Influence of riparian vegetation on near-bank flow structure and rates of erosion on a large meandering river

The dynamic process of meandering in alluvial rivers occurs through complex interactions among autogenic processes such as three-dimensional flow structure, channel planform geometry, and sediment transport. These internal processes can be strongly influenced by the geotechnical properties of the channel banks and floodplains, as well as riparian and in-channel vegetation, modifying rates of erosion and mechanism of bank retreat, often leading to complex planform geometries. While extensive research has been conducted on each of these processes independently, few studies have examined through detailed field measurements the combined effects and interactions between the internal processes and external forcings driving channel migration. Furthermore, most of the studies investigating the influence of bank material properties and vegetation have been conducted on small and moderately sized rivers with relatively simple planform geometry, or using simplified experimental flumes and numerical models. Thus, the influence of these external forcings on the meander dynamics of large rivers remain poorly understood. This dissertation research is organized into three separate investigations from two elongate meander loops with different riparian vegetation on a large river. The first study focuses on the spatial patterns of three-dimensional flow structure throughout these meander loops and examined the effects of near-bank large woody debris (LWD) on near-bank flow structure and boundary shear stress, and how the hydrodynamics varied during different hydrologic conditions. Data consist of time-averaged three-dimensional velocity measurements, which were obtained using a boat-mounted acoustic Doppler current profiler (ADCP) during varying hydrologic conditions. Patterns of depth-averaged velocity through the meander loop without near-bank LWD are fairly consistent with previous investigations of flow through elongate meander loops, however, LWD near the outer bank of the forested loop has a strong influence on the near-bank flow field. Specifically, the LWD produces a zone of low velocity against the outer bank that extends up to 40 m into the channel and over the entire flow depth, and creates several streamwise-oriented secondary cells. These effects from the LWD on the near-bank flow field prevent advection of high momentum fluid against the outer bank. In contrast, the roughness elements along the outer bank of the unforested bend are primarily large-scale topographic irregularities that are not effective at reducing flow velocities near the bank toe. The second study explores the various scales of outer bank form roughness produced from large-scale bankline irregularities and small-scale surface roughness, the influence of bank material properties and vegetation on scales of roughness, and how scales of roughness differ during variable discharge conditions and through time. Detailed morphology of the outer banks was obtained using terrestrial LiDAR during low flow conditions and multi-beam echo sounding (MBES) during near-bankfull conditions, and scales of roughness were evaluated using Hilbert-Huang Transform spectral analysis and root-mean-square analysis. Results show that scales of roughness along banks composed primarily of non-cohesive sediment vary as bank elevation increases and show a tendency for a dominant length scale of roughness, whereas banks composed of fine-grained silt and clay increase the resistance properties of the banks and promote uniform roughness vertically over the bank face and do not appear to have a dominant scale of roughness through the bend. Additionally, comparison between small-scale surface roughness obtained during subaerial and subaqueous conditions shows that bank roughness is considerably reduced during high flow conditions when the banks are inundated, most likely related to the removal of small woody and leafy vegetation during subaqueous and eradication of small-scale erosional features in non-cohesive bank materials. The third study examined the lateral and vertical heterogeneities in bank material properties and riparian vegetation between these two bends using various geotechnical tests, and a numerical model of bank retreat and repeat terrestrial LiDAR surveys to evaluate the capacity of bank material properties to modify the rates and mechanisms of bank retreat. Results show substantial differences in the characteristic grain size of the bank materials, soil cohesion, and critical shear stress necessary for sediment entrainment between the forested and unforested bends, and are highly variable within each bend, both laterally and vertically. Results also reveal that riparian trees are capable of enhancing bank stability through increased cohesion due to root-reinforcement, and that bedrock outcrops within the downstream limbs of both of these bends that are highly resistant to erosion. The findings from the model simulations of bank retreat show that the variations in bank material properties and riparian vegetation greatly contribute to rates of erosion and the style of bank failure, and suggest that hydrologic variability is an important factor influencing the erodibility of cohesive banks. For the unforested bend, the non-cohesive bank materials, lack of riparian and in-channel vegetation, and limited influence of the bank roughness elements produce high rates channel migration near the bend apex. However, on the downstream limb of this bend, the platform of bedrock exposed within the channel is strongly influencing patterns of near-bank flow and shear stress, leading to a small zone of deposition along the outer bank downstream of the bedrock. In contrast, at the forested bend, the high resistance of bank materials, stabilizing effects of riparian trees, and reduction of near-bank shear stress from increased flow resistance by LWD, limit extension of this bend near the apex. On the downstream limb where the highest shear stresses occur, the channel is confined by bedrock from the upland valley, restricting the downstream translation of the bend. In conclusion, the results from this research advance knowledge and understanding of how the interactions and feedbacks among three-dimensional flow structure, material properties of the banks and floodplains (sediment and bedrock), and vegetation characteristics near the outer bank influence the morphodynamics of meandering rivers. The findings also provide an empirical foundation for the refinement and calibration of numerical models aimed at predicting these morphodynamics in complex natural settings