# Rockfall Modelling in Forested Areas: The Role of Digital Terrain Model Grid Cell Size
## 3 Results
### Validation of the Modelled Rockfall Runout Zones with and without Forest
At DTMs with a smaller grid cell size, the changes in the terrain are captured in larger detail; therefore, the overestimation will be larger. Comparing the modelling results of RS1 to those of RS2, the differences between the shape of the modelled propagation area DTMs are not as evident, since both lateral sides of the propagation area have very similar surface slope and terrain features. The forest has the largest impact on rockfall propagation area at RS1, namely at DTM2--reducing it by 13%, followed by DTM1 (12%) and DTM5 (4%) (Figure 5). In the case of DTM10, the rockfall propagation area did not change between the modelling scenarios. Differences between the forest and no forest scenario at RS2 are almost negligible; forest reduced the rockfall propagation area by 4% at DTM2, 3% at DTM1, and 2% at DTM5, while at DTM10, the propagation area does not change. When observing the forest and no forest modelling results at RS1, it can also be stated for RS1 that at DTM1 and DTM2, the shape of the forest scenario changes according to the no-forest scenario, and the change is only observed for the maximum runout distances. On the other hand, at DTM5 and DTM10, the shape of the rockfall runout area is modelled correctly in both modelling scenarios; however, there are no major differences between the forest and no-forest scenarios. In general, all DTMs at RS1 provide comparable results with respect to the shape of the rockfall propagation. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{8}{c}{**ROCKFALL SITE 1**} \\ \hline  & \multicolumn{8}{c}{_scenario without forest_} & \multicolumn{8}{c}{_scenario with forest_} \\ \hline _DTM grid_ & \multirow{2}{*}{_AI_} & \multirow{2}{*}{_SI_} & \multirow{2}{*}{_D2 PC_} & \multirow{2}{*}{_TSS_} & \multirow{2}{*}{_AI_} & \multirow{2}{*}{_SI_} & \multirow{2}{*}{_D2 PC_} & \multirow{2}{*}{_TSS_} \\ _cell size_ & & & & & & & & \\
**DTM1** & 0.927 & 0.957 & 0.076 & 0.914 & 0.893 & 0.934 & 0.126 & 0.868 \\
**DTM2** & 0.927 & 0.947 & 0.076 & 0.894 & 0.897 & 0.931 & 0.110 & 0.862 \\
**DTM5** & 0.927 & 0.930 & 0.110 & 0.859 & 0.922 & 0.931 & 0.103 & 0.862 \\
**DTM10** & 0.922 & 0.932 & 0.100 & 0.864 & 0.928 & 0.939 & 0.090 & 0.877 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Validation results for modeling scenarios considering different DTM grid cell sizes and two modeling scenarios: with and without the effect of forest. Comparison of the Maximum Kinetic Energy and Maximum Passing Heights between DTMs and Modelling Scenarios  
Two fictive fences placed perpendicular on the slope direction were used to calculate the maximum kinetic energy (E_95 CI) and maximum passing height (Ph_95 CI). The results are shown in the form of a distance profile along both of the fences (Figure 6). Within the profile, it is possible to distinguish between two momentums: when the rocks in the model are in interaction with the forested area (trees) and when not. At both rockfall sites, the model is in interaction with forest within the first 40 m (RS1) and first 30 m (RS2), as marked on the distance profile. Observing E_95 CI results at both sites of the profile, the kinetic energy values experience high variations along the distance profile. The maximum kinetic energy in this part has smaller variations compared to the part of the profile that is not in interaction with the forest, and larger jumps in kinetic energy can be recognized at RS1 when observing the forested part. Focusing only on the forested part in RS1, the deviations between scenarios with/without forest can be recognized with the higher peaks being scenarios with forest. The DTM1 scenario with forest achieves the highest values of E_95 CI compared to other DTMs (903 kJ forest, 650 kJ no forest scenario). The maximum kinetic energies are the lowest at DTM10. Differences between scenarios with and without forest are smaller at RS2 compared to RS1. Absolute maximum values are achieved by DTM2 scenario with forest (1180 kJ). In the part of the slope with no forest, maximum kinetic energies are higher.