Frazil ice
![](http://upload.wikimedia.org/wikipedia/commons/thumb/c/c0/Frazil_ice_in_Yosemite_Creek.png/250px-Frazil_ice_in_Yosemite_Creek.png)
Frazil ice is a collection of loose, randomly oriented
Frazil ice is notorious for blocking water intakes[6][9][10] as crystals accumulate and build up on the intake trash rack. Such blockages negatively impact water supply facilities, hydropower plants, nuclear power facilities, and vessels navigating in cold waters, and can lead to unexpected shut downs of the facility or even collapse of the trash rack.
Formation
![](http://upload.wikimedia.org/wikipedia/commons/thumb/a/a1/Mouth_of_Blanda_river.jpg/220px-Mouth_of_Blanda_river.jpg)
When the water surface begins to lose heat rapidly, 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 small size relative to current speeds, it has an ineffective buoyancy and can be carried to the bottom very easily.
Through a process called secondary nucleation, 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 ice 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 or damage to objects such as trash racks. Since frazil ice is found below the surface of water, it is difficult to detect its formation.
Usually, the frazil ice accumulates on the
Frazil ice has also been demonstrated to form beneath temperate (or "warm-based")
Control
There are several ways to control frazil ice build up. They include suppression, mechanical control, thermal control, vibration, materials selection and damage mitigation.
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.[11] Sufficient area needs to be covered in order for this method to work, but it is still unknown what is meant by "sufficient". The St. Lawrence River is explicitly managed to create "flow conditions that help form a stable ice cover" to prevent frazil ice and subsequent ice jams.[12]
Mechanical control
These methods include stabilizing freeze without restricting water flow, such as implementing weirs and ice booms, 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.[citation needed]
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, it must be heated to a temperature above freezing. Electrical
Vibration
Although still in the experimental stages, blasting with dynamite is one form of vibrational control that will break loose any frazil ice accumulation. 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 and nearby residents might complain about sound pollution. For all these reasons, this method is not often used, except as an emergency last resort.[citation needed]
Materials selection
Man-made structures are often the objects to which frazil ice adheres. As such, the choice of materials for these structures should include consideration of ice adhesion. Steel structures, for example will rust, and rust-to-ice adhesion is very strong. Choosing a material with lower adhesion such as plastic, fiberglass, graphite or even an epoxy paint coating on the steel will reduce the adhesion probability. Although adhesion will still occur, using such materials makes other methods, such as raking, easier.[citation needed]
Damage mitigation
Damage could be reduced by protecting designated flood regions with mechanical structures.
See also
References
- ^ Bukina, L.A. (1967). "Size distribution of frazil ice crystals in turbulent flows". Izvestiya, Atmospheric and Oceanic Physics. 3 (1). Translated by Keehn, P.A.: 58–68.
- S2CID 246047589.
- ^ doi:10.1139/L07-086.
- ISSN 0165-232X.
- ^ S2CID 210569242.
- ^ OCLC 465638709. Archived from the originalon 17 April 2012.
- ISBN 978-1-4822-8308-2.
- ISBN 978-1-60223-101-6.
- ISSN 0165-232X.
- ISSN 0733-9429.
- .
- ^ International Lake Ontario - St. Lawrence River Board (25 May 2018). Observed Conditions & Regulated Outflows in 2017 (PDF). International Joint Commission. p. 21. Retrieved 3 May 2019.
- ^ S2CID 133902202.
External links
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