| Popis: |
As large wood (LW) supplied by landslides and debris flows is one of the main components of watershed ecosystems, the importance of quantifying the dimensions of the LW is evident. However, the low accessibility of disturbed channels after landslides and debris flows generally impedes accurate and quick LW investigations. Recent advances in photogrammetry techniques may overcome such issues. In this study, we used ortho-photographs acquired using a small unmanned aerial vehicle (UAV) to measure the lengths of LW (wood pieces > 0.2-m long and > 0.03-m diameter) entrapped mainly by closed-type check-dams. We focused on two channels that are 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 the both ends of a piece LW are satisfactorily extracted from an ortho-photograph acquired via the UAV, the LW 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 LW pieces were discharged from landslide scars in the broadleaf forest, whereas approximately 750 LW pieces were left in the landslide scars of the coniferous forest. The number of LW pieces in the landslide scars increased with the increase in the landslide area, suggesting that some LW pieces can be left even if large landslides occur. There were no significant changes in the lengths or locations of the entrapped LW, 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 LW rarely migrate even under extreme rainfall. |
