Hazard and risk assessment of the Stiksmoen unstable rock slope in Aurlandfjord (Flåm, Vestland)

Abstract

Stiksmoen is located on a west-facing slope at approximately 120 m a.s.l. in the southern end of Aurlandfjord (Flåm municipality, Vestland county). The unstable rock slope is bounded by a well-developed backscarp, wide open in the north and open but less developed in the southern sector. The lower limit is mapped close to the foot of the frontal cliff of the unstable rock mass. The upper and central part of the instability is characterized by the presence of several open cracks, 10 to 15 meters deep. High rockfall activity is observed in the frontal cliff, with the detached blocks forming a block field. Structural and geophysical surveys allowed for the reconstruction of the sliding surface of Stiksmoen and the computation of its volume (ca. 0.4 million m\u00B3). Extensometers, dGNSS, ground-based and satellite-based radar measurements show similar displacement rates in the upper-central part and on the lower-frontal part of the unstable rock mass. The obtained displacement rates range from 0.9-2.5 cm\/y towards the west, with a dip of ca. 30\u00B0. The degree of development of the Stiksmoen unstable rock slope, its structuralcharacteristics and displacement rate result in a high hazard level. A sudden collapse of the whole unstable rock mass would result in rocks propagating mostly towards the fjord and a small proportion in WSW direction (towards Viki). The direct impact of the rock mass into the fjord would cause a displacement wave that could result in the loss of lives. The high hazard level and the potential consequences in the direct and secondary impact area result in a high-risk level for Stiksmoen.