Debris-flow historical assessment using UAV-mounted multispectral cameras at Ohya landslide scar, Japan

Abstract Debris flows are geomorphic processes that cause significant damage to people and property because of their high velocity, destructive power, and long travel distances. The historical assessment of debris flow activity, which is important for planning effective countermeasures, is often challenging because of limited records and field data. In this study, we reconstruct debris flow histories using unmanned aerial vehicle-mounted multispectral cameras to propose a novel debris flow assessment approach. In the Ohya landslide scar in central Japan, the reflectance of red-edge and near-infrared regions, normalized difference vegetation index (NDVI), and normalized difference red-edge index increased with elapsed time after debris flow occurrence, as investigated using differences in digital elevation models over 13 periods. Changes in the spectral reflectance parameters were primarily caused by vegetation recovery along the debris flow paths. A simple linear regression model expressing the relationship between NDVI and the elapsed time after debris flow could reconstruct the debris flow history, including the timing of debris flows with annual accuracy and temporal changes in debris flow paths caused by avulsion. Notably, the model obtained from mid-summer images accurately estimated the elapsed time using spring and autumn images. These results suggest that multispectral imagery is a useful tool for reconstructing debris flow histories, particularly in areas where historical disaster records are scarce.