The influence of pioneer riparian vegetation on river processes from the plant to reach scale

Alluvial rivers have morphologies that are shaped to varying degrees by the character of the riparian vegetation they support. Floodplain vegetation produces bank cohesion, for example, which in turn is responsible for inducing river meandering that gives rise to in-channel bars suitable for pioneer vegetation recruitment. Once established, pioneer vegetation is inundated by channel-forming flows, where it interacts with flow and sediment transport processes. This dissertation quantifies interactions between in-channel pioneer vegetation, which is under-studied relative to floodplain vegetation, and river processes across spatial scales. At the seedling scale, I link field experiments measuring woody riparian seedling uprooting forces to numerical calculations of flow forces. Seedling uprooting sets the trajectory of vegetation-river interactions that may ensue if vegetation survives, becomes established and alters river morphodynamics at the patch, bar, and reach scales. I constrain the differential controls on seedlings’ resisting force, and show that substantial bed scour is required to uproot seedlings. These constraints on seedling uprooting conditions inform management strategies aimed at increasing or decreasing riparian species. I characterize relationships among topographic features created by vegetation patches on river bars and vegetation morphometric parameters. I show that flume-based hydraulic relationships poorly predict field observations. I also demonstrate that the signature of vegetation alters reach-scale morphology. This analysis, combined with one that characterizes the wavelengths of in-channel river topography, shows that vegetation and the topographic features it creates within a channel have a large influence on the distribution of shear stresses compared to other roughness features. Lastly, using a high-resolution hydrodynamic model that accounts for vegetation drag, I simulate the impact of vegetation succession on channel-bend and meander processes by changing the size and density of vegetation on an in-channel bar. A global sensitivity analysis shows that vegetation parameters are nearly as influential as channel characteristics in altering bend hydraulics. For a river reach, simulations show that a vegetated bar changes channel hydraulics and forces in a manner that would be expected to alter channel evolution, and explains qualitative observations of vegetation-mediated river morphologies. This research thus quantifies under which conditions pioneer seedlings can persist and alter channel topography, with implications for changing the morphology of rivers

Flow and scour constrainst on uprooting of pioneer woody seedlings

Scour and uprooting during flood events is a major disturbance agent that affects plant mortality rates and subsequent vegetation composition and density, setting the trajectory of physical-biological interactions in rivers. During flood events, riparian plants may be uprooted if they are subjected to hydraulic drag forces greater than their resisting force. We measured the resisting force of woody seedlings established on river bars with in situ lateral pull tests that simulated flood flows with and without substrate scour. We quantified the influence of seedling sizes, species (Populus and Tamarix), water-table depth, and scour depth on resisting force. Seedling size and resisting force were positively related with scour depth and water-table depth--a proxy for root length--exerting strong and opposing controls on resisting force. Populus required less force to uproot than Tamarix, but displayed a greater increase in uprooting force with seedling size. Further, we found that calculated mean velocities required to uproot seedlings were greater than modeled flood velocities under most conditions. Only when plants were either shallowly rooted or subjected to substrate scour (≥0.3 m) did the calculated velocities required for uprooting decrease to within the range of modeled flood velocities, indicating that drag forces alone are unlikely to uproot seedlings in the absence of extreme events of bar-scale sediment transport. Seedlings on river bars are most resilient to uprooting when they are large, deeply rooted, and unlikely to experience substrate scour, which has implications for ecogeomorphic evolution and river management

Riparian Vegetation and Sediment Supply Regulate the Morphodynamic Response of an Experimental Stream to Floods

Feedbacks between woody plants and fluvial morphodynamics result in co-development of riparian vegetation communities and channel form. To advance mechanistic knowledge regarding these interactions, we measured the response of topography and flow to the presence of riparian tree seedlings with contrasting morphologies in an experimental, field-scale, meandering stream channel with a mobile sand bed. On a convex point bar, we installed seedlings of Tamarix spp. (tamarisk) and Populus fremontii (cottonwood) with intact roots and simulated a bankfull flood, with each of eight runs varying sediment supply, plant density, and plant species. Vegetation reduced turbulence and velocities on the bar relative to bare-bed conditions, inducing sediment deposition when vegetation was present, regardless of vegetation density or species. Sediment supply also played a dominant role, and eliminating sediment supply reduced deposition regardless of the presence of vegetation. Unexpectedly, plant density and species architecture (shrubby tamarisk vs. single-stemmed cottonwood) had only a secondary influence on hydraulics and sediment transport. In the absence of plants, mobile bedforms were prominent across the bar, but vegetation of all types decreased the height and lateral extent of bedforms migrating across the bar, suggesting a mechanism by which vegetation modulates feedbacks among sediment transport, topography, and hydraulics. Our measurements and resulting insights bridge the gap between laboratory conditions and real dryland sand-bed rivers and motivate further morphodynamic modeling

Structure from Motion at Sheep Draw

In spring 2020, the world was hit by a pandemic that spread globally by March, causing universities and most of the world to move to remote means. Summer field camps, long hailed as a rite of passage in the geosciences, were cancelled throughout the US. The community moved quickly, with NAGT developing remote learning tools and arranging for sharing and collaboration between instructors and institutions. As such, UNAVCO (GETSI) and University of Northern Colorado embarked on a data collection campaign for a summer field course entitled Geoscience Field Issues Using High-Resolution Topography to Understand Earth Surface Processes – originally slated for in-person teaching. The team collected GNSS data, drone imagery for use in structure from motion, and terrestrial laser scanning from a site near Greeley, Colorado on the Poudre River. In this assignment, students conduct a Structure from Motion (SfM) survey of the Sheep Draw field site. The end product is a point cloud used in additional assignments to conduct analyses

The Influence of a Vegetated Bar on Channel-Bend Flow Dynamics -- Data Set #10

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2023: Members of UNC Learning Communities Share

In this learning session, members of UNC Learning Communities Amie Cieminski, Sharon Bywater-Reyes, Maurice Harris, and Jun Park will share their take-away messages about teaching, learning, and assessment from their participation in different learning communities