The importance of accounting vegetation on unsaturated slopes prone to rainfall-induced instabilities – a case study in Norway

In unsaturated steep slopes, the upper unsaturated zone may have a crucial role in the slope stability. In this work we studied a natural slope located adjacent to a railway track in Eastern Norway. Due to its steep inclination, the factor of safety should be always kept under observation. In addition, the climate in Norway is expected to become wetter and warmer, with increased snow melting. Thus the rainfall/snowmelt infiltration play an important role and needs to be monitored properly to avoid any failure. The slope is instrumented since 2016 and both volumetric water content (VWC) and the pore water pressure regime are monitored.

The modulus SEEP/W of the commercial software GeoStudio (GEO-SLOPE International, Ltd.) was used to model the transient seepage conditions of the slope for a 7-month monitoring period (from June 2019 to December 2019). Several analyses were carried out by changing the initial conditions and the boundary climate conditions of the slope. Regarding the initial conditions, two series of simulations were carried out, one with an initial calibration of the VWC distribution, another one without calibration, hence, by only locating the ground water table at a specific depth and by indicating the maximum negative head (as required by the model). The calibration, instead, consisted in starting the simulation considering a VWC distribution as closer as possible to the in-situ value.

For each series, a total of three simulations were carried out with different boundary climate conditions, respectively considering only rainfall/snowmelt (R), considering both rainfall/snowmelt and evaporation (Cl), and considering rainfall/snowmelt and evapotranspiration due to vegetation (V). Indeed, the slope is all covered by relatively dense vegetation, with shrubs and birch trees. For the simulations including evaporation and vegetation (Cl and V), the land-climate interaction boundary condition was adopted. Climate functions, such as the pairs temperature-time, relative humidity-time, wind speed-time, were obtained from a close meteorological station. The evaporation was determined by using the Penman-Monteith equation, including vegetation features in the case of vegetated slope. Preliminary results show that the initial calibration is important for the correct back-analyses of the measured data, and that the model is more accurate when accounting for climate boundary conditions and vegetation, which influence also the slope stability conditions.

The work has been part of the working package "Landslide triggered by hydro-meteorological processes" within the Center for Research-based Innovation (CRI) programme KLIMA2050 (), financed by the Research Council of Norway.