Author(s): Daniel E. Lawson; Edward F. Chacho; Bruce E. Brockett

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Abstract: Repetitive surveys and measurements from 1983 through 1986 of the ice-covered Tanana River near Fairbanks, Alaska, have shown that flow occurs in sub-ice channels that are separated by longitudinal bars composed of stratified, partly consolidated frazil ice of varying type and distribution. In contrast to hanging dams, these frazil bars extend up- and down-stream parallel to flow as well as from the base of the ice cover to the bed, and act as lateral walls for the sub-ice channels. Individual sub-ice channels may branch and reunite, thus forming a braided pattern beneath the ice cover. Longitudinal frazil bars apparently develop at locations characterized by lower velocities, such as where currents are diverted by irregularities in the bed or in the base of the ice cover. Once they are developed, shifting of the position of sub-ice channels and frazil bars suggests that bar faces are eroded by impinging currents. Increases in frazil bar dimension indicate winter growth and continuing deposition at other locations. Frazil bars are progressively eroded and destroyed as discharge, velocity and water temperature increase prior to spring break-up. Cross sections of sub-ice channels differ in dimension, configuration and hydraulic parameters, including velocity, discharge and sediment transport rate.

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Year: 1986