Abstract
Manndalen valley in northern Norway has a high density of unstable rock slopes. Geomorphological characteristics, geological structures and slope activity have been mapped for all 10 rock slope instabilities. Foliation is sub-horizontal and joints almost vertical throughout the entire valley, leading to the conclusion that geological structures do not allow for an easy kinematic failure process. However Interferometric Synthetic Aperture Radar (InSAR) data from the TerraSAR-X satellite indicate that six of the sites are displacing with average rates in between 3 and 26 mm/yr. The most active unstable rock slope in Manndalen is Gamanjunni-3, showing displacements of locally up to 5 cm/yr and clearly developed limits of the displacing rock mass. Two rock avalanche deposits have been dated to 10.6 ± 0.6 ka and 4.1 ± 0.3 ka BP. This revealed a frequency of ~5000 years for rock avalanches in Manndalen in post glacial time, supporting earlier conclusions that the Gamanjunni-3 site is prone for a failure in close future.
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