# Rockfall Modelling in Forested Areas: The Role of Digital Terrain Model Grid Cell Size
## 4 Discussion  
In this study, we investigated the influence of DTM grid cell size and the protection role of forest on modelling rockfall propagation and runout area. For the calibration of the rg indices (at DTM2 forest scenario), we have used four indices (AI, SI, D2 PC, TSS), even though the authors suggest that using one is enough for the parameter estimation. By calculating all indices, we conclude that there are differences between two groups of indices, namely AI and SI, and D2 PC and TSS. In general, the first two provide the results that indicate more successful models achieving higher values, while the other two achieve lower values in general. The difference is most likely due to the formulation of AI and SI that give more weight to TN values compared to D2 PC and TSS [60]. In the case of our study, TN presented a more \"stable\" part of the equation by not changing as much between the model calibration runs as the FP and FN values that would have a more important weight in the calculation of D2 PC and TSS. In the end, the combination of all four by using ranks was the option that was the most secure to ensure that all aspects of correctly and wrongly modelled areas were considered in the calibration as equally as possible. In order to capture the surface roughness in detail, the surface slope at both rockfall sites was divided into multiple subareas that were then given different rg values according  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**ROCKFALL SITE 1**} & \multicolumn{6}{c}{**SCENARIO WITHOUT FOREST**} & \multicolumn{6}{c}{**SCENARIO WITH FOREST**} \\ \cline{2-10}  & & **AI** & **SI** & **D2 PC** & **TSS** & **AI** & **SI** & **D2 PC** & **TSS** \\ \hline \multirow{3}{*}{**initial fall height (m)**} & 50 & 0.925 & 0.982 & 0.032 & 0.963 & 0.945 & 0.976 & 0.046 & 0.951 \\  
to the mean obstacle height and percentage of coverage. Differences between values of rg70, rg20, and rg10 between subareas are small, all falling in the size of the surface roughness classes between 10 cm and 1 m. At both rockfall sites, the largest rg values were in subarea 2, and this area was the most crucial on reducing the maximum runout zone. Area 2 is located between the forested and non-forested area, where the majority of larger rocks from previous events were deposited due to the obstacles such as trees, stumps, laying logs, concavities, etc. The combination of rg values calibrated for area 3 at RS1 and especially at area 4 at RS2 is mostly a consequence of a good performance of combination of values at subarea 1 and 2, since it is located out of the reach of the majority of rocks in the model. There could be some improvements in the calibration process. The calibration was done without the inclusion of soil types--based on the average rg values, soil type areas could also have been delineated additionally and calibrated together with the rg values [63], leading to the improved final result. When comparing the results of DTMs, we can observe that the maximum runout is underestimated at DTMs with a larger grid cell size. The same calibrated rg values were used at all DTMs; however, this indicates that perhaps the separate calibration of these parameters could be done for different grid cell sizes in order to achieve more realistic results. For example, for RS1--DTM5 and DTM10--it would have been better to use lower rg values because due to their cell sizes, the rocks in the model stop too soon, since the surface is \"more rugged\" according to the combination of DTM grid size and rg values. By decreasing the rg values, the surface would be smoother, allowing longer travel distances and reducing the underestimation of the model. In general, all DTMs predict the shape of the rockfall area accurately; at RS1, DTM1 and DTM2 are overestimating the lateral extent, and DTM5 and DTM10 are underestimating the maximum runout, while at RS2, all models overestimate in the lateral direction, DTM5 and DTM10 additionally at maximum runout.