QUANTIFYING POST-FIRE HILLSLOPE AND DEBRIS FLOW EROSION IN A STEEP CATCHMENT USING HIGH-RESOLUTION STRUCTURE FROM MOTION
We use an Unmanned Aerial Vehicle (UAV) and Structure from Motion (SfM) photogrammetry to easily derive point clouds at very high spatial resolution (sub-decimeter) within a steep ~1 km2 catchment. At the catchment scale, digital surface models (DSMs) produced from SfM allow rapid visualization and high-resolution quantification of debris flow scour volume and channel cross sections. Traditional field work, including deposit stratigraphy and mapping provide a check for our SfM methods. To quantify the diffusive hillslope components, we use handheld SfM with >400 pts/cm2 to create 2 mm DSMs at randomly distributed 1 m2 plots. Pedestals and markers allow us to characterize the volume of material removed by rainsplash and sheetwash within the catchment. Additionally, we use nested point, plot, and transect hillslope erosion measurements to relate differences in process to aspect and substrate. Rill density and cross-sectional area are recorded along 20 m transects and show that rilling is more extensive on finer-grained, north-facing slopes. Our investigation shows that very high resolution SfM datasets are well suited to quantify eroded volumes in a steep catchment, both on hillslopes and in channels. Our distributed measurements help couple diffusive hillslope and channelized post-fire erosion processes across the study area.