Assessment of UAV-Based Photogrammetry in Lahar Valleys to Estimate Boulder Potential Hazards-Field Experiment at Unzen Volcano (Japan)

Abstract

CITATIONS 0 READS 225 2 authors, including: Some of the authors of this publication are also working on these related projects: Heavy rainfall mass movements and drifted wood View project Urban geology-Forensic geology-Forensic geohazard View project Abstract: On volcanoes where lahars are triggered, the size of sediments and clasts is an important element to consider for hazards and risks. However, sediment analysed are mostly the fine fractions due to logistics issues. Recent advances in 3D capture technologies such as the terrestrial laser scanner (TLS) provide high precision topographical data that can be used to assess the largest clasts sizes, but instruments are still costly and are difficult to carry in the field due to their heavy weight. For this reason, the present contribution investigates the use of structure from motion (SfM) to assess the potential hazards due to large clasts transported by lahars. We used the SfM method from aerial photographs taken by UAV combined with ground control points, in order to obtain Orthophotos and DEMs to measure the size of boulders in the Gokurakudani gully, at Unzen volcano. Furthermore, the results are used for calculation of the prediction of behaviour of each boulder. Specifically, the critical shear stress and the physical energy of each boulder are calculated, showing that the shear stress range needed to transport such boulder range between 500 and 1200 Pascals, depending on their orientation in the valley and their size.