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Title:Meander migration with physically-based bank erosion
Author(s):Motta, Davide
Director of Research:Garcia, Marcelo H.
Doctoral Committee Chair(s):Garcia, Marcelo H.
Doctoral Committee Member(s):Abad, Jorge D.; Best, James L.; Langendoen, Eddy J.; Parker, Gary; Rhoads, Bruce L.
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Bank Erosion
Meander Migration
Floodplain Soil Heterogeneity
Computer Modeling
Meander Planform Shape
Migration Rates
Abstract:Interest in meander evolution has recently been revitalized with ongoing efforts worldwide to re-naturalize highly modified streams through re-meandering, with the consequent need of assessment tools. While research on meandering rivers has been very productive in the last five decades, among the components associated with the process of meander migration -- hydrodynamics, sediment transport, bed morphodynamics, and bank erosion -- the latter still needs a considerable amount of research, especially with regards to how the different processes responsible for bank erosion and retreat impact meander migration patterns. In this thesis, the model RVR Meander is further developed to analyze how the bank erosion processes of fluvial erosion, cantilever failure, and planar failure affect migration rates and shapes at reach scale. This research analyzes how the spectrum of meander planform shapes is affected by the type of functional form used for bank erosion and compares the model performance against the classic approach for meander migration based on the use of a dimensionless migration coefficient. It is shown that the new functional relation adopted allows for broadening the spectrum of bend shapes that can be obtained, simulating features like downstream skewness of meander bends, compound loops, ``rectangular'' shapes, and preferential migration of some portions of a bend. It is also shown that prediction of historically-observed migration can be improved using the proposed approach. Proceeding from homogeneous to heterogeneous floodplains, the effects of horizontal heterogeneity of floodplain soils on rates and shapes of meander migration are evaluated. It is shown that floodplain-soil complexity can greatly contribute to meander planform complexity. Using a stochastic approach, the parameters governing the impact of floodplain heterogeneity on bend statistics are identified: (i) local randomness of soil resistance to erosion, mainly controlling bend shape; (ii) cross-valley increase of soil resistance, mostly controlling lateral migration and bend skewness; and (iii) length scale of floodplain heterogeneity, with finer scale of soil heterogeneity associated with lower variability of migration. Introducing another element of complexity, the impact of bank mass failure processes (cantilever and planar failure), through combined horizontal and vertical heterogeneity of floodplain soils, is investigated, with the goal of determining if they can affect migration in a sustained fashion. Cantilever failure continuously impacts meander migration rates and shapes, as it is driven by fluvial erosion at the bank toe. It is shown that lateral and downstream migration and bend skewness are both affected to a degree depending on size and location of floodplain soil patches, thickness of the top, less erodible, bank layer, and characteristic residence time of slump blocks. Planar failure, although more episodic than cantilever failure, may impact meander migration depending on the horizontal and vertical variation of geotechnical properties such as soil cohesion, the vertical distribution of shear stress acting on the different bank layers, bank geometry profile, and river planform. Results show that, besides hydrodynamics and bed morphodynamics, the horizontal and vertical structure of the floodplain soil affects meander evolution. This should be considered when attempting to correlate field observations of soil distribution to the observed complexity of meander planform shapes.
Issue Date:2013-08-22
Rights Information:Copyright 2013 Davide Motta
Date Available in IDEALS:2013-08-22
Date Deposited:2013-08

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