Abstract
The present paper explores the combination of unmanned aerial vehicle (UAV) photogrammetry and three-dimensional geomechanical modeling in the investigation of instability processes of long sectors of coastal rocky cliffs. The need of a reliable and detailed reconstruction of the geometry of the cliff surfaces, beside the geomechanical characterization of the rock materials, could represent a very challenging requirement for sub-vertical coastal cliffs overlooking the sea. Very often, no information could be acquired by alternative surveying methodologies, due to the absence of vantage points, and the fieldwork could pose a risk for personnel. The case study is represented by a 600 m long sea cliff located at Sant’Andrea (Melendugno, Apulia, Italy). The cliff is characterized by a very complex geometrical setting, with a suggestive alternation of 10 to 20 m high vertical walls, with frequent caves, arches and rock-stacks. Initially, the rocky cliff surface was reconstructed at very fine spatial resolution from the combination of nadir and oblique images acquired by unmanned aerial vehicles. Successively, a limited area has been selected for further investigation. In particular, data refinement/decimation procedure has been assessed to find a convenient three-dimensional model to be used in the finite element geomechanical modeling without loss of information on the surface complexity. Finally, to test integrated procedure, the potential modes of failure of such sector of the investigated cliff were achieved. Results indicate that the most likely failure mechanism along the sea cliff examined is represented by the possible propagation of shear fractures or tensile failures along concave cliff portions or over-hanging due to previous collapses or erosion of the underlying rock volumes. The proposed approach to the investigation of coastal cliff stability has proven to be a possible and flexible tool in the rapid and highly-automated investigation of hazards to slope failure in coastal areas.
Highlights
The instability processes of sea rocky cliffs pose a significant hazard to communities and infrastructures located along the coastline
We propose a methodological workflow to perform stability assessments of coastal cliffs by means of the application of three-dimensional finite element models (FEM) using the geometrical information reconstructed from the combination of nadir and oblique images acquired by unmanned aerial vehicles (UAV)
The methodology described in the previous sections shows that the combination of UAV photogrammetry and geomechanical modeling is able to cover a gap in the investigation of the stability of long sectors of coastal rocky cliffs, so that an accurate three-dimensional finite element model has been developed based on a detailed geometrical survey of the rock surface
Summary
The instability processes of sea rocky cliffs pose a significant hazard to communities and infrastructures located along the coastline For those areas highly devoted to touristic and recreational activities, the occurrence of rock falls and slope failures can threaten the safety of people and compromise the local economies related to the touristic activities. Manual field measurements have instead frequently major drawbacks related to logistic difficulties, safety risks for the operators, and inaccessible sites. Being both cost- and time-consuming, manual surveys of fractures, discontinuities and geometric patterns of the main rock features are not always feasible and practicable over large extent as well as do not provide a complete picture of the cliff setting [2]. Whenever manual surveys are carried out over limited portion of the investigated sea rocky cliff, their representativity for the entire slope is doubtful
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