The energetics of an ablating headscarp in permafrost

Abstract

In arctic regions mass movements in thawing permafrost are common. Although different types of landslides have been identified, one type referred to as a "bimodal flow" has caused a significant amount of interest on account of the unusually high rate of mass wasting associated with this type of flow slide. The flow is characterized by a steep headscarp and a low angle mud flow or tongue at the base of the slide.Although more than one activity contributes to the retreat of the headscarp, the most important process is that of ablation. An obvious anomaly exists between the amount of heat required to sustain the high rates of observed ablation and the amount of heat available from net radiation.The components of the energy balance were identified and a field study was undertaken to quantify these terms. Instrumentation included radiometers, wet and dry thermocouples, anemometers, and a lysimeter to measure the latent heat of condensation of evaporation.The results of this study indicate that all the major terms in the energy balance equation combined to form the source of heat responsible for the high rates of observed ablation. Moreover, they provide a rational basis for controlling bimodal flow slides using different insulation materials.

Rate of Short-Term ablation of Exposed Ground Ice, Banks Island, Northwest Territories, Canada

“…Maximum ablation rates for 3 hand 24 h periods, for which comparable data are available, are somewhat lower (Table Ill). At Fort Simpson, an average ablation rate of 140 mmld was obtained for the period between 21 June and 21 July (Pufahl and Morgenstern, 1980). However, this figure was for horizontal retreat of the ice and would be reduced to about lOO mm/d of ablation measured orthogonally to the 45· face .…”

“…However, since unfrozen moisture is only a small fraction of the total moisture content (frozen and unfrozen) of the face of a typical ground-ice slump, this error is minimal (Pufahl and Morgenstern, 1980). The transformation of d into an energy flux then requires only the volumetric latent heat (represented by the terms to the right of d in Equation (4» which can be obtained through a measurement of the sediment content.…”

“…A comparison of the energy-balance data recorded at slump I and at Fort Simpson is complicated by the geometric transformation required for ablation in the latter data set. The contribution made by Q* to Q m at Fort Simpson was estimated to have been 43% over a 4 week period (Pufahl and Morgenstern, 1980). If the transformation due to the slope of the ice is applied to these data, this figure rises to approximately 62%.…”

“…Insgesamt stammen mindestens 60% der Energie, die fUr Ablation an dem Sturz verbraucht wird, aus der Netto-Strahlung. ablation rates or energy exchanges over exposed ground ice, exceptions for the former being Burn (unpublished) and Kerfoot (unpublished), and for the latter Pufahl and Morgenstern (1980). Most studies have concentrated on the measurement of head wall retreat of slulnps, since accurate assessment of ablation has been difficult without specialized instrumentation.…”

“…(Kerfoot, unpublished), and Fort Simpson, N.W.T. (Pufahl and Morgenstern, 1980). It is not possible to compare the ablation rates with more commonly measured head wall-retreat rates (e.g.…”