QUANTIFYING POST-FIRE HILLSLOPE AND DEBRIS FLOW EROSION IN A STEEP CATCHMENT USING HIGH-RESOLUTION STRUCTURE FROM MOTION

Climate change is projected to increase wildfire severity and frequency in the mountainous western United States during the 21^st century. In the South Fork Payette River drainage of Idaho, wildfire greatly impacts erosion regimes and sediment yields with implications for salmonid habitat, infrastructure, and Quaternary landscape evolution. Much previous work has focused on minimum deposited sediment volumes from post-fire debris flows and relies on laborious manual survey methods. Relatively few post-fire erosion studies have integrated sediment yields from hillslopes with debris flow volumes. Additionally, diffusive hillslope processes (e.g. rilling, sheetwash, rainsplash) are difficult to measure over appropriate spatial scales with existing approaches. We combine new remote sensing methods with nested field measurements, mapping, and stratigraphy to characterize and quantify post-fire erosion in a mountainous, granitic drainage.

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.