## 4 Result
### Stability of the Landslide  
The computed results for landslide safety factors under varying water levels are summarized in Table 7. At the normal water level, the stability is relatively high, with a safety factor of 1.062. However, when the water level drops to the dead water level, the safety factor decreases to 1.048. This suggests that a decrease in water level from the normal level to the dead level may trigger landslide instability and failure. This observation aligns with the recent sliding failure characteristics observed at the front edge of the Cheyiping ancient landslide deposit (H\({}_{1}\)) in June 2019, which were induced by a sudden drop in reservoir water level to a low level. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & & \multicolumn{3}{c}{**Net Correlation Coefficients**} \\ \multicolumn{2}{c}{**Stages (Reservoir**} & \multicolumn{3}{c}{**between Landslide Cumulative Displacement and**} \\ \multicolumn{2}{c}{**Water Level)**} & \multicolumn{3}{c}{**Time**} & \multicolumn{3}{c}{**Reservoir Water Level \(\boldsymbol{r_{12-3}}\)**} \\ \cline{3-5} \multicolumn{2}{c}{} & & **GTP-01** & **GTP-02** & **GTP-03** \\ \hline Fluctuating & 25 April 2019–30 May 2019 & 0.0386 & \(-0.0279\) & \(-0.0513\) \\ Descending & 2 June 2019–19 June 2019 & \(-0.7085\) & \(-0.5015\) & \(0.0578\) \\ Ascending & 19 June 2019–18 July 2019 & \(-0.0900\) & \(-0.1983\) & \(-0.3195\) \\ Fluctuating & 19 July 2019–13 August 2019 & \(-0.0140\) & \(0.0926\) & \(0.2071\) \\ Descending & 14 August 2019–30 August 2019 & \(-0.1918\) & \(-0.7201\) & \(-0.6813\) \\ Ascending & 31 August 2019–28 September 2019 & 0.2411 & \(-0.0907\) & \(0.0370\) \\ Fluctuating & 29 September 2019–16 January 2020 & 0.0303 & \(-0.0667\) & \(0.0319\) \\ Ascending & 17 January 2020–29 February 2020 & 0.0652 & \(-0.0803\) & \(-0.0222\) \\ Descending & 1 March 2020–15 April 2020 & \(-0.9656\) & \(-0.8039\) & \(-0.8593\) \\ Ascending & 17 April 2020–29 June 2020 & 0.0194 & \(0.1793\) & \(0.2114\) \\ Fluctuating & 30 June 2020–28 August 2020 & 0.1159 & \(0.1217\) & \(-0.2484\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Summary of net correlation coefficients between cumulative landslide displacements and water levels. The safety factors under the influence of solely reservoir water level fluctuations and under the combined effects of reservoir water level fluctuations and 30 mm/d rainfall show negligible variation. This indicates that, in this scenario, the fluctuation of the reservoir water level predominantly influences the landslide stability, while the impact of rainfall on the stability of this reservoir bank landslide is minimal. Figure 15 illustrates saturation maps at varying water levels. It is evident that with the decrease in reservoir water level, there is an overall reduction in the saturation of the soil above the groundwater level. The areas experiencing significant changes are predominantly located near the base of the landslide. Following the decrease in reservoir water level, the zero hydraulic headline exhibits a concave shape, leading to larger unsaturated zones near the base of the landslide. Figure 16 illustrates the distribution patterns of pore water pressure and total pressure on the sliding surface (horizontal distribution range approximately 122 m to 608 m) at different reservoir water levels. The figure indicates that reservoir water level fluctuations have a significant impact on the forefront of the landslide (516 m to 608 m). When the reservoir water level decreases from its normal level to its minimum level, both the pore water pressure and total pressure at the forefront of the landslide decrease. However, the influence range of the total pressure is slightly smaller than that of the pore water pressure. The decline in reservoir water levels has led to a significant reduction in pore water pressure within a 92 m range (516 m to 608 m) and total pressure within a 50 m range (558 m to 608 m).