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Freeze-up and Winter Ice Processes

This lecture provides an introduction to freeze-up and winter ice processes in rivers, including the formation of border ice, frazil ice, ice cover thickening, hummocky ice cover, anchor ice, frazil adhesion, continuous frazil production, hanging dams, snow ice, thermal ice, and aufeis accumulation.

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Freeze-up and Winter Ice Processes

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  1. Introductory lectures in River Ice Engineering Freeze-up and Winter Ice Processes

  2. Border Ice is usually the first type of ice to form in rivers. Border ice forms along the banks. It grows thermally, similar to lake ice. photo by S. Beltaos

  3. Border ice will form where velocities are less than about 0.1 m/s. photo by F. Hicks

  4. Frazil Ice: photo by R. Brown Frazil ice forms in turbulent, fast flowing water.

  5. Frazil Ice: CRREL photo Frazil ice usually occurs in the form of discs; however, needle shapes occur as well. microscopic photo by G. Tsang

  6. Frazil Ice Evolution: 1 2 3 • Frazil forms spontaneously throughout the flow depth in supercooled, turbulent water (Tw = -0.01ºC approximately). • Active frazil is very adhesive and flocculates to form frazil slush. • Once large enough to overcome turbulence, slush balls float to surface where the unsubmerged portion freezes to form “pans”.

  7. The frazil pans have rough upturned edges, resulting from collisions with each other. photo by S. Beltaos

  8. Pans may freeze together to form “rafts”. photo by S. Beltaos

  9. The rafts and pans increase in surface concentration, reducing the open water surface area and decreasing the rate of frazil production. photos by R. Gerard

  10. Once surface concentrations approach 100%, local frazil production ceases. photo by S. Beltaos

  11. Also, as surface concentrations approach 100%, “bridging” becomes likely. photo by R. Gerard

  12. Bridging tends to occur at natural flow constrictions Bridging is also sometimes referred to as “arching”. from “River Ice Jams”, 1995

  13. Tight bends and bridge constrictions are likely spots for ice “bridging” to occur. photo by R. Gerard

  14. “Juxtaposed” Ice Cover • once bridging occurs, a stationary ice front progresses upstream as incoming rafts and pans lengthen the accumulation • if the rafts and pans accumulate edge to edge, like a jigsaw puzzle, we have what is called a “juxtaposed” ice cover • the spaces between pans freeze, strengthening the accumulation

  15. “Juxtaposed” Ice Cover The rafts and pans have accumulated edge to edge. photo by S. Beltaos

  16. Ice Cover Thickening • Once frazil production stops, further heat loss causes freezing of the pore water within the frazil slush (i.e. completion of freezing of the submerged portion of the pans). • Once the frazil slush is frozen, further heat loss causes thermal growth of ice. This occurs in long, vertically oriented crystals and is known as “columnar” ice.

  17. The columnar crystal structure is exposed when the ice cover melts. We call this “candle ice”. photo by F. Hicks

  18. “Hummocky” Ice Cover • Sometimes the drag force of the flow, acting on the underside of the ice cover, is sufficient to collapse a juxtaposed ice cover. • When this happens, a “hummocky” ice cover is formed. • This results in a much thicker ice cover, and can frequently cause flooding.

  19. “Hummocky” Ice Cover also known as a “freeze-up jam” or a “frazil ice jam” Notice how rough the ice cover is, and how the ice has pushed up into the trees. (Bow River at Calgary, 1950; photo courtesy of Alberta Infrastructure)

  20. Freeze-up jams create problems for bridges (photo courtesy of Nfld. Government)

  21. forms when frazil particles freeze to gravel on the river bed. Anchor Ice… Anchor ice can fill pools, causing problems for fish. photo by R. Brown

  22. If sufficient frazil adheres to a rock, it may float the rock up and incorporate it into the ice cover. Frazil Adhesion Rock found in the Mackenzie River’s ice cover. photo by F. Hicks

  23. Frazil Adhesion Frazil adhesion to trash racks is a common problem at water intakes. (photos adapted from Ashton , 1986)

  24. Continuous Frazil Production In steep reaches where an ice cover does not form, frazil ice production will continue throughout the winter. V>1.5m/s (adapted from Ashton , 1986) The frazil produced will tend to accumulate under an ice cover which has formed in a flatter reach downstream.

  25. Hanging Dams These frazil slush accumulations, known as “hanging dams”, can become very large. this hanging dam contained more than 56 million cubic metres of frazil slush LaGrande River, Quebec, 1973 (Michel and Drouin, 1981)

  26. Snow Ice Snow ice forms when the weight of snow on an ice cover is sufficient for submergence and saturation to occur. Ice cover (with cracks) (adapted from Ashton , 1986)

  27. Ice Types This core sample illustrates the different types of ice. photo by F. Hicks snow ice frazil ice thermal ice

  28. (ice on top of ice) Aufeis This tributary’s stream flow has frozen, layer upon layer, over the winter. aufeis accumulation from “River Ice Jams”, 1995 The weight of aufeis has depressed the Mackenzie River’s ice cover.

  29. Aufeis deposits often develop in culverts. This is also known as a “culvert icing”. It can frequently result in culvert washout. photo by F. Hicks

  30. Mean freeze-over data on rivers (Allen, 1977)

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