Comparison of Length and Transport of Entrapped Woody Debris in Coniferous and Broadleaf Forests Based on Mapping Using Ortho-Photographs Acquired by Uncrewed Aerial Vehicle

Abstract Landslides and debris flows often result in woody debris from initiation and riparian zones, through their runout. Considering that woody debris is one of the main components of watershed ecosystems, the importance of quantifying its properties and transport is evident. However, the low accessibility of disturbed channels after landslides and debris flows generally impedes the accurate and quick investigation of woody debris. The recent advances in photogrammetry techniques and technology may overcome such issues. In this study, we used ortho-photographs acquired by a small uncrewed aerial vehicle (UAV) for measuring the lengths of woody debris entrapped mainly by closed-type check-dams. We focused on two channels, located in coniferous and broadleaf forests and affected by two different landslides events. The measurement accuracy was analyzed by comparing the lengths derived from the UAV method with direct measurements. When both edges of woody debris were satisfactorily extracted from an ortho-photograph acquired via UAV, the length of the woody debris with respect to coniferous trees can be measured with an accuracy of approximately ±0.5 m. However, some coniferous trees were captured by stand trees in the riparian zone, and the coverage by tree-crowns led to the underestimation by several meters of the extracted length of the entrapped woody debris. For broadleaf trees, most of the extracted lengths were shorter than the directly measured lengths. This is probably caused by the low visibility of both edges due to the complex structures of the root-wad and the tree-crown. Our results showed that there were no significant changes in the lengths and locations of the entrapped woody debris, in both sites, after seven months of the first UAV flight. In the coniferous forests, the rainfall that triggered landslides in 2017 exceeded the 100-year return level, which was obviously an abnormal intense rainfall. Although the 2019 rainfall event that occurred between UAV flights was not as much to the rainfall triggering landslides, rainfall intensities with different durations reached the second-highest value from 1976 to 2019, exceeding the 30-years return period. This suggests that most of the entrapped woody debris rarely migrate even under extreme rainfall.