Susceptibility maps for debris slides and debris flows

Susceptibility maps for debris slides and debris flows reveal potential runout areas for all type of soil landslides, with the exception of quick clay slides and large debris flows in river channels. The runout area includes all the area where the landslide still contains material that can be deposited. If the water content of the landslide is very high, the water can continue to travel farther than the solid material.

Picture: Extract from a susceptibility map for debris slides and debris flows. 

Susceptibility maps for debris slides and flows indicate the area that can be affected by these types of landslides. The maps are meant as a basis for initial hazard evaluation with respect to areal planning and land development in areas that have not been mapped in more detail. This is described in more detail in guidelines from the Norwegian Water and Energy Directorate (NVE). These maps do not say anything about the probability of landslides and therefore cannot be used in relation to the safety requirements of the building regulations.

A susceptibility map can be useful in conjunction with surveillance and preparedness in particularly vulnerable areas where more detailed maps do not exist. Similar maps for avalanches and rockfalls have previously been developed. The correct level of detail is displayed when the maps are approximately 1:50 000 scale.

Methods

The methods used in the creation of a susceptibility map are thoroughly described in an NGU report (Fischer et al., 2014). The maps are created using two different numerical topographic models; one for source areas and one for avalanche runout, where input parameters are manually calculated and adjusted according to the terrain shape, geology and former landslide activity in the area. Both the identification of source areas and discharge modelling is based on the National Mapping Authority’s latest digital elevation model (DEM) with 10 m resolution (2012).

To identify the source areas for landslides and debris flows, an empirical approach that analyses three topographical features is used; inclination, planar curvature and the size of the water catchment area for each DEM cell, using a GIS based tool (Rick Husband and Zimmermann, 1993). A cell is considered as a source area when it meets the threshold criteria for all three properties simultaneously. Threshold values are adapted for different regions that are separated by the following procedure: 1) The entire country's area is categorized into three classes according to how much earlier landslide activity has been observed from aerial photos; high, medium or low avalanche activity, 2) Similarly, areas are divided into three classes with respect type of soil cover; avalanche deposits, other soils or bare rock, 3) Two classes of elevation model source (FKB data or N50 data). Overall, this gives 18 different sets of thresholds.

Avalanche runout is calculated for each source area using a 'multiple flow direction' model (Flow-R; Horton et al., 2008; 2013), which takes into account the topography along the runout path. The model uses a probabilistic method to calculate which direction the material moves. The slope angle from the source region and a specified energy restriction in the model determine the avalanche runout area. The modelling does not take into vegetation and buildings. Other human encroachment, for example forest roads, will only affect the modelling of the runout to the extent the road is visible in the DEM. Fieldwork has been done at selected locations throughout the country to calibrate/validate the identification of source areas and the thresholds used for runout determination.