Sogn og Fjordane

Sogn and Fjordane is the county with the second highest number of historic rock avalanche events, and has historically the largest loss of life resulting from rock slope failures and related tsunamis. NGU has mapped around 23 unstable rock slopes that might form rock avalanches in the future.

Picture: View of the Osmundneset rock slope instability in Gloppen Municipality.

Sogn og Fjordane is a county in western Norway characterized by numerous fjords and valleys surrounded by high mountain sides. The steep mountain sides have led to several large rockslides and rock avalanches since the last glaciation. Some of these rock avalanches have caused deadly displacement waves in the fjords. There are, for example, seven large historical events documented at Loen Lake, all of them triggering displacement waves. Two of these lake tsunamis killed a total of 134 people in the years 1905 and 1936.

NGU has performed systematic mapping of unstable rock slopes in Sogn og Fjordane since 2007. Out of about 200 sites investigated, 23 unstable rock slopes with clear signs of past or present movements have been detected by NGU in collaboration with the former County geologist of Sogn og Fjordane and the Sogn og Fjordane University College. The systematic investigation of these unstable rock slopes is still ongoing and is expected to result in the hazard and risk classification of all the unstable rock slopes.

Periodic displacement measurements are currently being taken for 16 unstable rock slopes in Sogn og Fjordane, using for example differential Global Navigation Satellite Systems (commonly known as GPS), terrestrial laser scanning, satellite-based and ground-based InSAR or tape extensometers. Significant displacements have been measured on six unstable rock slopes: Flåm, Vidme and Viddalen in Aurland municipality, Osmundneset in Gloppen municipality, Lifjellet Øst in Hyllestad municipality and Ovrisdalen in Vik municipality. Those sites have average velocities ranging from 0.2 to about 3 cm/year.