## 6 Numerical Modeling
### Seepage Analysis  
Seepage analyses were conducted using the finite element method in the software SEEP/W [61]. These analyses aimed to investigate the variations in soil matric suction and positive pore water pressure distribution under different rainfall intensities and durations. Transient flow analyses were employed to determine the changes in matric suction and positive pore water pressure due to rainfall. The initial pore pressure distributions for the transient analyses were constituted based on the in-situ monitoring data, considering the average dry- and wet-period sections as well as groundwater levels. Numerical analysis for unsaturated flow through the soil was conducted by assigning two curves, known as the soil water characteristic curve (SWCC) and hydraulic conductivity \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Soil** \\ \hline Material Model & Mohr-Coulomb \\ \(c^{\prime}\) (\(\rm kPa\)) & 8 \\ \(\phi^{\prime}\) & 21 \\ \(\gamma\) (\(\rm kN/m^{3}\)) & 20 \\ SWCC & Figure 6 (average SWCC) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Geotechnical properties used in modeling the soil. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameters** & **Rock** \\ \hline Material Model & [PERSON] \\ \(\gamma\) (\(\rm kN/m^{3}\)) & 23 \\ \(\Phi_{\rm b}\) (\({}^{\circ}\)) & 0 \\ GSI & 30 \\ D & 0 \\ \(m_{\rm i}\) & 6.65 \\ \(\sigma_{\rm ci}\) (MPa) & 9.81 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Geotechnical properties used in modeling the rock. function (HCF), into the model that governs flow for unsaturated media. The average SWCC curve presented in Figure 6 was used in the analyses. The [PERSON] [53] hydraulic conductivity model, given in Equation (3), was implemented to calculate the hydraulic conductivity function. \[\mathrm{k}=\mathrm{k_{s}}\frac{\left\{1-\left(\alpha\varphi\right)^{n-1}\left[1 +\left(\alpha\varphi\right)^{n}\right]^{-m}\right\}^{2}}{\left[1+\left(\alpha \varphi\right)^{n}\right]^{\frac{n}{2}}} \tag{3}\]  
where k is the unsaturated hydraulic conductivity coefficient; \(\mathrm{k_{s}}\) is the saturated hydraulic conductivity coefficient; \(\alpha\), n and m are the fitting parameters obtained from SWCC; \(\varphi\) is the matric suction. In the characterization of HCF, the most important parameter required in addition to the soil water characteristic curve is the saturated hydraulic conductivity coefficient of the soil. The saturated hydraulic conductivity of the soil was measured as an average 1.76 \(\times\) 10\({}^{-7}\) m/s in laboratory tests. Regarding studies conducted in the literature, it has been stated that saturated hydraulic conductivities can change with test conditions, test type, and sample size even for identical soils [65; 66]. Various studies have reported that saturated hydraulic conductivities measured in field can be 10-10\({}^{3}\) times higher than those measured in the laboratory [67; 68]. In this study, rainfall infiltration analyses were performed for saturated hydraulic conductivities of 1.76 \(\times\) 10\({}^{-6}\) m/s, 1.76 \(\times\) 10\({}^{-7}\) m/s and 1.76 \(\times\) 10\({}^{-8}\) m/s and found to be 10-times smaller and greater than the lab saturated hydraulic conductivity to meet the possible conductivity values for the ground. Thus, the aim was to examine the effect of changes in the saturated hydraulic conductivity on the results and to consider the difference between field and laboratory conditions. Figure 14 shows the variation in hydraulic conductivity function with saturated hydraulic conductivities. Rainfall was measured between February 2021 and September 2021 in the Hisar landslide area. Most of the hourly rainfalls were below 20 mm during the monitoring. The maximum hourly rainfall was recorded as 50.6 mm on August 2021. The effects of various rainfall intensities and duration, including the ranges measured in the area, on the pore-pressure distribution and stability of the slope were investigated by seepage and stability analyses.