## 3 Results and Discussion
### Observation of Dam Deformation  
When installing the gabion retaining wall structure on the dam crest, the interface between the existing dam and the overtopping prevention structure becomes hydraulically vulnerable; therefore, the seepage behavior at the interface should be assessed. Accordingly, piping and deformation behaviors were monitored depending on each water level stage to evaluate the effectiveness of installing overtopping prevention structures for improving the stability of reservoir dams.  
Figure 10 illustrates the results of observing the deformation in the gabion structure and downstream slope at different stages: initial flood level (S1: 44 cm), flood level (S1: 44 cm), dam crest water level (S2: 51 cm), and overtopping water level (S3: 54 cm). As shown in Figure 10a, during the initial flood level (S1: 44 cm), there was no deformation singularity. However, over time, as shown in Figure 10b, some gaps appeared at the boundary between the gabion, the contact surface of the upper part of the dam crest, and, owing to drying shrinkage, the core. Because the effects of drying shrinkage increase when the core is exposed to the atmosphere for an extended period, the interconnection of the upstream and downstream gabions might require using strip material containing concrete or asphalt at the boundary between the core and gabions.  
Figure 8: PWP distribution at different water levels. (**a**) S1: height, 44 cm (t = 30 min); (**b**) S1: height, 44 cm (t = 24 h); (**c**) S2: height, 51 cm; and (**d**) S3: height, 54 cm.  
Figure 9: Total displacements of stepped gabion retaining wall. (**a**) S1: height, 44 cm (t = 30 min); (**b**) S1: height, 44 cm (t = 24 h); (**c**) S2: height, 51 cm; and (**d**) S3: height, 54 cm.  
Figure 10c illustrates water leakage results at the interface between the existing dam and the structure at the dam crest water level (S2: 51 cm). A geomembrane installed between the dam crest core and the gabions blocks seepage water flow, preventing it from spreading down the downstream slope. Figure 10d presents the results of the dam deformation and water leakage at the overtopping water level (S3: 54 cm). No changes were observed throughout the overtopping water level, which suggests that the designed system maintained a steady state. However, assessing the stable state requires verification through a comparison with the measured values inside the dam body when transitioning from the dam crest water level (S2) to the overtopping water level (S3).  
Overall, a comprehensive examination of flood prevention structures across different water levels revealed that overtopping prevention structures prevent overtopping and effectively block water leakage from the downstream slope. Furthermore, no dam deformation or water leakage was observed near the overtopping prevention structure during the experiment. Water leakage occurred solely through the toe drain located at the end of the slope on the downstream side. These results are highly meaningful, suggesting that the horizontal filter and toe drain contribute to constraining the expansion of the saturation region. Although some aspects, such as drying shrinkage, require improvement, the findings indicate the potential practical application of the proposed method in preventing disasters associated with reservoir failure.