Preliminary identification of hydro-meteorological conditions that trigger landslides in Norway

To reduce the impacts of debris flows, debris avalanches and slushflows the Norwegian Water Resources and Energy Directorate (NVE) is operating a forecasting and early warning service that issues daily landslide warnings to local authorities and public in general. Already in the first 5 years of operations, it has been observed that the most relevant landslide-triggered hydro-meteorological conditions (LHMC) vary between regions and seasons. Two different approaches have been tested to further explore this observation. Using a heuristic approach, based on observations, region and season specific LHMC have been identified. These conditions are defined by the spatial and temporal distribution of different hydro-meteorological parameters (e.g. rainfall, snowmelt, soil saturation, etc.), landslide occurrence, as well as other synoptic conditions (i.e. information about location and paths of low- and high-pressure systems, coincidence of atmospheric rivers, strong wind, extreme events, etc.). The landslide data are obtained from the national mass movements database available at www.skredregistrering.no, while historical hydro-meteorological data are recorded as 1km2 grid maps at seNorge.no.The analysis confirmed that water, in form of rainfall (also convective), snowmelt, high soil saturation or a combination of them, is the main triggering mechanism of landslides. In total eight hydro-meteorological conditions have been found to be most relevant for landslide occurrence. Each LHMC is described based on certain criteria like: main exposed areas, temporal distribution (season and month), general weather description and type of weather prognosis, duration of the condition, other synoptic information, list of dates when the condition was observed and caused landslides, general description of the main hydro-meteorological parameters, number and type of landslides, information about other associated hazards, evaluation of the landslide hazard index performance and recommendation about the most appropiate warning level.Separately, a quantitatively evaluation was also tested, in a selected region, by using rain as main triggering factor, and the Grosswetterlagen (GWL) weather pattern classification through exploratory and statistical analysis, to see how this can be used as integrated tool in the operational service. In this work, the applied analytical process is described. The hydro-meteorological conditions and their predictability are also shortly described, by presenting some recent examples. Finally, it is explained how the LHMC are integrated in the daily forecasting operations. Ideas for improvements will be discussed.