Frazil, Anchor, and Rime

It is 90^o F outside today and although this is just the end of summer and there has been no ice forming on our river, in the interests of cooling off, it seemed on this particular late summer day a good time to think about ice. What is there to say about ice? Ice is ice, right?

Well the basics of ice are the same regardless of the type of ice formation. The motion energy of water molecules drops as the temperature drops. Once the temperature hits 32^o F, the molecules in pure water slow enough to link up with each other. Along with less excitation at the molecular level at lower temperatures, water is a polar molecule and its slight electrical charge attracts and holds other molecules of water. At a low enough temperature, the molecules link together and form a hexagon matrix and at that point, once liquid flowing water becomes hard brittle ice.

Water expands some nine percent when it freezes because water molecules lock further apart in the matrix than when they are bouncing around and colliding with each other as a liquid. The temperature of ice can go well below 32^o F but no further changes take place in its physical properties. Impurities in water usually drop the temperature at which water freezes. Pure water freezes to clear ice with any color caused by impurities of some kind including the air bubbles trapped in common white ice.

However, when you get beyond the basics of ice, there are different sorts of ice that form in different ways, under different conditions, in different locations in a river or on land, and each can have different effects. Large institutions like the Cold Regions Research and Engineering Laboratory right here in the upper Connecticut River valley conduct research on ice to determine the ecological impacts of ice on rivers, how to protect water supplies, and emergency management to prevent flooding.

Most ice we experience is the kind that forms on top of a river, stream, lake, or puddle. Ice forms this way because of another physical property of water. As water cools, it becomes denser, but as soon as it freezes, it becomes lighter consequently, most ice we see is floating on top of denser water but there are circumstances where ice does the unexpected.

Frazil ice forms in flowing, supercooled streams. Frazil ice is not solid but a collection of unattached, randomly oriented needle shaped ice crystals. Frazil resembles slush and because of its light refracting properties sometimes appears oily when seen on the surface. Frazil ice can cover the entire surface of a stream and when it encounters an object in the flow, it can build into an ice blockage that backs up the flow of the stream and as with solid ice, the jam can cause flooding.

Frazil ice will adhere to objects even under water, especially if the objects themselves are below the freezing point. Frazil ice clumping will continue to extend upstream, increase in width until the accumulations bond together and block the water. In larger rivers, frazil ice can form below the surface so it is difficult to detect its formation. Along with flooding, accumulations of frazil ice can damage facilities like the trash racks collecting detritus in front of hydroelectric dams.

Anchor ice forms on the bottom of streams. Despite the density properties of water that keeps most ice on the surface, sometimes during periods of extreme cold the flow induced roiling of the water that keeps the surface unfrozen, suprecools the water temperature below freezing. When the supercooled water encounters stones or other objects found on the stream bottom that are at freezing temperature, the water will freeze to those objects. Frazil ice in the stream flow may adhere to and thereby increase the rate of formation of anchor ice.

Anchor ice in rivers can harm fish species whose eggs over winter in redds on the bottom of streams while the eggs mature and hatch. Anchor ice blocks water flow around the eggs and the lack of water flow denies the eggs the dissolved oxygen carried in the water they need to survive.

Then there is rime ice. Rime ice deposition occurs on land objects. It coats the objects quickly, often during weather conditions of heavily saturated moist air and high winds. Unlike frost that forms from water vapor, rime ice forms from water droplets in mist or light rain. If the temperature of the droplets is below the freezing point, the droplets will adhere to a cold below freezing surface. Rime ice is unlike the feathery formation of frost in that rime is hard and depending on conditions can be thick, heavy and white or clear in color.

When the freezing rain or drizzle event is light and brief, the rime is thin and causes only minor damage with dead branches on the road and the like. When thick buildups take place, real problems with power and communication outages and dangerous walking and driving conditions take place. Major ice damage has occurred on our upper valley ridges when clouds encased our ridgelines for extended periods under cold and windy conditions.

It is still 90^o outside but thinking about frazil, anchor, and rime ice brought a nice chill to the air.

David L. Deen is Upper Valley River Steward for the Connecticut River Watershed Council. CRC is celebrating over 60 years as a protector of the Connecticut River.