Frazil ice

Frazil ice is a collection of loose, randomly oriented, plate or discoid ice crystals formed in supercooled turbulent water . Its formation is common during the winter in rivers and lakes located in northern latitudes, and usually forms in open-water reaches of rivers where and when the heat exchange between the air and the water is such that the water temperature can drop below its freezing point (typically not lower than -0.1 °C in rivers). As a rule of thumb, such conditions may happen on cold and clear nights, when the air temperature is lower than −6 °C (21 °F). Frazil ice also forms in oceans, where it is often referred to as grease ice when floating on the surface. Frazil ice is a collection of loose, randomly oriented, plate or discoid ice crystals formed in supercooled turbulent water . Its formation is common during the winter in rivers and lakes located in northern latitudes, and usually forms in open-water reaches of rivers where and when the heat exchange between the air and the water is such that the water temperature can drop below its freezing point (typically not lower than -0.1 °C in rivers). As a rule of thumb, such conditions may happen on cold and clear nights, when the air temperature is lower than −6 °C (21 °F). Frazil ice also forms in oceans, where it is often referred to as grease ice when floating on the surface. Frazil ice is notorious for blocking water intakes 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. 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 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. Frazil ice has also been demonstrated to form beneath temperate (or 'warm-based') glaciers as water flows quickly downhill 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 tritium — produced by nuclear weapons testing and therefore almost entirely absent in ice frozen before 1945 — 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.