APPLICATION OF STRUCTURE FROM MOTION (SFM) PHOTOGRAMMETRY TO QUANTIFY SEDIMENT MOBILIZATION FOLLOWING WILDFIRE BLOWDOWN EVENTS

Tree throw, the action of a tree falling over and pulling up sediment via it’s connected roots, can be a significant process in the downslope movement of sediment in forested mountain regions. The role of fire-related blowdowns in mobilizing sediment from floodplains to stream channels remains unexplored. In 2016, the Pioneer Fire burned nearly 260 square miles within the central Idaho Batholith, and resulted in valley bottom patches of blowdown within Clear Creek. In this study, we examine the application of photogrammetry as a method for quantifying the volume of sediment potentially communicated from floodplains to streams via tree throw. Within six 200-m long reaches we randomly subsampled five rootwads (sediment and organic material at the base of a fallen tree) for photogrammetry per reach. Photosets were collected at each rootwad and used to construct three-dimensional models using the Agisoft Structure from Motion (SfM) software. We then used CloudCompare to determine volumes of sediment and visible organic material. GPS tagging of the rootwads, control point tagging of each rootwad, and repeat photos allow us to quantify the loss of sediment volume, and thus the potential tree-thrown input of sediment into Clear Creek. Rootwad dimensions were also measured in the field to estimate total volume. Cores were taken to determine amount of organic material (roots) present. Evaluating the usefulness of photogrammetry in quantifying sediment supplied from rootwads over time has the potential to further our knowledge of postfire geomorphic change.