Skredkartlegging i Uskedalen, Kvinnherad kommune, Sunnhordland

Exhaustive knowledge of avalanche trigger mechanisms, dynamics and run-out length is of paramount importance when planning infrastructure. Recognizing active avalanche processes in an area is crucial to determine which mitigation measures that should be applied, as the different processes have various trigger mechanism and destructive potential. Avalanche processes are rapid mass-wasting of debris, snow and water, each process yielding a different geomorphological and sedimentological imprint on the colluvial fans and aprons. A thorough mapping of avalanche deposits has been conducted in Uskedalen, Kvinnherad municipality. The aim is to delineate the different avalanche processes’ geomorphological and sedimentological effects, as well as their role in the Holocene evolution of the landscape and their present activity. Various avalanche processes were recognized through extensive field observation of sedimentological properties and examination of geomorphological features in terrain models created in ArcGIS. The result of the avalanche mapping is presented in a Quaternary geological map, as well as a new map product from the Norwegian Geological Survey (NGU) – high resolution quaternary geological map. In addition to the mapping, a ~60 cm sediment core was retrieved from lake Fjellandsbøvatnet to study the potential of a reconstruction of the Holocene debris flow activity. Eight debris flow events were recognized in the core using high-resolution 3D-models derived from CT-scan data. Deposits of rockfalls, rockslides, high- and low-viscosity debris flows, earth slides, slush avalanches and slab avalanches were recognized in Uskedalen. A proposition of the activation of the various avalanche systems in accordance to the wastage of the ice-sheet is presented. Observations indicate high activity of rockfalls, rockslides and high-viscosity debris flows in the paraglacial period following the deglaciation. There are indications that all the above processes have been active throughout the Holocene, apart from rockslides, which may be restricted to the paraglacial activity of the early Holocene