The unstable rock slope, Stampa, above the village of Flåm, Norway,
shows signs of both active and postglacial gravitational deformation
over an area of 11 km2. Detailed structural field mapping, annual
differential Global Navigation Satellite System (GNSS) surveys, as
well as geomorphic analysis of high-resolution digital elevation
models based on airborne and terrestrial laser scanning indicate
that slope deformation is complex and spatially variable. Numerical
modeling was used to investigate the influence of former rockslide
activity and to better understand the failure mechanism. Field observations,
kinematic analysis and numerical modeling indicate a strong structural
control of the unstable area. Based on the integration of the above
analyses, we propose that the failure mechanism is dominated by (1)
a toppling component, (2) subsiding bilinear wedge failure and (3)
planar sliding along the foliation at the toe of the unstable slope.
Using differential GNSS, 18 points were measured annually over a
period of up to 6 years. Two of these points have an average yearly
movement of around 10 mm/year. They are located at the frontal cliff
on almost completely detached blocks with volumes smaller than 300,000
m3. Large fractures indicate deep-seated gravitational deformation
of volumes reaching several 100 million m3, but the movement rates
in these areas are below 2 mm/year. Two different lobes of prehistoric
rock slope failures were dated with terrestrial cosmogenic nuclides.
While the northern lobe gave an average age of 4,300 years BP, the
southern one resulted in two different ages (2,400 and 12,000 years
BP), which represent most likely multiple rockfall events. This reflects
the currently observable deformation style with unstable blocks in
the northern part in between Joasete and Furekamben and no distinct
blocks but a high rockfall activity around Ramnanosi in the south.
With a relative susceptibility analysis it is concluded that small
collapses of blocks along the frontal cliff will be more frequent.
Larger collapses of free-standing blocks along the cliff with volumes
> 100,000 m3, thus large enough to reach the fjord, cannot be ruled
out. A larger collapse involving several million m3 is presently
considered of very low likelihood.