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 debris flows generally impedes accurate and quick wood-piece investigations. Thus, remote-sensing measurements for wood pieces are necessitated. Focusing on sub-watersheds in coniferous and broadleaf forests in Japan (the CF and BF sites, respectively), we measured the lengths of wood pieces supplied by landslides (> 0.2 m length and > 0.03 m diameter) from orthophotos acquired using a small unmanned aerial vehicle (UAV). The measurement accuracy was analyzed by comparing the lengths derived from the UAV method with direct measurements. The landslides at the CF and BF sites were triggered by extremely heavy rainfalls in 2017 and 2018, respectively. UAV flights were operated during February and September 2019 at the CF site and during November 2018 and December 2019 at the BF site. Direct measurements of wood pieces were carried out on the date of the respective second flight date in each site. When both ends of a wood piece are satisfactorily extracted from an orthophoto acquired by the UAV, the wood-piece lengths at the CF site can be measured with an accuracy of approximately ±0.5 m. At the BF site, most of the extracted lengths were shorter than the directly measured lengths, probably because the complex structures of the root wad and tree crown reduced the visibility. Most wood pieces were discharged from landslide scars at the BF site, but at the CF site, approximately 750 wood pieces remained in the landslide scars approximately 19 months after the landslide occurrence. The number of wood pieces in the landslide scars of the CF site increased with increasing landslide area, suggesting that some wood pieces can be left even if large landslides occur. The lengths and locations of the entrapped wood pieces at both sites were not significantly changed between the two UAV flight dates. However, during this period, the rainfall intensities around the CF site measured by the closest rain-gauge of the Japan Meteorological Agency reached their second highest values from 1976 to 2019, which exceeded the 30-year return period. This suggests that most of the entrapped wood pieces rarely migrated even under intense rainfall.

Generation and Subsequent Transport of Landslide-driven Large Woody Debris Induced by the 2018 Hokkaido Eastern Iburi Earthquake

“…It is possible to acquire LWD information from orthophoto images acquired using unmanned aerial vehicles (UAVs) (Tsunetaka et al, 2021). Aerial photographs and elevation data after the earthquake were acquired twice, immediately after the earthquake (15 September 2018) and approximately 2 years after the earthquake (20 October 2020).…”

Do standing trees affect landslide mobility on forested hillslopes in Japan?

“…While this information holds for large landslides, there is still discussion concerning what controls the mobility of smaller landslides (< ~10 5 m 3 ). Debris flows formed by landslides entrain sediment and trees around the runout area as they descend (e.g., Johnson et al, 2000; Miller et al, 2003; Wohl et al, 2009; Ruiz‐Villanueva et al, 2014, 2016; Piton & Recking, 2016; Swanson et al, 2021; Tsunetaka et al, 2021). Entrained trees are broken into pieces, which potentially change the flow regime of the debris flows, depending on their size and composition ratio (e.g., Benda et al, 2003; Jackson & Sturm, 2002; May & Gresswell, 2003a).…”