Dynamics of River Ice


River ice processes such as break-up and jamming are driven by climatic factors such as the seasonal flow pattern (the flow hydrograph), the degree and thickness of the winter ice cover, and the shape of the stream (stream morphology) (Beltaos, 2008). At the beginning of the cold season, river ice formation is triggered by both hydrodynamic (water flow) and atmospheric conditions. This can lead to three different freeze-up processes: (1) Ice formation in a smoothly-flowing (laminar) river, (2) formation of frazil ice crystals in turbulent flow, and (3) snow precipitation in water near the freezing point. The ice type depends on the formation process. Columnar ice (black ice) is formed in laminar flow and spreads horizontally, along the shore and perpendicularly to the center of the channel. Frazil ice (white ice) is generated in turbulent waters: Frazil crystals appear when the entire water column is mixed and below the freezing point. As they grow, frazil crystals stick together and form frazil slush, which can float to the surface and form ice pans or adhere to the bottom or any obstacle and form anchor ice. The ice pans will then aggregate to form floes which will eventually consolidate to form a stable cover of randomly oriented ice blocks. This type of ice is located principally downstream of rapids.

Ice break-up starts when there is a definite breaking and movement of ice due to melting, higher flow, or rising water levels. A thermal break-up is initiated when heat input to the ice cover reduces its size and strength, thus weakening it and melting it in place. A mechanical break-up is initiated by increased flow from rainfall or snowmelt, causing uplift and tension in the ice cover. When the pushing force of the water exceeds the strength of the ice cover, it breaks. The manner of break-up depends on a subtle trade-off between ice deterioration due to warm weather and ice cover rupture due to increased discharge (Hicks, 2007).

An ice jam is an accumulation of broken ice which restricts or blocks the flow of water. Ice jams form when the volume of moving ice exceeds the transport capacity of the river. This can occur due to channel narrowing, sharp bends in the river, or when the ice meets an obstacle such as a bridge or a strong intact ice cover. Upstream of an ice Jam, water levels may increase to high levels and cause severe flooding. When the jam is released, a steep water wave, a "jave", progresses downstream, resulting in flooding, breaking of downstream ice covers and potential damage to Infrastructure (see Video below). The severity of the break-up and ensuing ice jam is dependent upon the water levels during the preceding freeze up, the thickness of the ice cover at the end of the winter, the weather condition during the breakup period, the water velocity and the spring runoff (Beltaos, 2008). Ice-induced floods have lower recurrence than open water floods but cause larger damage. For example, in New Brunswick (Canada), ice-jam induced flooding accounts for 42% of all floods, but they are responsible for almost 70% of the flood damages (Humes and Dublin, 1988).

Download Video: MP4, WebM, Ogv

Video:) Ice jam release event, Athabasca River, Alberta, Canada, 2007. Faye Hicks, www.riverice.ca.

References

  • Beltaos, S. (2008). River Ice Breakup. Water Resources publications, Chelsea, Michigan, 462 p.
  • Hicks, F. (2009). An overview of river ice problems: Cripe07 guest editorial. Cold Regions Science and Technology 55, 175-185.
  • Humes, T.M. and J. Dublin (1988). A comparison of the 1976 and 1987 Saint-John River Ice Jam flooding with emphasis on antecedent conditions. Proceedings of the 5th Workshop on River Ice/Ice jams, June 1988, Winnipeg, Canada, 43-61.

Material for this page was provided by Thomas Bergeron and Yves Gauthier, Institut national de la recherche scientifique.