Hazard evaluation of rock avalanches; The Baraldsnes - Oterøya area

Abstract

The report provides an evaluation of the hazard related to Skår\/Baraldsnes area in accordance with a contract with Hydro Technology and Projects. Geological and historical data on rock-avalanche events demonstrate that the high-risk areas are concentrated to the inner part of deep fjords of western Norway. The studied locations, Baraldsneset, Oppstadhornet and Hellenakken, are situated outside the high-risk areas. Major avalanches cannot be excluded from this region, but the probability is much lower. There are no indications of larger-scale instability structures on Skårahornet near Baraldsneset, and the probability of large rock avalanches from this near area considered very low and at least lower than a yearly probability of 10-4. Debris flows triggered along Kloppelva at the western slope of Skårahornet can reach the fjord in the western part of the considered area and a hazard zone with an estimated yearly of 10-4 is present. Several areas exhibiting collapsed bedrock characterize the 700-m-high slope towards Oppstadhornet. There, distinct large blocks show instense fracturing and overall down-slope sliding both along the foliation and on cross-fractures. Surface disturbanches at two locations indicate recent movement; these failures are younger than ca. 11500 years. The volume of the entire failure is estimated to be c. 20 mill m3 (scenario 1), including a 2-3 mill m3 large steep easter part (scenario 3) that is characterized by fractures and fissures showing evidence of movements. A potential more limited slide from the upper parts of the collapsed area (scenario 2) may influence a mass of 5-7 mill m3. This part constitutes the unstable top layer modelled by NGI. Estimates of potential run-out distances indicate that a large rock avalanche involving the entire area may go 0,7 - 1,2 km into the fjord (scenario 1), whereas the uppermost top of the slide may reach 0