# Rockfall Modelling in Forested Areas: The Role of Digital Terrain Model Grid Cell Size
## 4 Discussion
The shape of the area is more complex and detailed at DTM1 and DTM2 as a result of greater interaction of the rocks and surface slope [29] in the model in comparison to DTM5 and DTM10, where it is possible to observe the impact of grid shape of raster on the actual rockfall runout shape [32; 35]. All models overestimate the extent at the lateral side of the rockfall outline, but it is more expressed on the smaller grid cell sizes, which can be explained by smaller roughness at larger DTMs and lack of topographic channeling [64]. The effect of forest is the most evident at RS1, while it did not have an important role at the RS2. At RS1, forest reduces the maximum runout area at DTM1 and DTM2, and it also changes the shape of the modelled propagation area, while at the other two, it can practically be neglected. Even though we found field evidence that forest had a crucial role on stopping rocks at RS2, it did not have that effect in the simulation. The following is mostly due to the distance of the forest from the source area and the initial fall height with which the model gets high kinetic energy. The rockfall site is surrounded by forest, and in the areas where it should have reduced the maximum runout area or lateral extensions, the distance from the source is less than 30 m. When the rocks hit the area of forest within the model (Figure 6b), the maximum kinetic energy (between 1000-2000 kJ) is too high and the trees would not be able to dissipate that much energy [8; 22]. At RS1, the distances to the forest are larger (>100 m), and the kinetic energy is lower (between 500 and 800 kJ) (Figure 6a) when rocks hit the forested area; therefore, forest is able to reduce their impact to some extent, as it can be observed in Figure 5.  
Changes in the modelled rockfall runout area will not be as evident between different DTM grid cell sizes when the surface slope is more smooth and there are no major terrain features that stand out [30; 35]. The same trend can be observed when modelling with or without forest. At the scale of one rockfall area only, DTM1 and DTM2 detect different movement of rocks within the model due to forest effect. The model used in this paper is more intended for use on the local scale, meaning that changes in the DTM grid cell size will be expressed quicker than in the case of the models that are intended for the use on the regional level. This is mainly due to the different inclusion of the terrain features into the model, where in the case of regional models, the topography is accounted for in a more general way (e.g., energy line angle principle) with all DTM grid cell sizes than by such complex 3D models such as Rockyfor3D. Forest not only reduces the rockfall propagation and runout area at all DTM grid cell sizes but also reduces the maximum kinetic energy [30; 64; 65; 66; 67]. When forest is not included into model, maximum kinetic energies changes along the profile are quicker at DTMs with higher grid cell size (DTM5, DTM10), while with lower DTMs, the changes are more gradual. With larger grid cell sizes (DTM5 and DTM10), the slope geometry will be smoother and simplified, resulting in larger deviations in both kinetic energies, since there are less micro-topographic features that could absorb kinetic energy during the movement [29; 68]. The observed differences between DTMs in the maximum kinetic energy at the area outside the forest at RS1 were smaller, indicating that in smoother areas, the influence of DTM grid cell size on the results is smaller than on more rugged surfaces [30]. If forest is included into the model, kinetic energies experience less changes in the maximum values, since the obstacles (trees) are the same for all models.