to cause operational problems. Electrical connection heating systems have also been used, but these have a potential safety problem. Other active methods are also available. A lot of warm water byproducts come from several nearby water facilities and the warm water is wasted. Redirecting this water to potential frazil ice accumulation regions could raise the water temperature 0.1-0.2°C, often enough to prevent supercooled water from developing.
[edit] Vibration
Although still in experimental stages, a form of vibration control that can be used is blasting with dynamite, which will break any frazil ice accumulation. When blasting, there are a lot of things to take into consideration. The charge must be precise such that the ice breaks, but surrounding structures and environment are not harmed. Safety of the blasting also is important. As well, nearby residents might complain about sound pollution. For all these reasons, this method is not often used.
[edit] Prior Thought
Anthropogenic structures are often the objects which frazil ice adheres to. As such, the materials of these structures should be thought of prior to placing in water. Steel structures, for example, will rust and the rust to ice adhesion is very strong. Choosing a material with lower adhesion such as plastics, fibreglass, graphite or even an epoxy paint coat overtop the steel will reduce the adhesion probability. Although adhesion still will occur, using such materials makes other methods, such as raking, easier.
[edit] Damage Mitigation
Damage could be reduced by promoting designated flood regions with mechanical structures.
[edit] References
Daly, Steven F. Cold Region Technical Digest No. 91-1 "Frazil Ice Blockage of Intake Trash Racks". March 1991.
Lawson, D. E., Strasser, J. C., Evenson, E. B., Alley, R. B., Larson, G. J., and Arcone, S. A., 1998, Glaciohydraulic supercooling: a mechanism to create stratified, debris-rich basal ice: I. Field evidence: Journal of Glaciology, v. 44, no. 148, p. 547-562.
