## 5 Discussion  
### Debris Flow Trigger Mechanism  
ERT measurements in combination with DP HPT logging and sediment coring have provided detailed insights of subsurface structures upslope of the fissure zone. We assume that the potential interflow lens at a 3 m depth below the surface that is characterized by significantly low resistivity values (Figure 5, lower) plays a crucial role for the trigger mechanism of the event. At a comparable depth, DP log BIE HPT 2 shows the lowest HPT values of all the logs, indicating high permeability in sand-dominated SU VI (blue shading in Figure 6) framed by significantly higher pressure values, indicating dense, clayey (impermeable) periglacial slope deposits (base of SU III) and weathered bedrock (SU IVa), respectively. In addition, slope deposits reach their greatest thickness of 4.9 m within the investigated transect. In comparison to DP log BIE HPT 1, where permeability significantly decreases in 0.8 m b.s., DP log BIE HPT 3 shows an overall higher permeability related to lose limestone debris in SU VII. These observations are in accordance with reports from the late evening of 14 July by local residents who observed disappearing water masses in the field upslope to the northeast of the scar and a concentration of surface runoff in the depth line where site BIE HPT 2/BIE 2 is located. In addition, during sediment coring in February 2022, we observed continuous water discharge within the scar at a depth that corresponds to the position of the assumed interflow (Figure 9).  
Figure 8: Modeled runout and deposition behavior of the debris flow with calculated trigger areas and photographic comparison to the in situ situation.  
The described phenomena of different permeability and surface runoff are directly connected to the normal fault, separating the Lower Muschelkalk in the east from the Upper Buntsandstein in the west, which is also indicated in ERT transect BIE ERT 1 (cf. Figure 5). The hillslope debris flow was triggered exactly in the position of the normal fault. One may assume that the material in this tectonic position was already characterized by less stability.  
Overall, we hypothesize that the strongly increased pore water pressure and the enhanced interflow during the tremendous rainfall event led to a severe loss of cohesiveness and, finally, to a sudden movement of the substrate as an initial landslide movement with the sliding surface below the interflow layer. In addition, strong surface runoff with the associated drag force further liquefied and mobilized the initial landslide mass, accelerating the downslope movement. The surface runoff modelled with r.sim.water is shown in Figure 10, indicating that the largest amount of surface runoff was concentrated in the depth line prior to the mass movement. To sum up, all factors lead to a development from an initial translational slide into the high-energy and widespread hillslope debris flow.  
As well as the described potential trigger mechanisms, one has to assume that the forest path just upslope of the scar also played an important role (Figure 2e). Compaction in the course of such infrastructural measures reduces the infiltration capacities and rates. In addition, the morphological change between the path and slope leads to the formation of an artificial convex embankment in a downslope direction. The risk of hillslope debris flows increases if the surface runoff concentrates on these embankments with significantly steep artificial slopes [49].