FLOW STEERING AND RIVER ENGINEERING OF WOODY RIPARIAN TREES QUANTIFIED WITH GROUND-BASED LIDAR

Woody riparian trees recruit on river bars, and may influence bar topography during flow events by steering flow and altering sediment transport dynamics. The presence of alternate bars is known to topographically steer flow, but the contribution of riparian vegetation to flow steering remains unquantified. We investigated vegetation-related flow steering using high resolution ground-based LiDAR scans of riparian trees and associated topography. We scanned trees of various growth stages and morphologies (Populus and Tamarix) and post-processed vegetation scans for hydrodynamic vegetation density that dictates how much flow is steered around versus penetrated through trees. We compared vegetation density to the volume of sediment eroded upstream and deposited downstream of trees, and compared these metrics to literature predictions obtained from flume experiments with artificial vegetation. Our results indicate individual Tamarix trees allow more flow to pass through, because of their multi-stem morphology, and therefore have longer length scale, higher volume downstream deposits. Individual Populus trees block more flow, causing more steering and greater upstream scour, but less downstream deposition. The presence of Tamarix results in enhanced deposition of sediment compared to Populus, and therefore partitions shear stress available for geomorphic work in rivers because of both drag on stems and drag on associated deposits, heightening their ability to engineer their own environment. These novel calculations of the relative influence of vegetation and associated topography on river engineering inform outstanding questions about vegetation-morphodynamic feedbacks, with implications for plant community and landscape evolution in a multitude of riverine settings.