# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 5 Discussion  
This study focuses on the influence of geometry on stability evolution during the impounding of a reservoir by analyzing different real and generic deep-seated rock slide models. In addition, a block model was developed to introduce the fundamental mechanisms and to provide a tool for calculations. Our study, which only focuses on the overall geometry, led to several implications when assessing rock slide hazards in the surrounding areas of reservoirs. At the beginning, the FoS continuously decreases until a threshold reservoir level with the lowest FoS is reached. From there, the FoS increases again until the rock slide is fully submerged. Practically, the impact of this reduction can be very different. In some cases, the reduction in the FoS can lead to the acceleration of the total slide or only slabs of it, or the formation of a new active slide. In the worst case, although extremely seldom, sudden failure of a slide can occur [5], and in contrast, case studies also show that there can be no observable or measurable impact on pre-existing slides [9]. In order to be able to assess the actual behavior of specific case studies, further investigations and numerical modeling are absolutely necessary. We found a clear tendency that the decrease in the FoS is larger for rock slides where the majority of the mass is arranged at the foot of the slope. In contrast, rock slides whose mass is mainly located in the central to higher sections of the slope are less affected at the beginning of reservoir impounding. Nevertheless, the change in the FoS can also be very pronounced, but later when a larger height ratio, for example at \(\rm h/h_{0}=0.5\), is reached. Thus, the height of the final operational reservoir level in relation to the total height of the rock slide has a significant influence on the maximum change in stability. Depending on the rock slide geometry, i.e., translational, rotational, or compound, the maximum reduction in the FoS can be considerable and, in our case, reaches almost 12%. Since this study is based only on a few case studies, it is quite conceivable that even larger reductions are possible. It is also noteworthy that the greatest reduction in the FoS occurred for the Vajont case study, and for this slide, sudden failure did occur during the third impounding cycle [30]. Even if the FoS increases again at higher water levels, the threshold water level must have been passed during the initial impoundment. In this case, the threshold water level has to be taken for stability assessment and not the maximum reservoir level. Consequently, finding the most unfavorable water level characterized by the greatest reduction in the FoS is recommended by carrying out calculations with different water level heights. As a result, an FoS reservoir level height curve representing the relationship between height and slope stability can be obtained. It should also be noted that deep-seated rock slides are often separated into individual slabs bounded by shear zones of fault gouge and breccia. In these conditions, each slab should be subjected to an impounding analysis and not just the entire rock slide. It is also necessary to assess whether the existing rock slide mass is prone to the formation of new slabs, especially at the foot of the slope. Lithological and structural zones of weakness as well as a high degree of fragmentation of the rock slide mass are predestined for progressive slope processes. Slab formation increases the complexity in terms of the temporal variable deformation behavior, slope stability, and the overall hydrogeological situation. Soil-like basal and internal shear zones separating individual slabs or the bedrock from the rock slide mass are spatially heterogenous in terms of shape, thickness, and hydro-mechanical coupled properties. Even highly advanced numerical modeling techniques are only suitable to a limited extent to satisfactorily simulate the diverse geometrical and hydro-mechanical coupled aspects of such rock slides when separated into variable active slabs.