Inner structure of anchor ice and ice dams in steep channels

Abstract

Dynamic ice growth (composed primarily of anchor ice and ice dams) is the dominant process in steep (slope>0.3%) channels during winter. The ice growth (in all its manifestations) regulates water levels, flow resistance, bathymetry, flow rate, water quality and fish habitat. Empirical models of the processes are available but in order to develop more physically-based models, more knowledge about the nature of ice in these channels is required. This paper presents original data from two channels of the Montmorency River watershed known for their intense anchor ice and ice dam development activity. The data include time-lapse photographs, underwater photographs, water level records, and water and air temperatures collected during winter 2012–2013. Moreover, anchor ice and ice dam samples were analyzed with thin sections and using computed axial tomography (CAT) scan technology. This study aims at understanding the development processes of both anchor ice accumulations and ice dams by investigating their crystal types and sizes, their growth mechanisms, patterns, and orientation, and their porosity.Two main types of ice crystals were observed: columnar ice crystals and fixed-frazil ice crystals. Relatively large columnar ice crystals grew upwards (away from the bed) perpendicularly to the local flow surface and were only observed in ice dam samples. Fixed-frazil crystals originated from the deposition and/or interception of drifting frazil particles that became “fixed” to the bed and to existing ice accumulations. The data suggest that in-situ growth of fixed-frazil ice crystals was the dominant process (accounted for the most ice development) for both anchor ice and ice dams. Fixed-frazil ice consists of a bonded mass of interlocking plate shaped crystals demonstrating a preference to be perpendicular to the bed and water surfaces. The mass displays some variability in porosity (mean value 41%). When analyzed using CAT scan slices, the interlocking plates appear as a tree-like or dendritic structure. The average size of resulting crystal segments and the maximum size of individual whole crystals within fixed-frazil ice structures were significantly more important for ice dam samples than for anchor ice samples.