Landslides triggered by snowmelt, Eastern Norway

Landslides in intermediate soils are generally triggered by natural factors affecting saturation and porewater pressure. Main factors controlling this are infiltration from rainfall, in cold regions also from snowmelt, and groundwater flow, including artesian porewater pressure. These factors are generally difficult to predict, as landslides often appear on places where “no-one was expecting it”. In year 2000, a landslided occurred at Negarden Sander after long-term rainfall. In April 2018, new landslides occurred in the same area, which is the main topic of this paper. After a long winter, there was a sudden change to summer temperatures. During few weeks, a snow cover of almost one meter vanished, equivalent to a water column of more than 200 mm, and resulted in 4 small landslides in a limited area, one each day from 20-23 April 2018. The topsoil consists of layers of intermediate soils, typically coarse silt to fine sand, overlying less permeable clay layers to large depth. Potential frost effects were evaluated to assess whether ground freezing influenced subsequent infiltration of meltwater. The conclusion was that the ground below the snow cover was probably unfrozen, leaving the ground open to infiltration when the intense snowmelt started. Based on site-specific soil properties, slope stability prior to snowmelt was found to have some safety margin under “normal” conditions (without extreme infiltration). Infiltration analyses done for the year 2000 landslide supports the conclusion that infiltration of snowmelt brought the stability to critical levels. Local variations are assumed to control the time of triggering of the four landslides on consecutive days, and at varying positions in the slopes.