Massrörelser initierade av extremnederbörd Ett exempel från Andøya i Nordnorge

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Abstract
This paper discusses some of the important factors influencing mobilization of rapid mass-movement (slides, flows) on steep (35°–40°) mountain slopes affected by extreme precipitation. A large area, including the Lofoten islands and the adjoining mainland, was affected by a violent rain-storm on 6 October 1959. The rain caused several slides and mudflows, most of which occurred on slopes exposed to the strong southwesterly winds. The geomorphological effects of the rain-storm can still clearly be seen as almost unvegetated scars in the slopes. One reason for the catastrophic effects of the 1959 rain-storm was the high precipitation during the preceding months (152 % of the normal precipitation in September) which had saturated the soil at the time of the rain-storm. Judging by the morphology of the erosion scars, the rapid mass-movement of soil always started with several successive slides. The slides were often initiated below basin-like, shallow depressions in the upper parts of the slopes or at other parts of the slope where drainage became concentrated. The small-scale morphology of the slopes seems to be more important than the mechanical composition of the soil in slide initiation. As the soil-water content increased downslope, the transport was transformed into mud-flows. The deposition of sediments shows that the mud-flows in some cases were followed by a terminal fluvial phase of lesser significance. The transport processes primarily relocated the solid material on the slopes and very little appears to have been transported away.

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