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The Cryosphere An interactive open-access journal of the European Geosciences Union
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Preprints
https://doi.org/10.5194/tc-2020-161
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/tc-2020-161
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  13 Jul 2020

13 Jul 2020

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This preprint is currently under review for the journal TC.

Continuous in situ measurements of anchor ice formation, growth and release

Tadros R. Ghobrial1 and Mark R. Loewen2 Tadros R. Ghobrial and Mark R. Loewen
  • 1Department of Civil and Water Engineering, Laval University, Quebec, G1V 0A6, Canada
  • 2Department of Civil and Environmental Engineering, University of Alberta, Edmonton, T6G 1H9, Canada

Abstract. In northern rivers, turbulent water becomes supercooled (i.e. cooled to slightly below 0 °C) when exposed to freezing air temperatures. In supercooled water, frazil (small ice disks) crystals are generated in the water column and anchor ice starts to form on the bed. Two anchor ice formation mechanisms have been reported in the literature: either by the accumulation of suspended frazil particles, which are adhesive (sticky) in nature, on the river bed; or by in situ growth of ice crystals on the bed material. Once anchor ice has formed on the bed, the accumulation typically continues to grow (either due to further frazil accumulation and/or crystal growth) until release occurs due to mechanical (shear force by the flow or buoyancy of the accumulation) or thermal (warming of the water column which weakens the ice-substrate bond) forcing or a combination of the two. Although detailed laboratory experiments have been reported to study anchor ice, but very few field measurements of anchor ice processes have been reported. These measurements have relied on either sampling anchor ice accumulations from the river bed, or qualitatively describing the observed formation and release. In this study, a custom-built imaging system (camera and lighting) was developed to capture high-resolution digital images of anchor ice formation and release on the river bed. A total of six anchor ice events were successfully captured in the time-lapse images and for the first time, the different initiation, growth and release mechanisms were measured in the field. Four stages of the anchor ice cycle were identified, namely: Stage 1: initiation by in situ crystal growth, Stage 2: transitional phase, Stage 3: linear growth, and Stage 4: release phase. Anchor ice initiation due to in situ growth was observed in three events and in the remainder the accumulation appeared to be initiated by frazil deposition. The Stage 1 growth rates ranged from 1.3 to 2.0 cm/hr and the Stage 2 and 3 growth rates varied from 0.3 to 0.9 cm/hr. Anchor ice was observed releasing from the bed in three modes referred to as lifting, shearing and rapid.

Tadros R. Ghobrial and Mark R. Loewen

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Tadros R. Ghobrial and Mark R. Loewen

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Continuous In-situ Measurements of Anchor Ice Formation, Growth and Release Mechanisms in Rivers Tadros Ghobrial; Mark Loewen https://doi.org/10.7939/DVN/6X5ATL

Tadros R. Ghobrial and Mark R. Loewen

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Short summary
Anchor ice typically forms on river beds during freeze-up and can alter river ice regime. Most of the detailed knowledge on anchor ice mechanism has been attributed to lab experiments. This study presents for the first time, insights into anchor ice initiation, growth, and release in rivers using an underwater camera system. Four stages of growth and modes of release have been identified. These results will improve modeling capabilities in predicting the effect of anchor ice on river ice regime.
Anchor ice typically forms on river beds during freeze-up and can alter river ice regime. Most...
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