Numerical modeling study on meandering and cutoff dynamics

Li, Zhi

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https://hdl.handle.net/2142/115668 Copy

Description

Title

Numerical modeling study on meandering and cutoff dynamics

Author(s)

Li, Zhi

Issue Date

2022-03-11

Director of Research (if dissertation) or Advisor (if thesis)

Garcia, Marcelo H.

Doctoral Committee Chair(s)

Garcia, Marcelo H.

Committee Member(s)

• Parker, Gary
• Tinoco, Rafael O.
• Rhoads, Bruce L.
• Langendoen, Eddy J.

Department of Study

Civil & Environmental Eng

Discipline

Civil Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• river
• meander
• cutoff
• riverbed
• sandbar
• bank erosion
• hydrodynamics
• morphodynamics
• numerical modeling

Abstract

Meandering rivers never cease shaping the Earth's surface. Rivers choose to meander because a straight channel cannot maintain its initial planform due to bend instability; thus, the channel sinuosity will gradually increase with time, and this process is termed meander evolution. When the sinuosity is sufficiently large, cutoffs play a limiter's role in reducing sinuosity and reducing planform complexity. Though previous studies have attempted to explore the problem of meandering and cutoff dynamics through theoretical analysis, field observation, laboratory experiment, and numerical modeling approaches, it is still not fully solved today as a fundamental science problem, mainly because it is related to complex hydrodynamics (secondary current, turbulence, etc.) and morphodynamics (bedload transport, bank erosion, etc.). The theory of the formational mechanisms of the two types of cutoffs, neck cutoffs and chute cutoffs, is not fully established, either. Hence, this research attempt to contribute a more profound understanding of meandering and cutoff dynamics through numerical modeling approaches. There are three topics in this dissertation. The first topic concerns an improved method of a riverbed topography generator for constant-width meandering rivers. The present model, named pyRiverBed, computes the synthetic riverbed topography through a group of analytical solutions, which is based on the linear bend theory. pyRiverBed can facilitate meandering river researchers to interpolate their field-measured bathymetric data using the synthetic bed, design their non-flat bed laboratory flumes for experiments, and initialize their hydrodynamic and morphodynamic numerical models. It can also guide stream restoration projects on designing a channel with a morphodynamic equilibrium bed. The quality of generated synthetic bed topography is evaluated through the comparison against both laboratory experiment data and field-measured data. The meander migration submodel is validated using a real-world channel migration and neck cutoff event. The validation results prove that the synthetic riverbed's accuracy is reasonably good, and the meander migration submodel can successfully predict meander migration and neck cutoff. The second topic focuses on the development of a novel hybrid deterministic-stochastic bank erosion numerical model that integrates a two-dimensional hydrodynamic model (deterministic) with a morphodynamic model (deterministic) and a bank erosion model (stochastic). The model solves the 2D Shallow Water Equations and the standard k-epsilon turbulence model. Bedload transport is estimated using the Meyer-Peter and Muller formula, and bed evolution is solved using the Exner Equation. The stochastic bank erosion model makes use of a new method to evaluate the bank erosion risk. The model is applied to a cutoff event in the Maiqu River on the Qinghai-Tibet Plateau in China. Flow field evolution and bed evolution in the cutoff channel suggest that the model can successfully simulate bank erosion processes during the cutoff channel evolution, and bank topographic irregularities are reasonably captured. Implications of the findings include the effect of both bank erodibility properties and statistical realizations. A newly introduced calibration parameter, the ratio of mesh size to the coupling period between the bank erosion model and the hydrodynamic & morphodynamic models, is not as intuitive as in traditional bank erosion models. The proposed approach makes it necessary to estimate the size of a typical slump block, or 10-30\% of the cutbank height, to set the mesh size. This could be a limitation of the model if field observations are not available. The third topic investigates the hydrodynamics of chute cutoffs at the laboratory scale through well-designed numerical experiments. Past work has simplified the hydrodynamics of a chute cutoff as a river bifurcation cohering with a river confluence, and identified the dominant effects of the channel width ratio and the diversion/junction angles. However, a more comprehensive study is needed to understand the hydrodynamics of a chute cutoff as a whole. The model developed in this work solves the three-dimensional Reynolds-averaged Navier-Stokes (3D RANS). The model is calibrated with flume experiment data from a physical model used to analyze the hydrodynamics before and after a chute cutoff. The modeling results agree with experimental observations and provide additional insights into the existing conceptual models for chute cutoff morphodynamics.

Graduation Semester

2022-05

Type of Resource

Thesis

Copyright and License Information

Copyright 2022 Zhi Li

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