Assessment of the relation between structural and morphological setting as condition for coastal slope stability

The influence of the structural and morphological setting along coastal slopes is a well-known topic. In particular, the relationship between slope and bedding attitude as well as fracture spatial distribution play an essential role in the evaluation of the slope morphodynamic evolution within large areas. The proximity of the sea, the high slope inclinations and situationally the occurrence of landslides are some of the difficulties encountered in studying coastal slope, making it necessary to support the traditional field surveys with remote sensing and geomorphometric techniques. The use of spatial analysis and in particular the assessment of morphometric variables, could be a valid approach to investigate coastal slope involved by gravitational processes (Piacentini et al., 2021) and could be integrated with geometric and spatial analysis of joints. This work presents a multi-disciplinary approach to determine the predisposing role of the bedding and rock mass discontinuities in controlling the morphoevolution of rocky coasts. Previously proposed methods, (Grelle et al., 2011; Santangelo et al., 2015; Francioni et al., 2018), make it possible to carry out a semi-quantitative analysis to identify structural control domains in areas with uniform lithologies. We propose to integrate these approaches with the acquisition, by means of geo-structural, geomorphological and UAV (Unmanned Aerial Vehicle) surveys, of geometric and spatial discontinuity data, integrating values of geomechanical parameters gathered on different lithologies (Uniaxial Compressive Strenght, Schmidt Hammer, Leeb hardness) and performing a kinematic analysis of the slopes. The proposed approach has been tested on the western coastal sector of the Central Adriatic Sea (Italy) consisting of several lithologies from Cretaceous limestones and marls to the Upper Messinian sandstones and marls and interested by several gravitational processes. Highly detailed surveys using a low altitude flight platform have been carried out to obtain a 3D outcrop model to be analysed in pair with a Digital Terrain Model (DTM) derived by aerial LiDAR (Light Detection and Ranging) data, with a ground resolution of 2x2 m. The 3D model allows to identify several joint families, to implement them to those measured in the field, and successively to compare these located sets with two morphometric variables (Slope and Aspect) computable by spatial analysis in Geographic Information System (GIS) software. The relative relationship between joint families and slope geometry highlights different sectors that can be considered kinematically stable or instable for planar and wedge sliding. The comparison procedure has been implemented in a Python script (ArcPy), which allows a semi-automatic application of the repeated calculation procedures required. The fine-tuned calculation procedure represents a useful tool to provide preliminary information when assessing the likely hazard of coastal instability.