Effects of ice content on the thermal erosion of permafrost : implications for coastal and fluvial erosion
Résumé
The effects of ice content and water flow on thawing and erosion of non-cohesive permafrost banks were investigated through laboratory experiments. A critical ice content was identified as associated with turbulent flows (20% and 80% for Reynolds numbers of 15 900 and 12 700, respectively), above which thermal erosion results in ablation and a decrease in ice content increases ablation. Below the critical value, the ablation model over-estimates the erosion rate as the removal of thawed sediments occurs episodically, possibly due to the imbrication of sand grains. The ablation model applies to rivers with non-cohesive banks, high ice contents and subject to high-velocity water flows. The model may explain differential erosion that results in massive ice layers in relief. At other sites, however, the retreat rate increases with ground ice content. This behaviour can be explained for sandy permafrost with a relatively low ice content and heterogeneous sand sizes and shapes subject to relatively low-velocity water flow. These results apply only to perenially frozen sands, as even a small percentage of cohesive material would modify the relationships described. Copyright (c) 2011 John Wiley & Sons, Ltd.