Ice permeability
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Ice permeability |
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The in-situ permeability of the sea-ice cover has been determined employing a method based on the hydrological bail test, described by Freitag and Eicken (in prep.; to be submitted to J. Glaciol.). In short, this involves drilling of 0.14-m-diameter holes into the ice cover to a given depth level, inserting a packer tube open at the bottom and top end (with a tight seal along the entire vertical wall of the hole ensured by an inflatable rubber sleeve), and removing brine from the hole with a valve bail. This reduction in the local hydraulic head induces brine or meltwater flow into the hole through the bottom surface. From time-series measurements of the instantaneous water level in the hole with an ultrasonic sensor (vertical resolution +/- 5 mm), the intrinsic permeability of the ice layers underlying the hole can be derived based on Darcy's Law. Flow focussing into the hole due to anisotropic sea-ice permeabilities (assuming a ratio between permeabilities in the vertical to those in the horizontal direction of 10:1 based on previous field and lab measurements; details in Freitag and Eicken, in prep.) has been corrected for with an analytical expression derived from 3-dimensional fluid flow simulations. |
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Photographs showing dispersal of tracer (ca. 5 g Fluoresceine) on day 198 in a ridged area (Ridge mass balance site visible in background in upper left panel; local hydraulic gradients on the order of 0.01 to 0.1) 25, 40, and 110 min after release. A significant portion of the flow is confined to the highly permeable uppermost ice layer (see photograph of core taken at point shown in upper panel - note that the two tracer plumes are result of the confluence of two separate tracer releases). Meltwater from a large fraction of the ridge flank drained into a puddle visible in the foreground (lower panel) that was in hydraulic connection with the sea.
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The seasonal evolution of ice permeability and meltwater transport is shown schematically in this figure, indicating the ablation of the snow cover and the concurrent appearance and deepening of melt ponds. Early- and mid-season drainage of melt ponds resulted in formation of false bottoms and under-ice ponds. Low ice permeabilities during the early melt season resulted in horizontal meltwater transport over larger horizontal distances (10s to 100s of meters, see photographs of tracer experiments above). An overall increase in the macroscopic (controlled by pore space) and large-scale (controlled by development of cracks and thaw holes) permeability of the ice cover promoted vertical meltwater percolation during the second half of the melt season. |
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