Long‐term precipitation and slide activity in south‐eastern Norway, autumn 2000

Abstract

AbstractIn autumn 2000 high amounts of precipitation caused a large number of landslides in south‐eastern Norway. The precipitation reached more than 400% of the normal monthly values in November. In addition to the precipitation, groundwater monitoring showed unusual high levels. High air temperatures in the mountains caused the precipitation to fall as rain instead of snow, causing additional runoff in the catchments. In this paper, 90 landslides are presented that were studied and evaluated by the Norwegian Geotechnical Institute (NGI) in the counties effected by the slide activity. The landslide database serves as start point for an analysis of the connection between slide activity and precipitation. The analysis shows a high variability of the observed precipitation just a few days before the slides. This variability decreases significantly around 50 days prior to the slides when the results stabilize at about 400% of normal precipitation in a 50‐day period. Comparison of these results to long time observations gives an estimate of return periods for the precipitation events. The 53 days accumulated precipitation has the highest return period of 132 years. This suggests that the high landslide activity in autumn 2000 in south‐eastern Norway was caused by a long‐term infiltration of large amounts of precipitation rather than high rainfall intensities during a short period. Copyright © 2007 John Wiley & Sons, Ltd.