RAPID CHARACTERIZATION OF STREAMBED MICROTOPOGRAPHY IN MOUNTAIN CHANNELS: METHODS AND APPLICATION USING STRUCTURE-FROM-MOTION PHOTOGRAMMETRY (Invited Presentation)

Erosion and sediment transport in mountain rivers depend to first order on the size distribution of channel-bed sediment and flow resistance, both of which are encoded in streambed microtopography. Recent advances in structure-from-motion (SfM) multi-view photogrammetry have radically lowered the cost and effort of acquiring high-resolution topographic surveys, which are particularly well-suited for characterizing streambed microtopography at dry or low-flow conditions. Minimal equipment and power requirements make field surveys of typically inaccessible channels feasible, allowing for rapid collection of baseline datasets for quantifying bed-state and assessing landscape change. Here we outline a workflow focused on efficient field data collection of streambed microtopography using ground-based SfM methods. We present field techniques for scaling and referencing datasets with minimal equipment, and evaluate optimal scales of analysis for point-cloud-based processing and mesh generation using statistical properties of streambed microtopography. We apply this methodology to field examples in Pennsylvania, California, and Taiwan to: 1) evaluate connections between streambed microtopography and grain size distribution and particle shape; 2) quantify boulder mobility and erosion using repeat surveys; and 3) integrate field-surveyed streambed microtopography into computational fluid dynamics models to predict flow resistance.