Figure 1A: Interpreted oblique air photo from "Norge i 3D" of Svaddenipun above Rjukan. Three different scenarios (A, B, C) with different volumes were characterized that show different stages of development and displacement rates.
Telemark is a county in southern Norway characterized by high relief contrasts with valleys surrounded by high mountain sides that are often occupied by fjord lakes. Telemark was classified as lowest hazard category for unstable rock slope mapping in the Norwegian plan for landslide hazard mapping. Nevertheless, three potential high-risk objects have been reported in this county. These are Svaddenipun Mountain above Rjukan (Figure 1A), Kassen Mountain above Bandak lake and Håkåneset Mountain above Tinnsjø lake (Figure 2). All sites are characterized by clear indications of rock slope deformation and the impact area of the rockslide or related displacement wave will reach settlements with larger population making these sites potentially high risk sites.
Mapping of those rock slopes started in 2011 and displacement measurements in 2012. At the three unstable rock slopes, rock slope deformation is evidenced by the opening of cracks. However, displacement rates are very low or below uncertainty limits of GPS measurements (in general <3 mm) after an interval of 2 years of measurements. The lake floor was mapped at both Bandak and Tinnsjø lake (Figure 2), revealing at Bandak lake that a prehistoric rock avalanche has occurred in post glacial times (Figure 3), which certainly triggered a large displacement wave. At Tinnsjø lake, our lake-floor mapping revealed evidence that the instability continues far below the water level of the lake (Figure 2), however, our multi-sonar did not reach to the deepest part of the lake and further mapping is still outstanding that will attest the depth of the instability and show if prehistoric post-glacial failures have occurred at that site.