Debris flow causes entrainment of stand-woods located in the initiation and riparian zones, and, consequently, may include dozens of percentage of woody debris by volume (Johnson et al., 2000; May and Gresswell, 2003a; Lancaster et al., 2003). In addition to its destructive impact on life and infrastructure (e.g., Ruiz-Villanueva et al., 2013), woody debris in debris flow can alter the flow regime because of their irregular shape that can be entrapped around obstacles, leading to anomalous deposition of sediment and inundations (e.g., May, 2002; Lancaster et al., 2003; Tang et al., 2018; Booth et al., 2020). Therefore, the accumulation of woody debris results in structural peculiarities around the channel networks (e.g., Keller and Swanson, 1979; Woodsmith and Buffington, 1996; Montgomery et al., 1996; Nakamura and Swanson, 2003). This contributes to changes in the ecosystem, the channel morphology, and the sediment flux through woody-debris preservation and decay (e.g., Wallace and Benke, 1984; Lisle, 1995; Montgomery et al., 1995; Gurnell et al., 2001; Comiti et al., 2006; Ruiz-Villanueva et al., 2016). Hence, the river form and function are determined by the interaction between water, sediment, and wood (Nakamura et al., 2017; Swanson et al., 2020). Quantifying woody debris is important for assessing its impacts on ecological, geomorphological, and fluvial conditions.
Many previous studies focusing on in-situ channels have contributed to unraveling the role of woody debris in various spatiotemporal-scales and environmental settings, but most of these approaches required direct field measurements, such as local monitoring (e.g., Manners et al., 2007), tracking of woody debris (e.g., Ravazzolo et al., 2015; Wyżga et al., 2017), and field experiments using artificial woody debris (e.g., Haga et al., 2002). Taking into account the diversity of forests around channels (e.g., age, species, and density of trees), the accumulation of field data is obviously an effective approach. However, the necessity of human effort in the field hinders data acquisition at inaccessible areas (e.g., headwater channels and disturbed areas immediately after landslides and debris flows). In practice, the field data acquisition is difficult for a large scale area exceeding a sub-basin size. The improvement of field measurement techniques with respect to woody debris remains a critical issue.
To address the measurement issues of woody debris, remote sensing approaches using three-dimensional data have been applied. The Use of LiDAR (light detection and ranging) data clearly reduces the processing time required for mapping the logjam and large woody debris (Kasprak et al., 2012; Abalharth et al., 2015; Atha and Dietrich, 2016). However, such technology is expensive and therefore available only in some regions. Alternatively, photogrammetry based on structure from motion multi-view stereo (SfM-MVS) using UAV (uncrewed aerial vehicle) has been proven time-efficient compared to classical field surveys (Sanhueza et al., 2019). This approach overcomes data availability issues and is relatively low-cost. Nevertheless, most tests were conducted in low-land and flood plains rather than low-accessibility areas such as steep channels (e.g., Sanhueza et al., 2019). As the accuracy of SfM-MVS is remarkably influenced by complex surfaces and obstacles, such as steep slopes, large reliefs, and vegetation coverage (e.g., Fonstad et al., 2013; James and Robson, 2014), many unresolved uncertainties remain over the application of the SfM-MVS approach in steep and complex targets, such as woody debris in channels.
Aerial photography being one of the traditional two-dimensional data sources may more or less provide meaningful information on woody debris. Even satellite images from Google Earth are being used as accurate for mapping woody debris (Atha, 2013; Ulloa et al., 2015). Hence, the efficacy of aerial photography is evident, but the accuracy and effort of mapping depend on the image quality. In this respect, it is expected that small UAVs allow acquiring high-resolution aerial photographs at low cost because of the lower flight-altitude and higher portability compared to conventional aerial vehicles. Moreover, because flights of small UAVs can overcome inaccessibility issues and cover several kilometers depending on the flight design, it is a fairly attractive tool for obtaining woody debris measurements in low-accessibility areas.
In addition, although entrapped woody debris is often quickly removed to avoid unexpected damages in the downstream area due to its transport, UAV flights can be carried out immediately after rainfall events that result in a large amount of woody debris. Conversely, the risk of secondary impacts arising from woody debris entrapment has not been evaluated properly so far, because it has been difficult to conduct field surveys immediately after intense rainfall involving woody debris supply. Therefore, mapping woody debris would enable investigating the possibility of woody debris transport due to subsequent rainfall, even if it is carried out based on a simple method using aerial photography. Nevertheless, the potential to measure woody debris based on aerial photographs acquired via UAV has not been thoroughly examined due to lack of sample cases.
In this study, we analyze the accuracy of UAV-based measurements in the case of entrapped woody debris. Two regions are selected representing two forest types, coniferous and broadleaf forests. In both regions, large amounts of woody debris were supplied through landslides and debris flows triggered by a single rainfall event. This research has two main objectives: (1) to analyze the capability of ortho-photographs acquired via UAV to measure the lengths of entrapped woody debris, and (2) to investigate the transport potential of entrapped woody debris based on rainfall analysis. Based on the results, we discuss the effectiveness of UAV measurements and how woody debris behave after their entrapment.