Characterizing reach-scale flow resistance in mountain streams using structure-from-motion surveys and computational fluid dynamics simulation

Understanding flow hydraulics in mountain streams is important for assessing flooding hazard, and for quantifying sediment transport and bedrock incision in upland landscapes. In such settings, reach-scale flow resistance is sensitive to grain-scale roughness in channel bed sediment cover, which has traditionally been characterized by particle size distributions derived from time-consuming point counts performed in the field. However, developing a general framework for quantifying frictional relationships in mountain channels has proven a significant challenge. Here we combine millimeter-scale Structure-from-Motion (SfM) surveys of bed topography, traditional point counts of surface clast size and shape, and computational fluid dynamics (CFD) simulations in order to better evaluate the important scales of roughness for turbulent flow in mountain rivers. We focused our field surveys on gravel, cobble, and boulder bedded channels in Southern California and Central Pennsylvania spanning a wide range of grain size, sorting, and shape, with the goal of deriving empirical relationships between metrics of bed microtopography roughness and particle size and shape distributions. The resulting reach-scale topographic models were then used in large eddy simulation CFD experiments to quantify flow behavior and bed resistance. By analyzing bed microtopography using structure function analysis, we identified three scaling regimes that correspond to roughness length scales important for constraining the geometric complexity required for CFD simulations. Our preliminary results highlight the potential for rapid estimation of reach-scale microtopography using SfM methods that in addition provides a more direct measure of flow resistance than particle size statistics. Additionally, SfM methods enable a similarly straightforward assessment of vegetation and bedrock channel roughness, with broad applications in fluvial geomorphology.