GEOMORPHIC AND STRUCTURAL MAPPING FOR REMOTE BEDROCK RIVER CORRIDORS IN THE TAIWAN CENTRAL RANGE USING PAIRED OUTCROP AND KILOMETER SCALE UAV SURVEYS

Quantifying the role of sediment and rock properties on river incision is important for testing hypotheses about the connection between surface and deep Earth processes. However, field observations necessary for discriminating among competing models of bedrock river incision have been rare due to time intensive field work, particularly in steep terrain. Recent advances in small unmanned aerial vehicle (UAV) technology and structure from motion photogrammetry enable mapping at high enough resolution to resolve patterns in sediment cover, channel geometry, and rock properties, while mapping across large enough spatial scales to integrate across reach scale heterogeneity.

Here we present data from paired UAV photogrammetry and field surveys of over 40 kilometers of 18 bedrock river channels in the Taiwan Central Range. The resulting 3D river corridor models have a resolution of 1-5 centimeters, and are tied to detailed structural measurements at accessible bedrock outcrops and patch scale observations of channel morphology and sediment properties. Although narrow canyons provide challenges to UAV photogrammetry workflows due to limited GPS coverage and ground control, it is possible to generate usable, continuous survey data in areas that in some places are otherwise inaccessible. Additionally, we have developed workflows for processing the resulting large datasets to visualize and quantify spatial patterns in bedrock structure, channel morphology, and sediment grain size. Preliminary results indicate that channel width, orientation, and slope are tied to transitions in bedrock lithology and foliation orientation, and that major tributary junctions are pinned at lithologic changes. Where repeat surveys have been conducted, point cloud differencing reveals patterns of individual boulder mobility; all our surveys act as baselines for future change detection, providing a new window into erosion and sediment transport processes in bedrock rivers.