Comparison of length and dynamics of wood pieces in streams covered with coniferous and broadleaf forests mapped using orthophotos acquired by an unmanned aerial vehicle

Abstract

As wood pieces supplied by landslides and debris flows are one of the main components of ecological and geomorphic systems, the importance of quantifying the dimensions of the wood pieces is evident. However, the low accessibility of disturbed channels after landslides and debris flows generally impedes accurate and quick wood-piece investigations. Recent advances in photogrammetry techniques may overcome such issues. In this study, we used orthophotos acquired using a small unmanned aerial vehicle (UAV) to measure the lengths of wood pieces (> 0.2-m long and > 0.03-m diameter) entrapped mainly by check-dams. We focused on channels that are located in coniferous and broadleaf forests and affected by two different landslide events. The measurement accuracy was analyzed by comparing the lengths derived from the UAV method with direct measurements. When the both ends of a wood piece are satisfactorily extracted from an orthophoto acquired via the UAV, the wood-piece lengths of coniferous trees can be measured with an accuracy of approximately ±0.5 m. For broadleaf trees, most of the extracted lengths were shorter than the directly measured lengths, probably due to the low visibility arising from the complex structures of the root wad and tree crown. Most wood pieces were discharged from landslide scars in the broadleaf forest, whereas approximately 750 wood pieces were left in the landslide scars of the coniferous forest. The number of wood pieces in the landslide scars increased with the increase in the landslide area, suggesting that some wood pieces can be left even if large landslides occur. There were no significant changes in the lengths or locations of the entrapped wood pieces, at either site seven months after the first UAV flight. In the coniferous forests, the rainfall that triggered landslides in 2017 exceeded the 100-year return level, which was an abnormally intense rainfall. Although the 2019 rainfall event that occurred between UAV flights did not provide enough rainfall to trigger landslides, rainfall intensities with different durations reached the second-highest value from 1976 to 2019, exceeding the 30-year return period. This suggests that most of the entrapped wood pieces rarely migrate even under extreme rainfall.