Rainfall induced landslides is one of the most common natural hazard worldwide. Intensive rainfall and rapid snow melting caused the two landslides in Jordalsdalen in Fjærland in Western Norway.
Debris flow is a global phenomenon and occurs in all regions with steep relief and occasional rainfall. They are one of the most frequent mass movement processes and play an important role in moving sediment from steep lands and into river systems.
Their high flow velocity, impact forces, and long runout, combined with poor temporal predictability, cause debris flows and debris avalanches to be one the most hazardous landslide types.
The terminology debris is related to soil containing more than 20 % gravel and coarse sizes. Debris flow is a very rapid to extremely rapid flow of saturated non-plastic debris in a steep channel.
Instability of steep slopes commonly results in the occurrence of debris flows. The initiation area in both debris flows in Jordalsdalen consisted of unconsolidated material overlying bedrock. Water commonly contributes to the instability of steep slopes. Infiltration of rainfall and melt water from snow produced increased soil weight and decreased soil strength to the point that the mass began sliding downslope.
Western Norway is a region in an exposed climatic position with much rain in the spring and fall, and even a lot of snow in the winter season, especially at high elevations. The Intergovernmental Panel on Climate Change (IPCC) predicts that this region in Norway will be expected to experience more frequently intensive rain and snow falls in the future. If this future scenario is consisting the area will probably be exposed for a number of debris flows.
Jordalsdalen is a valley between steep mountainsides and the area is a constant potential hazard zone. It is therefore important to mitigate the effect of debris flows in this area and assess the situation of future events.