The evolution of rockfall phenomena depends on both rock mass and slope characteristics: variations in cover, morphology or vegetation induced by intense weather events can have huge consequences. Investigating these effects is required to assess the effectiveness of existing protection structures. This study focuses on the Bazena case (Brescia, Italy), a slope subject to rockfall phenomena, crossed by a provincial road connecting two valleys. Originally the slope was covered by a coniferous forest, which was almost completely destroyed by an extremely intense storm in 2018. In this context, the ASFORESEE model was applied to understand the value of the protection provided by the forest, over a period of 25 years. The output of ASFORESEE and other evaluations here proposed show that the forest played an active role in protecting the provincial road against rockfall phenomena. This work aims to highlight the potentiality of combining nature-based solutions and flexible barriers.
The study of rockfall phenomena, especially through numerical models, usually requires three steps: first, the definition of the features of the rock mass where the unstable blocks are located; secondly, the quantification of one or more reference values for the falling blocks; lastly, the characteristics of the slope must be described according to the employed approach. The definition of the rock mass properties usually is derived from in situ measurements through contact or non-contact geomechanical surveys, aiming at a proper definition of the geometric properties (i.e., orientation and spacing of discontinuities), with added information related to mechanical features (i.e., friction angle and cohesion of discontinuities) that can be either derived from empirical relations or measured in laboratory tests. This step is important in defining two aspects: the kinematic possibility of a block movement and the stability of such block. The first aspect is purely geometrical and is traditionally studied through Markland's kinematic tests, which identify the possible types of movement unstable blocks can be subjected to. This process is usually performed manually employing stereographic projections: the test needs to be performed for each orientation of the rock face, making it a time-consuming approach in case of geometrically complex or articulated situations. To speed up this process, the algorithm described in Taboni et al. (2022) was employed. As it is common practice when dealing with rockfall, stability analyses were not performed; the hazard was evaluated by simulating the trajectories of a large number of falling blocks. The definition of the reference values for block volume is a crucial issue to be solved. The problem can be approached in two ways, either describing the block volume distribution measured directly below the rock face and along the slope or from previously recorded events, or deriving it from the geometric structure of the rock mass itself. Considering also that there is no consensus in the scientific literature over the choice of the representative volume (i.e., average, modal, or specific frequency percentage) (Umili et al. 2020), two values were chosen so that a significant portion of the distribution could be taken into account. Lastly, the characterization of the slope is a key feature for numerical simulations and can be subdivided into three elements: morphology of the slope, mechanical properties, and vegetation. The first parameter is readily available using Digital Terrain Models (DTMs). The mechanical properties of the slope are generally expressed in terms of restitution coefficients, the values of which are tabulated for many different types of materials. Vegetation is not considered in most cases, employing a more conservative approach that does not account for the protective effect it can provide. In this study, we tried to evaluate the positive effects of vegetation by studying a slope subjected to an extreme weather event in 2018 that removed the forest cover almost completely. The slope is located close to the Alpe di Bazena, in the Breno municipality (Brescia province, Northern Italy): the slope is crossed by a provincial road connecting two valleys, therefore of significant importance at local level. The direct effects on rockfall were quantified with and without vegetation using Rockyfor3D code (Dorren 2016), which allows for full 3D numerical simulations. The ASFORESEE model (Accastello et al. 2019) was then applied to express the economic value of the protection provided by the vegetation assuming an optimal condition of the forest.
