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Title:The fluvial dynamics of compound meander bends
Author(s):Engel, Frank
Director of Research:Rhoads, Bruce L.
Doctoral Committee Chair(s):Rhoads, Bruce L.
Doctoral Committee Member(s):Best, James L.; Parker, Gary; Garcia, Marcelo H.
Department / Program:Geography & Geographic InfoSci
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):River meander
Acoustic Doppler Current Profiler (ADCP)
Acoustic Doppler Velocimeter (ADV)
Bank Erosion
Fluvial geomorphology
Abstract:The dynamic evolution of the planform of meandering rivers often leads to the development of compound loops with multiple lobes of maximum curvature, also known as compound meander bends. At present, the interaction among spatial patterns of mean flow, turbulence, bed morphology, bank failures and channel migration in compound loops is poorly understood. In particular, field studies of this interaction over the timescale of planform evolution are lacking. The research presented here examines the co-evolution of flow, bed morphology, and channel planform in two compound meander bends. Careful study is given to the interaction of processes over differing spatial and temporal timescales. Results suggest that patterns of flow, sediment entrainment, and planform evolution in compound meander bends are more complex than in simple meander bends. Moreover, interactions among local influences on the flow, such as outer bank blocks, local topographic steering, and locally high curvature, tend to cause compound loops to evolve toward increasing planform complexity over time rather than stable configurations. The research is comprised of three field investigations. The first study examines the co-evolution of flow, bed morphology, and channel planform in a compound meander loop and relates patterns of near-bank velocity and turbulence to planform change within the loop. Data consist of repeat surveys of channel change in a compound loop over an 11-year period, coupled with Acoustic Doppler Velocimeter (ADV) measurements of 3-D instantaneous velocities for similar magnitude flows at the beginning and end of this period. Results confirm that this compound loop is highly dynamic with major changes in planform occurring over the 11-year period. Spatial patterns of near-bank velocity and turbulence correspond to patterns of bank erosion and channel migration within the loop; however, these patterns are not strictly a function of planform curvature. Instead, local factors, including deflection of the flow by point bars and failed bank blocks, can enhance or inhibit the development of high near-bank velocities and turbulence kinetic energy. The loop has elongated and become more asymmetric over time—a pattern of development consistent with patterns of near-bank velocities and turbulence at the beginning of the study period. The pattern of near-bank velocities and turbulence for measurements at the end of the period indicate that the loop will continue to elongate in the near future, supporting the hypothesis that compound loops change progressively over time rather than evolving into a stable configuration. In the second study, Acoustic Doppler Current Profiler (ADCP) measurements of the time-averaged flow structure are examined to evaluate the influence of channel curvature and hydrologic variability (i.e., stage) on the structure of flow within a compound loop and to relate changes in bed morphology to flow structure at various flow stages. Local increases in centerline curvature (or decreases in dimensionless radius of curvature) within the upstream lobe of the bend reduce outer bank velocities at morphologically significant flows, creating a region that protects the bank from high momentum flow and corresponding high bed shear stresses. The upstream lobe of the compound loop, which has a dimensionless radius of curvature about one-third less than that of the downstream lobe, also has an average bank erosion rate less than half of the erosion rate for the downstream lobe. Relatively high bank erosion rates within the downstream lobe corresponds to the shift in a core of high velocity and the zone of high bed shear stresses toward the outer bank as flow moves through the two lobes. This pattern of erosion provides a mechanism for continued migration of the downstream lobe in the near future. The distribution of bed material sizes within the multi-lobed bend corresponds to the spatial pattern of bed shear stress magnitudes, indicating that bed material sorting within the bend is governed by bed shear stress. In the last study, field data are used to analyze the characteristics of turbulence near the outer bank of a compound meander loop, and based on this analysis, a conceptual framework of the structure of turbulence in the near outer bank region of meander bends is developed. Specifically, the study determines the structure of turbulence at the outer bank of an actively migrating meander bend, and evaluates the spatial variability of turbulence characteristics in relationship to the curvature-induced mean flow field. Results show that the structure of turbulence is linked to curvature-induced effects through the progressive advection of high momentum fluid toward the outer bank as flow moves through successive lobes of the loop. Contrary to straight-channel bed-generated turbulence, where streamwise turbulent fluctuations are the predominant contributor to Reynolds stresses, curvature-induced secondary circulation in the study bend enhances the strength of cross-stream and vertical turbulent fluctuations, leading to increased contribution to the Reynolds stresses from these components. Additionally, large roughness elements, such as failed bank material, can disrupt the curvature-induced pattern of turbulence. Bank blocks reduce flow velocities and turbulent stress in the immediate lee of the block, a finding supporting the bank-toe protection hypothesis that large roughness elements can protect the outer bank from fluid forces and reduce bank erosion rates (Carson and Kirkby 1972). Furthermore, the findings support the assertion that the effects of local topographic features on erosion and deposition within compound bends can be more important than reach-scale effects associated with channel curvature.
Issue Date:2014-05-30
Rights Information:Copyright 2014 Frank Engel
Date Available in IDEALS:2014-05-30
Date Deposited:2014-05

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