Freeze-up and Freeze-up and Winter Ice Winter Ice ProcessesProcesses

Introductory lectures in

River Ice EngineeringIntroductory lectures in

River Ice Engineering

Border IceBorder Ice is usually the first type of ice to form in rivers. 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. Beltaosphoto 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:

Frazil ice forms in turbulent, fast flowing water.Frazil ice forms in turbulent, fast flowing water.

photo by R. Brown

Frazil Ice:Frazil Ice:

Frazil ice usually occurs in the Frazil ice usually occurs in the form of discs; however, needle form of discs; however, needle shapes occur as well.shapes occur as well.

microscopic photo by G. Tsang

CRREL photoCRREL photo

Frazil Ice Evolution:Frazil Ice Evolution:

1.1. Frazil forms spontaneously throughout the flow depth in Frazil forms spontaneously throughout the flow depth in supercooled, turbulent water (supercooled, turbulent water (TTww = -0.01ºC approximately). = -0.01ºC approximately).

2.2. Active frazil is very adhesive and flocculates to form frazil slush.Active frazil is very adhesive and flocculates to form frazil slush.

3.3. Once large enough to overcome turbulence, slush balls float to Once large enough to overcome turbulence, slush balls float to surface where the unsubmerged portion freezes to form “pans”.surface where the unsubmerged portion freezes to form “pans”.

1 2 3

The frazil pans have rough upturned edges, The frazil pans have rough upturned edges, resulting from collisions with each other. resulting from collisions with each other.

photo by S. Beltaosphoto by S. Beltaos

Pans may freeze together to form “rafts”.Pans may freeze together to form “rafts”.

photo by S. Beltaos

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

photos by R. Gerardphotos by R. Gerard

Once surface concentrations approach 100%, local frazil Once surface concentrations approach 100%, local frazil production ceases.production ceases.

photo by S. Beltaosphoto by S. Beltaos

Also, as surface concentrations approach 100%, Also, as surface concentrations approach 100%, “bridging” becomes likely. “bridging” becomes likely.

photo by R. Gerardphoto by R. Gerard

Bridging tends to occur at natural flow Bridging tends to occur at natural flow constrictionsconstrictions

from “River Ice Jams”, 1995from “River Ice Jams”, 1995

Bridging is also sometimes referred to as “arching”.Bridging is also sometimes referred to as “arching”.

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

photo by R. Gerardphoto by R. Gerard

““Juxtaposed” Ice CoverJuxtaposed” Ice Cover

• once bridging occurs, a stationary ice front once bridging occurs, a stationary ice front progresses upstream as incoming rafts and progresses upstream as incoming rafts and pans lengthen the accumulationpans lengthen the accumulation

• if the rafts and pans accumulate edge to edge, if the rafts and pans accumulate edge to edge, like a jigsaw puzzle, we have what is called a like a jigsaw puzzle, we have what is called a “juxtaposed”“juxtaposed” ice cover ice cover

• the spaces between pans freeze, strengthening the spaces between pans freeze, strengthening the accumulationthe accumulation

The rafts and pans have accumulated edge to edge.The rafts and pans have accumulated edge to edge.

““Juxtaposed” Ice CoverJuxtaposed” Ice Cover

photo by S. Beltaosphoto by S. Beltaos

Ice Cover ThickeningIce Cover Thickening

• Once frazil production stops, further heat Once frazil production stops, further heat loss causes freezing of the pore water loss causes freezing of the pore water within the frazil slush (within the frazil slush (i.e.i.e. completion of completion of freezing of the submerged portion of the freezing of the submerged portion of the pans).pans).

• Once the frazil slush is frozen, further heat Once the frazil slush is frozen, further heat loss causes thermal growth of ice. This loss causes thermal growth of ice. This occurs in long, vertically oriented crystals occurs in long, vertically oriented crystals and is known as and is known as “columnar” “columnar” ice.ice.

The columnar crystal structure is exposed when the ice cover melts.The columnar crystal structure is exposed when the ice cover melts.

We call this “candle ice”.We call this “candle ice”.photo by F. Hicksphoto by F. Hicks

““Hummocky” Ice CoverHummocky” Ice Cover

• Sometimes the drag force of the flow, Sometimes the drag force of the flow, acting on the underside of the ice cover, is acting on the underside of the ice cover, is sufficient to collapse a juxtaposed ice sufficient to collapse a juxtaposed ice cover.cover.

• When this happens, a When this happens, a “hummocky”“hummocky” ice ice cover is formed.cover is formed.

• This results in a much thicker ice cover, This results in a much thicker ice cover, and can frequently cause flooding.and can frequently cause flooding.

““Hummocky” Ice Cover Hummocky” Ice Cover also known as a “freeze-up jam” or a “frazil ice jam”also known as a “freeze-up jam” or a “frazil ice jam”

(Bow River at Calgary, 1950; photo courtesy of Alberta Infrastructure)(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.Notice how rough the ice cover is, and how the ice has pushed up into the trees.

Freeze-up jams create problems for bridges Freeze-up jams create problems for bridges

(photo courtesy of Nfld. Government)(photo courtesy of Nfld. Government)

Anchor Ice…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. Brownphoto by R. Brown

Frazil AdhesionFrazil AdhesionIf sufficient frazil adheres to a rock, it may float the If sufficient frazil adheres to a rock, it may float the

rock up and incorporate it into the ice cover. rock up and incorporate it into the ice cover.

Rock found in the Mackenzie River’s ice cover.

photo by F. Hicksphoto by F. Hicks

Frazil AdhesionFrazil Adhesion

Frazil adhesion to trash Frazil adhesion to trash racks is a common racks is a common problem at water intakes.problem at water intakes.

(photos adapted from Ashton , 1986) (photos adapted from Ashton , 1986)

Continuous Frazil ProductionContinuous Frazil Production

In steep reaches where an ice cover does not form, frazil In steep reaches where an ice cover does not form, frazil ice production will continue throughout the winter. ice production will continue throughout the winter.

The frazil produced will tend to accumulate under an ice The frazil produced will tend to accumulate under an ice cover which has formed in a flatter reach downstream. cover which has formed in a flatter reach downstream.

(adapted from Ashton , 1986) (adapted from Ashton , 1986)

V>1.5m/s

Hanging DamsHanging Dams

These frazil slush accumulations, known as These frazil slush accumulations, known as “hanging dams”“hanging dams”, can become very large. , can become very large.

LaGrande River, Quebec, 1973 (Michel and Drouin, 1981)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

Snow ice forms when the weight of snow on an Snow ice forms when the weight of snow on an ice cover is ice cover is sufficientsufficient for submergence and for submergence and

saturation to occur. saturation to occur.

(adapted from Ashton , 1986) (adapted from Ashton , 1986)

Ice cover (with cracks)

Ice TypesIce Types

snow ice frazil ice thermal icesnow ice frazil ice thermal ice

This core sample illustrates the different types of ice.

photo by F. Hicksphoto by F. Hicks

AufeisAufeis (ice on top of ice)(ice on top of ice)

aufeis accumulationaufeis accumulation

The weight of aufeis has depressed the Mackenzie River’s ice cover.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”, 1995from “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. Hicksphoto by F. Hicks

(Allen, 1977)

Mean freeze-over data on rivers