Form and function of anabranches and floodplain secondary channels in lowland meandering river systems

Shukla, Tanya

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https://hdl.handle.net/2142/125823 Copy

Description

Title

Form and function of anabranches and floodplain secondary channels in lowland meandering river systems

Author(s)

Shukla, Tanya

Issue Date

2024-07-12

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

Rhoads, Bruce L

Doctoral Committee Chair(s)

Rhoads, Bruce L

Committee Member(s)

• Anders, Alison
• Cienciala, Piotr
• Garcia, Marcelo H

Department of Study

Geography & GIS

Discipline

Geography

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Anabranching Rivers
• Floodplain Secondary Channels
• United States Midwest

Language

eng

Abstract

Alluvial rivers exhibit diverse forms that represent adjustments to the prevailing and past water and sediment supply. Based on planform, rivers have been classified into two groups that include single-channeled meandering rivers, and multi-channeled anabranching and braided rivers. Braided rivers have small, transient in-stream bars and differ from anabranching rivers that are characterized by stable, alluvial islands that divide flow into multiple channels at discharges up to bankfull. Rivers in historically glaciated and intensively managed landscapes of the United States Midwest are generally meandering but locally can have well developed anabranches or incipient secondary channels perched on the floodplain surface. Secondary channels in meandering river systems are important features connecting the main channel to the floodplain. Despite recognition of anabranching configurations and floodplain secondary channels across the world, particularly in large river systems, the prevalence, form, and processes within anabranches and floodplain secondary channels in lowland alluvial rivers of the US Midwest have not been extensively documented. The objectives of this dissertation are to 1) quantify the abundance, planform morphology and stream power domain of anabranching reaches relative to meandering reaches in the US Midwest, 2) characterize the form and fluvial processes in floodplain secondary channels along a meandering river reach and investigate the role of secondary channels in anabranch development, and 3) examine the consequences of anabranch development within meandering rivers with respect to river stability. The first study quantifies the prevalence, planform morphology, and stream-power domain of anabranching reaches along numerous lowland meandering rivers in the Midwest. Anabranch prevalence is explored for five representative watersheds using Google Earth aerial imagery and airborne lidar data. Less than 1% or 164 km of the total stream length (~42000 km) in these watersheds exhibits anabranching, but no watershed is devoid of anabranching reaches. Most anabranching reaches are characterized by a two-channel looping configuration, although complex anabranches with more than three channels exist at a few locations. Mean bifurcation (70°±29°) angles of the anabranching reaches are higher than those of braided rivers (40°-60°), with maximum angles as high as 150°, whereas mean confluence angles (60°±28°) fall within the range of junction angles in stream networks (45°-75°). Stream-power analysis indicates that the occurrence of midwestern anabranching reaches along meandering rivers is significantly related to local increases in channel slope. The stream power domain of most midwestern anabranching reaches falls within that of meandering rivers, although some reaches have power similar to that of braided or wandering gravel-bed rivers. When considered collectively, multi-thread reaches juxtaposed with meandering reaches do not fit neatly into existing classifications of anabranching rivers. The second study examines the geomorphological characteristics (channel form, material properties, wood jams) and bed-material mobilization potential of secondary channels on the floodplain of a meandering river in Illinois, USA. It also compares these attributes to those of the main channel. Results show that secondary channels are at most about one-third the size of the main channel, but also vary in size over distance. Channel dimensions tend to be greatest near the proximal connection of secondary channels to the main channel, corroborating the effectiveness of hydraulic action to produce scour where flow from the main channel enters secondary channels. The beds of secondary channels consist mainly of mud in contrast to sand and gravel on the bed of the main channel. Secondary channels connected to the main channel at both ends have more abundant active wood jams than those connected only at the proximal end. Flow from the main channel enters secondary channels at sub-bankfull stages, but maximum mobilization of cohesive bed material in secondary channels only occurs during flows that exceed the average bankfull stage in the main channel. Overall, secondary channels are active conduits of flow, sediment, and large wood on floodplains and at least some tend to be erosional and may potentially evolve into anabranches. The third study in this dissertation builds on the first two and applies the Telemac suite of solvers to model flow, sediment transport, and bed evolution across a juxtaposed meandering-anabranching reach. Steady state simulations are run under bankfull conditions on an unstructured, fine mesh to resolve flow patterns at the three confluences and bifurcations. Model results are satisfactory in simulating field conditions of flow, bed grain size distributions, and channel morphology at key locations, however, improvements can be made, particularly by better constraining bed morphology and sediment fluxes. Results indicate that despite lower flow rates in the anabranching channels relative to the adjacent single-thread meandering reach, bed-material transport continuity is maintained through the anabranching-meandering interface. Over time, all channels in the anabranching reach evolve toward stability, except the downstream portion of one large anabranch, where the rate of aggradation increases with time. This reach has the potential to become abandoned as flow is captured by a secondary channel that seems to be incising into the floodplain. The results here support the argument that anabranching reaches within otherwise meandering river systems are relatively stable and can persist for long periods of time (many decades). Factors that influence stability of the anabranching reach over decadal time scales evident in repeat aerial imagery include low bankfull stream power and high resistance of bank material. Overall, results presented in this dissertation lead to an improved understanding of the environmental conditions in which anabranching of rivers occurs, processes and forms of floodplain secondary channels along meandering reaches of rivers, and dynamics of juxtaposed anabranching and meandering rivers. This improved understanding of river dynamics contributes to basic theoretical knowledge of the relationship between river forms and processes. Indeed it also has crucial implications for the management of lowland rivers in the agricultural Midwest because it provides insights into the role of floodplain secondary channels in river-floodplain connectivity and of anabranching in overall river system stability.

Graduation Semester

2024-08

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/125823

Copyright and License Information

Copyright 2024 Tanya Shukla

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