QUANTIFYING FLUVIAL AND STRUCTURAL CONTROLS ON BEDROCK AND CHANNEL WIDTH IN TAIWAN

Understanding the processes controlling bank erosion in bedrock rivers is important for predicting patterns in channel cross-sectional geometry, which influences sediment transport and river incision. In general, the width of bedrock rivers scales similarly to that of alluvial rivers and can be modeled at a regional scale; however, the controls on bank geometry, which determine the local relationship between shear stress and flood magnitude, are poorly known. This knowledge gap arises mainly because of challenges in measuring high-resolution channel geometry at scales large enough to integrate local variability. In this study, we take advantage of a suite of high-resolution 3D datasets in Taiwan built with uncrewed aerial vehicle (UAV) photogrammetry to directly characterize bedrock walls along river corridors. Previous analysis of this dataset showed that bedrock channel width can vary by an order of magnitude over distances of only a few channel widths. We examined two reaches with different patterns in bedrock width and lithology. We directly measured wall roughness and bank angle, connecting it to variability in channel width and bedrock lithology. Even along individual reaches, we saw bank angles ranging from 20 to 90 degrees with substantial variability in wall roughness and channel width. To identify the processes of what controls bedrock wall erosion, we made observations of fluvial sculpting by abrasion and plucking versus hillslope mass wasting. Preliminary observations indicate that bedrock structure sets channel width and bank angle when there are multiple lithologies present. By distilling what processes control bedrock wall erosion and geometry, we can further the knowledge on how bedrock rivers work.