Laser measurement and calculation of elevation changes in the debris-flow source area in Atami City before and after the disaster

A debris flow occurred due to heavy rain in Izusan, Atami City, Shizuoka Prefecture on July 3, 2021. In order to understand the situation of the debris flow, Geospatial Information Authority of Japan (GSI) measured the elevation using a laser scanner mounted on an unmanned aerial vehicle (UAV) three days after the disaster. In this area, airborne laser measurements were conducted as public survey in 2009 and 2019. These past data enabled GSI to calculate the elevation changes by comparing the results of laser measurements between three periods, including before and after the disaster. As a result, detailed and quantitative information including volume of residual soil is obtained. The results were released through GSI website on the same day as the UAV measurement. In this paper, its calculation process and results are reported.