Morphological Description of Erosional and Depositional Landforms Formed by Debris Flow Processes in Mainland Norway

Abstract

The predominantly alpine landscape of Norway is situated along the Atlantic Ocean and most of Norway has today a relatively maritime climate with high both summer and winter precipitation. This is a generally favourable setting for precipitation induced debris flows given that there are sediments available for mobilization. This condition is also fulfilled since the last ice age left a landscape with valley sides covered with extensive deposits of glacial till. In addition to the till, extensive glaciofluvial deposits are found in the lower parts of the valleys and bedrock in high elevation areas are often draped with in situ weathering material. Weathering, rock falls and snow avalanches are active processes which continue to contribute more sediment to the slopes systems, available for remobilization by different types of debris flows. The varying morphological imprint of debris flow processes are readily identifiable in the landscape and have influenced the general development of valley sides and bottoms since the last ice age. Debris flows initiate and develop differently depending on the large-scale morphology of the slopes, the properties of the available sediments and the hydrological condition at initiation. Debris flow terminology is complex and may be based on several factors relating to release mechanism and type of movement within the flow. Here we use primarily morphological characteristics and thus describe and divide the Norwegian debris flows into two major types: A. Open-slope debris flows and B. Fluvial channel-dependent debris flows. The first type is further subdivided into: A1, channel forming debris flows; A2, widening debris flows and A3, high-viscosity debris flows. In addition, we mention and discuss larger scale landforms where debris flow processes play a significant role: C. Debris flow fan systems and D. Multi-process debris flow fans. We describe the general setting, release and movement mechanisms for the different types and the resulting morphological characteristics, using illustrated examples from around Norway.