# Rockfall Modelling in Forested Areas: The Role of Digital Terrain Model Grid Cell Size
## 2 Materials and Methods  
### Study Area  
Our study involved modelling at two rockfall sites in the Trenta valley, which are located in the Julian Alps in the northwestern part of Slovenia (Figure 1a,b). Geotectonically, the valley belongs to the upper part of the Southeastern Alps or thrust unit of the Julian Alps [37; 38]. The thrust unit consists of sediment rocks from the Lower Triassic and Cretaceous with carbonate rocks from Upper Triassic being predominate. The dominant direction of faults is transverse dinar (NE-SW), while prominent faults are also present in the Dinartic direction (NW-SE) and are shifted along the transverse dinar faults. Less significant faults are in the direction of N-S and E-W. The prevailing bedrock in the valley,which is also present at the locations of both rockfalls, is the Dachstein limestone of the Norian-Rhaetian age. The limestone layers are 0.5 to 5 m thick. Typically, limestone is passing into dolomite in vertical and lateral direction [39]. Tectonically damaged rocks form very steep and even subvertical slopes that are covered by the _Anemono trifoliate-Fagetum_ forest community [40], which predominantly has the protection function.  
Rockfalls in the studied area mostly occur due to the freeze-thaw cycle occurring in fractured bedrock in the spring, and extreme rainfall especially in autumn [41; 42; 43].  
Figure 1: (**a**) The study sites are located in the northwestern part of Slovenia in the Julian Alps. (**b**) Both rockfalls are a part of the Trenta valley. (**c**) Orthophoto image with marked source, and propagation and runout area of rockfall site 1. (**d**) Orthophoto image with marked source, and propagation and runout area of rockfall site 2. (**e**) Transverse profile of rockfall site 1. (**f**) Transverse profile of rockfall site 2.  
As Slovenia lies on the seismically active area between the Alps and the Mediterranean Sea, where the Adriatic/Apulian Sub-plate hits the Eurasian tectonic plate, forming the Periadriatic Seam, rockfalls in this part of the Alps can also be earthquake-induced ([PERSON] et al., 2017; [PERSON] et al., 2018). At the first rockfall site (RS1) (Figure 1c), the last large rockfall event occurred in April 2017, when approximately 29,000 m\({}^{3}\) of coarse material was released. RS1 is located between 940 and 1160 m a.s.l. (Figure 1e), and its runout covers an area of approximately 19,300 m\({}^{2}\). The source area is located at a vertical rocky cliff (fall height \(\approx\)150 m), and the majority of the material was deposited directly below the cliff, while individual fragmented blocks were deposited further down the slope. Previous rockfall activity can be distinguished by the color of the rocks (older rocks are darker), and by vegetation cover (older rocks are overgrown by low shrubs and dwarf pines). A similar situation is present at the second rockfall event (RS2) (Figure 1d), which lastly occurred in March 2020 when approximately 11,000 m\({}^{3}\) material collapsed from a rock cliff (fall height \(\approx\)50 m) and impacted the area of 9500 m\({}^{2}\). RS2 is located between 890 and 1010 m a.s.l. (Figure 1f). Previous activity is also noticeable by the older rockfall deposits and the rock-induced damages in the surrounding forest. Rockfalls at this site occur in minimum on average every 8.5 years ([PERSON] et al., 2018). Both rockfalls are located in the vicinity of housing and infrastructure; consequently, the forest on these slopes plays an important role in reducing the rockfall runout.