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

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

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

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

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

Frazil Ice Evolution: 1.Frazil forms spontaneously throughout the flow depth in supercooled, turbulent water (T w = -0.01ºC approximately). 2.Active frazil is very adhesive and flocculates to form frazil slush. 3.Once large enough to overcome turbulence, slush balls float to surface where the unsubmerged portion freezes to form “pans”. 123

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

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

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

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

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

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

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

“Juxtaposed” Ice Cover once bridging occurs, a stationary ice front progresses upstream as incoming rafts and pans lengthen the accumulationonce 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 coverif 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 accumulationthe spaces between pans freeze, strengthening the accumulation

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

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 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.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.

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

“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.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.When this happens, a “hummocky” ice cover is formed. This results in a much thicker ice cover, and can frequently cause flooding.This results in a much thicker ice cover, and can frequently cause flooding.

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

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

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

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

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

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

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

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

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

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

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

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