## 7 Results and Discussion
### Rainfall Infiltration Stability Analyses
For the landslide area, I-D thresholds were represented by power functions, with \(\beta\) in the range of \(-\)0.631 to \(-\)135.8 and \(\alpha\) in the range of 13.803 to 5 \(\times\) 10\({}^{260}\), based on numerical consideration (See Figures 20 and 21). Further, in some ranges of rainfall intensity, particular constant rainfall durations were obtained. The empirical I-D thresholds were defined as more limited ranges of \(\beta\) and \(\alpha\) when compared to the numerical I-D thresholds proposed in this study. These differences mostly arise from the initial conditions and saturated/unsaturated hydro-mechanical properties of the soil, which are not considered in the empirical I-D thresholds but are considered in numerical analysis methods. Figure 21: Rainfall intensity–duration thresholds for wet period initial conditions. Figure 20: Rainfall intensity–duration thresholds for dry period initial conditions. In Figures 20 and 21, it can be clearly observed how the hydraulic conductivity, rainfall intensity and duration affect the I-D thresholds. Decreases in the saturated hydraulic conductivity shifted the rainfall intensity-duration thresholds to the right for both initial conditions. It is also observed that a longer duration of rainfall with the same rainfall intensity was required to trigger the landslide, as the saturated hydraulic conductivity of soil decreased. Rainfall thresholds were obtained as approximately vertical lines for the saturated hydraulic conductivity of 1.76 \(\times\) 10\({}^{-8}\) m/s in the dry-period initial conditions and as vertical lines for all saturated hydraulic conductivities in the wet-period initial conditions (see Figures 20 and 21). This is due to the overlapping of the factor of safety curves given in Figures 18 and 19 for the mentioned rainfall intensities and hydraulic conductivities. The maximum rainfall intensity-duration data points recorded in the dry and wet periods during the observation period in the study area (indicated with green points) and the rainfall intensity-duration data points of landslides that occurred in the past in the region (indicated with red points) were added to the Figures 20 and 21 to verify the established threshold model. The maximum rainfall intensities measured in the dry and wet periods in the study area were 50.6 mm/h and 10.2 mm/h, respectively. After the mentioned rainfall intensities, no total failure was observed in the field. The maximum rainfall intensity-duration data point recorded during the dry period was just to the right of the rainfall threshold obtained for the saturated hydraulic conductivity value of 1.76 \(\times\) 10\({}^{-6}\) m/s, with a factor of safety of 1.50 (see Figure 20). The maximum rainfall intensity-duration data point measured in the wet period was between the rainfall thresholds obtained for saturated hydraulic conductivities of 1.76 \(\times\) 10\({}^{-6}\) m/s and 1.76 \(\times\) 10\({}^{-7}\) m/s and closer to the rainfall threshold with higher permeability (see Figure 21). I-D points of past landslides observed in the region (red points) were also located between the mentioned rainfall thresholds. Except for only one landslide in the wet period, all I-D points of past landslides were to the right of the maximum rainfall intensity-duration data points. An exception can be caused by the differences in saturated hydraulic conductivity at that landslide location. As observed in Figures 20 and 21, it seems highly probable that the permeability in the field was between 1.76 \(\times\) 10\({}^{-6}\) m/s and 1.76 \(\times\) 10\({}^{-7}\) m/s. In this case, landslide-risk situations are reached if rainfall over 10 mm/h for dry periods lasts between 0.85 h and 8.37 h for an FoS = 1.5 and 1.67 h-17 h for an FoS = 1.0 depending on permeability. On the other hand, at rainfall intensities less than 10 mm/h, longer durations are needed for a landslide to occur.