[H-GM02] Geomorphology

[HGM02-01] Spatial analysis between underground cultural heritage and surface microtopography using high-density point cloud data: a case study of the Taya Cave, Yokohama Pref., Eastern.

*Yu Takagi¹, Takuro Ogura², Yasuhiko TAMURA³, Chiaki T. Oguchi⁴, Yuichi S. Hayakawa⁵, Masato Sato⁶, Tsuyoshi Hattanji⁷ (1.Graduate School of Science and Technology, Degree Programs in Life and Earth Sciences, University of Tsukuba, 2.Graduate School of Education, Hyogo University of Teacher Education, 3.The Executive Committee for the Preservation of TAYA CAVERN, 4.Graduate School of Science and Engineering, Saitama University, 5.Faculty of Environmental Earth Science, Hokkaido University, 6.National Research Institute for Earth Science and Disaster Resilience, 7.Faculty of Life and Environmental Sciences, University of Tsukuba)

Keywords:UAV-LiDAR, TLS, GIS, High-Density Point Cloud Data

There are underground cavities around Japan, including coal and stone mining sites and underground shelters. Shallow sinkholes are phenomena in which subsurface cavities deteriorate collapse, and expand, resulting in sinkholes. Although backfilling is a standard measure to deal with shallow sinking, if the cave is an object of preservation, like an underground cultural heritage site, an alternative measurement including non-destructive measurements is needed. LiDAR measurement is non-destructive method of determining cavities' shape and location. In addition, LiDAR measurement acquire high-density point cloud data and create a DEM from the data for analysis in a GIS. Thus, this study clarifies the location between Satoyama's underground cultural heritage and surface microtopography and the underground cave combining UAV-LiDAR data and TLS data. The study area is Taya Satoyama and Taya Cave, a registered historical site from Yokohama City.
The cave has many shallow sinkholes, and many reliefs are damaged. Topographic data of the satoyama surface (Satoyama DEM) and the cave surface (cave DEM) were created from the acquired high-density point cloud data. The soil layer thickness from the top of the cave to the ground surface and geomorphic quantities were calculated based on two DEMs: slope and TWI (Topographic Wetness Index). The distribution of the damage phenomena in the caves was sorted out through field surveys. The distribution of trees and tree trunk diameter in Satoyama were measured from the point cloud data. As the results, sinkholes, cave collapse, wall delamination, and cracks were observed in the Taya ceve. an average soil cover height of 5.86 m, an average slope of 23.14°, and an average TWI of 2.56 above the Taya Cave. Where sinkholes occurred, the average soil cover was thin (2.57 m), and the average TWI was significant (3.34). As the limit height of collapse determined from the physical properties of the ground is 2 m, this indicates that the risk of sinkholes is high when the soil cover is less than 3 m in the Taya cave. Where cracks occurred, the average TWI was significant (3.16), and the density of small-diameter trees was high. Because of the high TWI and favorable conditions for trees, the tree growth rate is fast. The formation and expansion of cracks are caused by root pressure resulting from the growth of tree roots. Cracks are thought to have formed because of the distribution of young trees with high root pressure.