10:45 AM - 11:00 AM
[HDS09-07] Collapse risk of sea cliff in Yonaguni Island:
Geomorphological analysis based on high-resolution three-dimensional measurement
Keywords:RTK-UAV, SfM-MVS, DEM, Yaeyama Group, Ryukyu Group, Sea cave
Buildings or roads constructed around sea cliffs have a fear of collapse as the cliffs recede. Since the erosion of sea cliffs is influenced by various conditions such as geology, lithology and wave conditions, it is important to conduct comparative studies in multiple regions to assess the risk of collapse. In this study, we consider the risk of collapse by conducting high-resolution three-dimensional measurements of geomorphology of the sea cliffs in Yonaguni Island, focusing on the geology and lithology. The geology of the sea cliffs is Miocene clastic sedimentary rocks (Yaeyama Group) and Pleistocene reef limestone (Ryukyu Group).
RTK (Real Time Kinematic) -UAV was utilized for Photogrammetry. Metashape Professional (Agisoft) was used for SfM-MVS processing. The three-dimensional model was created with a spatial resolution of about 5 cm. The GIS software QGIS and the 3D CAD software Rhinoceros (McNeel) were used for the geomorphological analysis.
The geomorphological analysis revealed that the erosion of the sea cliffs in Yonaguni Island occurs selectively where the lithological weak layers are subjected to waves, and that the geomorphology varies depending on the rock structure and the thickness and the continuity of the weak layers. As for a single collapse, the measurements of the boulders at the cliff foot revealed that the shapes and scales of the collapse differ between the clastic sedimentary rocks and the limestone. In the clastic sedimentary rocks, the thickness of the sandstone layer is important in determining the scales of the collapse, and the shapes of the collapsed boulders vary from plate-like to massive depending on the thickness of the sandstone layer and the arrangement of the high-angle joints. In contrast, in the limestone area, most of the boulders were massive, and the sizes tend to increase with the cliff height. In addition, the three-dimensional surveying in this study revealed that where the sea caves develop in the limestone, there is a risk of localized cave-in as the underground space exists under the road.
As described above, the geology and lithology have a great influence on the collapse and geomorphology of the sea cliffs and caves, therefore the accumulation of the analyses and comparisons of geomorphology of sea cliffs leads to a better understanding of the possible future collapses and their dangers.
RTK (Real Time Kinematic) -UAV was utilized for Photogrammetry. Metashape Professional (Agisoft) was used for SfM-MVS processing. The three-dimensional model was created with a spatial resolution of about 5 cm. The GIS software QGIS and the 3D CAD software Rhinoceros (McNeel) were used for the geomorphological analysis.
The geomorphological analysis revealed that the erosion of the sea cliffs in Yonaguni Island occurs selectively where the lithological weak layers are subjected to waves, and that the geomorphology varies depending on the rock structure and the thickness and the continuity of the weak layers. As for a single collapse, the measurements of the boulders at the cliff foot revealed that the shapes and scales of the collapse differ between the clastic sedimentary rocks and the limestone. In the clastic sedimentary rocks, the thickness of the sandstone layer is important in determining the scales of the collapse, and the shapes of the collapsed boulders vary from plate-like to massive depending on the thickness of the sandstone layer and the arrangement of the high-angle joints. In contrast, in the limestone area, most of the boulders were massive, and the sizes tend to increase with the cliff height. In addition, the three-dimensional surveying in this study revealed that where the sea caves develop in the limestone, there is a risk of localized cave-in as the underground space exists under the road.
As described above, the geology and lithology have a great influence on the collapse and geomorphology of the sea cliffs and caves, therefore the accumulation of the analyses and comparisons of geomorphology of sea cliffs leads to a better understanding of the possible future collapses and their dangers.