# Influence of Rock Slide Geometry on Stability Behavior during Reservoir Impounding
## 1 Introduction  
Mountainous regions with steep slopes and high relief gradients provide a good terrain for hydropower generation. Worldwide, numerous dams are built and valley sections are impounded. However, numerous case studies show that reservoir impounding often affects the surrounding slopes in terms of groundwater flow and stability behavior [1]. Slope stability reduction resulting from an impounding reservoir may lead to the formation of new deep-seated rock slides or the reactivation and acceleration of pre-existing ones. Numerous case studies document the formation of a new first-time rock slide or the acceleration of a slowly moving deep-seated rock slide as well as the reactivation of an ancient rock slide caused by the operation of a dam reservoir [1; 2; 3; 4; 5; 6; 7; 8; 9]. One of the most catastrophic events in this context occurred in Italy in 1963 at the Vajont reservoir, where a rock slide mass with a volume of 280 million m\({}^{3}\) suddenly failed, causing a giant water wave that destroyed many villages, leading to about 2000 fatalities [5]. With the exception of such very rare disastrous events, there are numerous cases documented worldwide where pre-existing dormant or very slowly moving rock slides have been reactivated or accelerated by dam reservoir operation [2; 3; 6; 7; 8; 9]. Reactivated or accelerated rock slides typically require comprehensive monitoring programs and, in some cases, even the application of stabilization measures, such as the construction of a costly tunnel drainage system. It is therefore advisable to select the location of dams and the storage areas behind them on the basis of comprehensive feasibility studies, in order to avoid or minimize any influence from pre-existing deep-seated landslides [10]. In light of a worldwide boom in the construction of hydroelectric power plants--i.e., at least 3700 major dams, each with a capacity of more than 1 MW, areeither planned or under construction [11]--the impacts of first-time reservoir impounding, annual reservoir management, and rapid drawdown need to be understood profoundly in order to assess the short- and long-term stability behavior [12; 13; 14] and perform slope risk management [15]. Research questions arise, mostly site-specific, concerning the influence of (i) the hydro-geological situation, (ii) the 2D/3D geometry and related type of movement (kinematics), (iii) slab development and shear zone characteristics, and (iv) the hydro-mechanically coupled parameters of the rock slide mass as well as the shear zones in the temporal variable stability evolution of such deep-seated rock slides. Apart from others, it is necessary to understand to what extent the geometry of rock slides influence the movement and stability evolution during initial impounding and later stages of reservoir management, a factor that has received little attention to date with regard to a systematic view. For the assessment, it is important to know whether the geometry (shape) of the rock slide has a minor or major influence on the stability development for the corresponding operational reservoir levels. The implications of this are manifold and have a significant impact on the proposed in situ investigation program, which is needed to assess the behavior during first-time impounding [10]. Furthermore, for already existing dam reservoirs located in the surroundings of active or inactive deep-seated rock slides, comprehensive assessment of long-term stability requires robust data of the geometry to determine the influence of cyclic loading due to reservoir infilling and lowering. Since different overlapping factors influence the stability in the natural environment, such as impounding, rapid drawdown, heavy precipitation, snowmelt, or others, it is necessary to know the amount of influence of these individual factors in order to make reliable assessments in the event of unexpected changes in slide movements [1].