Wednesday, October 27, 2010

Frazil Ice

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.

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.

No comments:

Post a Comment