Permafrost

Theme 1. Permafrost engineering, properties of frozen
soils, model development, and their applications
Influence of snow meltwater infiltration on active layer movement in
steep alpine scree slopes within the discontinuous mountain permafrost
zone
10
A. Rist 1,2 , M. Phillips 1 , W. Haeberli 2
(1.WSL - Swiss Federal Institute for Snow and Avalanche Research SLF, Davos, Switzerland;
2. Physical Geography Division, Department of Geography, University of Zurich, Switzerlan)
Abstract: The phenomenon of slow creep of seasonally and perennially frozen slopes has
already been known for a long time. In the European Alps numerous researchers have studied
creeping processes of slopes situated in the discontinuous permafrost zone. Profiles of
downslope displacements have usually been measured in vertical boreholes, several tens of
metres deep, using an inclinometer, with time intervals of at least several weeks between each
measurement. In these studies the creep rate of scree for a given stress level was found to be
mainly a function of ice content and temperature. However, the active layer is a special case:
since its ice content can be quite low in mountain permafrost regions due to the well drained
nature of coarse grained debris, infiltration of snow meltwater has to be considered as a cause
for downslope movement. As the active layer is most susceptible to destabilization related to
global warming, the influence of meltwater infiltration on the downslope displacement of the
active layer has been investigated in the field.
The study site is situated in a 37° steep, NW exposed scree slope at a height of about
3000m a.s.l. in the Upper Engadin Valley, Eastern Swiss Alps. The existence of permafrost at
this site had previously been proven by temperature measurements in boreholes. Meltwater
infiltration rates were measured using a lysimeter with an area of 4m 2 . To observe the influence
of water infiltration on volumetric soil water content and hence on slope deformations, TDR
probes were buried at various depths in the active layer. The downslope displacements of the
active layer were recorded with an in-place-inclinometer, which was installed in the middle of
the active layer in a borehole. A measurement interval of two hours enabled a high temporal
resolution monitoring of the three parameters. Data were collected over two snow melt periods
(2004 and 2005).
The results show a clear correlation between the meltwater infiltration rate, water content
and the downslope displacement of the active layer. As a consequence of the period of
continuous meltwater infiltration at the beginning of June, the water content and the downslope
displacement of the active layer increased sharply. Over the subsequent six months the average