On 10th of June 2011 a high amount of precipitation fell on southern Norway, the result of this precipitation in a combination with increased snowmelt cause a flood, the aspects of the flooding will not be discussed here because it has already been documented by others. The precipitation event triggered landslides in southern Norway, and caused several important roads to close.
Six landslides were identified in the study area of Veikledalen; several other landslides in the area were observed but are not part of this thesis. The landslides have been interpreted to have been triggered by precipitation, in the form of intense rainfall with or without antecedent rainfall. Rainfall thresholds were used to determine the triggering by precipitation. The landslides were determined to have started as debris slides which quickly transformed into debris flows.
Triggering zones of the six debris flows were identified and they are triggered on the western slope of the valley where the slope angle changes from a shallower angle to a steeper angle. Four debris flows were triggered at approximately the same elevation, while two debris flows were triggered at a lower elevation.
Erosion and deposition volumes have been estimated, and slope angles where deposition and erosion occurs have been measured. This can help determine the slope angle at which the debris flow transition from erosion to deposition. A possible connection between confinement of flow, by channelized flow, and erosion, and a connection between an open slope and deposition can also be observed in the study area. A combination of slope angle and confinement of flow is probably the important factors that determine the transition for a debris flow from erosion to deposition.
Debris flows are a threat to human life and property, the initial volume of the flow might be low but due to erosion the volume increases greatly. Debris flow can cause damage from impact forces, and can transport large objects for example a tractor or a car, which can cause damage due to collision with another object. Observations in the study area suggest that debris flows could transport boulder with a diameter of 4 meters, heavy tractors, and several cars. Processes that could support these objects in the debris flows are discussed.
Historical data from the area indicate that debris flows have occurred in the area previously, and with the possibility from climatic models that precipitation will increase in the study area the frequency of these events might increase.