Types of River Ice - Two basic types of ice found in rivers
- Columnar
- Thermally grown
- "Black ice" or "blue ice"
- Tends to occur in slower flow
- Very clear, allows solar penetration
- Ice consisting of columnar shaped grain. Ice growth continues downward in long, vertically oriented crystals after a cover has been established.
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- Fine-grained
- Frazil or snow
- "White ice"
- Tends to form in open, super-cooled water that is fast flowing or turbulent
- Resists solar penetration
- Fine crystals of ice suspended in turbulent, super-cooled water.
- Water turbulence inhibits crystal coagulation into an ice sheet.
- May accumulate on water bed surface or submerged objects forming anchor ice.
- Also called 'needle ice'.
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River Ice Formation
Thermal Ice Formation
- Thermal ice tends to occur in slower flow.
- Very clear, allowing for solar penetration
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- Border Ice
- Forms along, and is attached to, river banks.
- May also be attached to other obstacles such as bridge piers.
- Calm water flow provides best conditions for growth.
- Water level changes or surface waves can fracture existing border ice or detach it from the shore.
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Frazil Ice Formation
- Frazil ice tends to deposit...
- Where the stream slope changes from steep to mild
- Downstream from location that are tubulent enoguth to remain open most of the winter (e.g. rapids)
- The implications of frazil deposition...
- The thicker the ice, the longer it will take to break up
- Potential jam location
- Increases ice volume
- When to be concerned about thicker than normal frazil desposition
- Sudden period of intense cold when there is little to no ice cover to insulate the water surface
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- Frazil Pans / Pancake Ice
- Rounded, flat accumulation of frazil ice with a rough, raised rim.
- Shape and rim are due to repeated collisions with other frazil pans.
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- Frazil Rafts
- Collections of frazil pans that have collided and frozen together.
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- Ice Bridge
- Continuous ice cover of limited size extending from shore to shore like a bridge.
- Tends to occur at natural flow constrictions.
- Also referred to as 'arching'.
- Can occur as frazil pans accumulate and nearly cover water surface.
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- Juxtaposed Ice
- An edge to edge accumulation of frazil rafts and pans that appears similar to a jigsaw puzzle.
- As the spaces between the pans and rafts freeze, the ice cover is strengthened.
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- Hanging Ice
- In areas where an ice cover does not form, frazil is continuously produced.
- As the slush moves into a flatter, slower moving portion of the stream, the frazil will accumulate under the ice cover.
Can result in a hanging dam.
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- Anchor Ice
- Ice attached to the channel bed.
- Forms when turbulent flow transports frazil crystals or supercooled surface water deeper into the flow, allowing ice to build on underwater objects.
- Can develop into extensive ice blankets.
- Remains bonded to surface until ice buoyancy sufficient to lift attached material or until water warms, weakening the bond between the bed and ice.
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- Aufeis Ice
- Ice on ice.
- Forms when water from a spring, stream or culvert runs on top of existing ice and freezes.
- This occurs wherever there are continuous sources of water and freezing temperatures.
- Stream aufeis can fill the channel with ice that forces streamflow out of the banks to cause localized flooding.
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- Snow Ice
- Ice on ice.
- Weight of snow on ice cover enough to cause submergence, saturation and freezing.
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Freeze-up
Period of initial formation of a continuous ice cover on a body of water.
Predominated by border and frazil ice.
- What to look for as river starts to freeze up
- Is there border ice? How far from the shore has it grown?
- Is there moving ice? What types are present?
- Frazil slush?
- Frazil pans?
- Fragmented sheet ice?
- Large sheets?
- How much of the open channel does moving ice cover?
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- What to look for after ice cover forms
- How smooth (or rough) is the surface?
- How high are the surface variations?
- Is there anything to indicate the ice is starting to decay? Is ice snow covered?
- Is there melting snow?
- Is the ice beginning to melt or rot?
- Do you see 'candled' (or honeycombed) ice?
- Are there cracks in the ice?
- Are they parallel to shore? How far out?
- Are they perpendicular to shore?
- Is the ice fractured (parallel and through the ice) near the banks?
- How far from shore?
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- Sheet Ice
- Smooth, continuous ice cover.
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- Hummocked Ice
- Flow under the ice collapses the juxtaposed ice cover which then refreezes.
- Result is a thicker ice cover.
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Break-up
Period of disintegration of ice cover.
- Thermal break-up
- Weather factors most important
- Warmer air temperature
- Warmer water temperature
- Reflectance of the ice surface; surface color influences absorption of sunlight
- Direct sunlight plays a large role
- Ice 'rots' or melts in place, much like on a pond or lake
- Water on ice decreases reflection, may promote melting
- Development of open water allows more heat to enter the flow, melting ice from below.
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- Mechanical break-up
- Physical factors most important.
- Ice cover is pushed higher or lower, causing it to break into pieces
- Hydrodynamic forces acting on ice cover exceed cover strenght
- Results from an increase in stream discharge... increased energy to the system
- Precipitaiton event
- Snowmelt event
- Dam operation... large, sudden increase in releases
- Rule-of-Thumb...A stage (water level) increase of 1.5 to 3 times the thickness of the ice is needed to lift, break and transport the ice cover.
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- What to look for to indicate ice is breaking up
- Are there cracks in the ice?
- Are they parallel to shore? How far from shore?
- Are they perpendicular to shore?
- How far apart are the cracks on average?
- When did the ice begin to move?
- How long did it take for the ice to clear out?
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- Decay - Candle Ice
- Rotten, columnar-grained ice.
- Melt occurs at the edges of individual ice crystals.
- Eventually, these crystals begin breaking away from the main ice body.
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- Rotten Ice
- Ice in an advanced stage of disintegration.
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- Hinge Crack
- Typically, hinge cracks form along both banks.
- In narrow channels, a single crack may form down the middle of the channel.
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- Transverse Crack
- Cracks across a streambed formed as the ice cover lifts with the rising water level.
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- Crack
- Fracture formed in an ice cover or floe that does not divide it into two or more pieces.
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Ice Clearing
- As the water level continues to increase, ice will be lifted until at some point it will be freed from the geometry of the channel and move downstream.
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- Ice Run
- Flow of ice with the current of a stream or river, particularly with break-up.
- May be light or heavy, and may consist of frazil, anchor, slush or sheet ice.
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- Ice Ledge
- Narrow fringe of ice that remains along the shores of a river after break-up.
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Many thanks to Dr. K. White with the Cold Regions Research and Engineering Lab and
Dr. F. Hicks with the University of Alberta, Edmonton for their photo contributions.
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