Applications of UAV Digital Photogrammetry in landslide emergency response and recovery activities: the case study of a slope failure in the A-7 highway (S Spain)

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Abstract The disruption of vehicle traffic on important transport infrastructure has a major economic impact. Therefore, the instability of highway cut-slopes poses a significant risk for major infrastructures crossing mountainous regions and rocky coasts. The use of Uncrewed Aerial Vehicles (UAVs) is of great help in the prevention of road cut-slope failures and in emergency and rehabilitation of sites impacted by this type of landslides. Here, we show an example of UAV operations in emergency response and recovery in a highway cut-slope failure. We analyzed a failure that occurred on 11 March 2021 in a 52-m-high cut-slope located at 354.3 km of the A-7 Highway (S Spain). We also examine the applications of these analyses to help in risk evaluation and management. We acquired 3050 images and 6 videos with UAVs during 5 field surveys from 11 to 31 March 2021 to follow the evolution of the destabilized cut-slope. The videos taken by UAVs were of great help in the emergency response phase to guide the first steps in the removal of unstable rock blocks. The acquired images served to generate sequential Digital Elevation Models (DEMs) and 3D models (i.e., point clouds and tiled models) using the Digital Photogrammetry technique. These models were used subsequently to identify changes in the topography and calculate volumes of both destabilized and excavated material. The volumetric analyses indicate that 6200 m3 of rock collapsed, producing a deposit of debris of 10,900 m3, and after the partial failure, 15,100 m3 of the slope remained unstable. Therefore, the volume of rock mobilized by the landslide was about 21,300 m3. The recovery activities involved the removal of the collapsed debris and the re-profiling of the slope. This involved the excavation, transportation, and deposition of 83,400 m3 of rock to reduce the slope’s inclination from 64° to 45–33°. We then analyzed the follow-up work generating sequential DEMs during the cut-slope excavation in order to forecast the approximate date of the highway reopening. Thus, an average excavation speed of 1075 m3/day was estimated, and with this value, by March 31, it was possible to initially forecast that the highway could be reopened in the first week of June, not far from June 10, the date on which it was finally reopened. These results were obtained with reduced equipment costs and field time over topography conventional methods, which indicates the usefulness of UAVs and Digital Photogrammetry (UAV-DP) in emergency response and recovery situations provoked by unstable slopes in roads.