# Rockfall Modelling in Forested Areas: The Role of Digital Terrain Model Grid Cell Size
## 5 Conclusions  
As already confirmed in the literature [24; 69; 28] and by the results of this study, forest should be considered as part of rockfall hazard assessment. However, forest is not always as efficient at reducing rockfall propagation area, maximum kinetic energies, or maximum passing heights, especially if the forest is too close to the rockfall source area or the rockfall event is too large to be stopped by forest. A combination of rockfall simulation tools that consider the forest protection effect can enable us to identify areas of forest that are insufficient for stopping rocks and where we consequently need additional protection measures [24; 28; 67; 70]. These data can help with planning the location and capacity of the required protection measures (e.g., the height of the nets and their capacity) and the associated costs [24; 71].  
Nevertheless, the decision on the protective effect of forest and additional protection measures will be strongly correlated with the DTM grid cell size. Even though the grid cell size \(1\times 1\) m is not recommended by the model regarding the final results compared to larger grid cell sizes, based on our results, we can conclude that \(1\times 1\) m and \(2\times 2\) m were the most suitable grid cell sizes. They most accurately portrayed the protection effect of forest, while at other DTM grid cell sizes, the effect of forest in reducing rockfall propagation and runout area is practically neglected. If considering the computational time needed for modelling, \(2\times 2\) m is more efficient. Grid cell sizes of \(5\times 5\) m and \(10\times 10\) m would be too coarse in order to be used in planning of protection measures on a local scale. In the results, we can also observe that with those two DTMs, the maximum kinetic energy and bounce heights differ greatly from \(1\times 1\) m and \(2\times 2\) m grid cell sizes, and they express major differences along the rockfall slope. It is also important to state that a smaller DTM grid cell size might be very sensitive to small changes in elevation, which can be observed in the rockfall simulation model results as high fluctuation/variability of e.g., maximum kinetic energy or bounce heights in a small area. Yet, it is still more preferable to use \(1\times 1\) m or \(2\times 2\) m for the calculation of maximum kinetic energies and passing height than the other two, especially when planning protection measures, since the other two grid cell sizes provide reliable simulations on those data. Thus, the decision on DTM grid cell size must be consistent with the main goal of the final modelling results.  
Conceptualization, B.Z., M.M. and M.K.; methodology, B.Z., M.M. and M.K.; formal analysis, B.Z.; data curation, B.Z.; writing--original draft preparation, B.Z.; writing--review and editing, M.M. and M.K.; visualization, B.Z.; supervision, M.M. and M.K.; project administration, M.K.; funding acquisition, M.K. All authors have read and agreed to the published version of the manuscript.  
This research is a result of the work on the Interreg Alpine Space project GreenRisk4 ALPs (ASP 635) and was also supported by the Pahernik foundation. The second author acknowledges the financial support from the Slovenian Research Agency (research core funding No. P2-0180).  
Not applicable.  
The data presented in this study are available on request from corresponding author.  
The authors declare no conflict of interest.