Rest-Norway

The rest-Norway project focuses on mapping potentially high-risk objects in counties where systematic mapping has not yet started. So far mapping activities have concentrated on four mountains with rock slope deformation and/or prehistoric rock slope failures in Telemark.

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.

Utsnittet fra bildet over. Fra østflanken av fjellet i Rjukan, hvor sprekkene i dag har opptil en meters åpning  og er flere titalls meter dyp.
Figure 1B: A closer look on the picture above. This is from the east of the actual mountain in Rjukan.  The cracks is about 1 meter wide, and tens of meters deep.

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.

Skyggerelieffet av de ustabile fjellpartiene ved Håkaneset viser to ustabile partier, der et er delt inn i tre forskjellige scenarioer. Håkaneset 1 har deler som ligger over og under vann (den hvite linjen i midten er vannkanten). Den er større i volum, og kjennetegnes ved en stor åpning av en graben langs toppen.
Figure 2: View from N of a hill shade of the instabilities on Håkåneset Mountain showing two instabilities one of which was divided into three scenarios. Håkåneset 1 has a large subaerial and a large subaquatic part (water line is the white line in the middle). It is larger in volume and characterized by a large opening of a graben along the top.

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.

Vår batymetriske kartlegging av Bandak dokumenterte at et postglasialt fjellskred skjedde nær østenden av vatnet. Avsetningene ligger rett under det ustabile fjellpartiet ved Kassen.
Figure 3: Our bathymetric mapping of lake Bandak revealed evidence that a post glacial rock avalanche occurred close to the E end of the lake. The deposits lie right below the Kassen instability.