Sensitivity analysis of vegetation-induced flow steering in channels

Morphodynamic feedbacks result in alternating bars within channels, and the resulting convective accelerations dictate the cross-stream force balance of channels and in turn influence morphology. Pioneer woody riparian trees recruit on river bars and may steer flow and alter this force balance. This study uses two-dimensional hydraulic modeling to test the sensitivity of the flow field to riparian vegetation at the reach scale. We use two test systems with different width-to-depth ratios, substrate sizes, and vegetation structure: the gravel-bed Bitterroot River, MT and the sand-bed Santa Maria River, AZ. We model vegetation explicitly as a drag force by spatially specifying vegetation density, height, and drag coefficient, across varying hydraulic (e.g., discharge, eddy viscosity) conditions and compare velocity vectors between runs. We test variations in vegetation configurations, including the present-day configuration of vegetation in our field systems (extracted from LiDAR), removal of vegetation (e.g., from floods or management actions), and expansion of vegetation. Preliminary model runs suggest that the sensitivity of convective accelerations to vegetation reflects a balance between the extent and density of vegetation inundated and other sources of channel roughness. This research quantifies how vegetation alters hydraulics at the reach scale, a fundamental step to understanding vegetation-morphodynamic interactions.