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

Abstract

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.

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

“…The processed images from each anchor ice event were combined in time-lapse videos to help visualize the results. An example of such videos for Event C is available for download at https://doi.org/10.7939/DVN/6X5ATL (Ghobrial and Loewen, 2019). Using these videos, the process of anchor ice formation, growth, and release was separated into four stages: Stage 1: initiation by in situ growth; Stage 2: tran-sitional phase; Stage 3: linear growth; and Stage 4: release phase.…”

Referee review of Ghobrial & Loewen manuscript tc-2020-161

“…This system is a major advance over what is described in Ettema (2013, 2016) This made it possible for the authors to measure the increase of anchor-ice mass thickness in addition to measuring individual ice crystal growth. Ghobrial and Loewen reference Kempema and Ettema (2013) but appear to dismiss their reported ice crystal growth rates of ∼1-4 cm/hr on the basis that the wedge wire screen was placed in the water column where heat transfer was greater relative to the bed. They suggest this might explain the higher ice-crystal growth rates reported by Kempema & Ettema relative to their findings (1-3 cm/hr) (P12L24-29, P: page number, L: line number).…”