The morphology of alluvial sand dunes

Cisneros, Julia

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https://hdl.handle.net/2142/110809 Copy

Description

Title

The morphology of alluvial sand dunes

Author(s)

Cisneros, Julia

Issue Date

2021-04-15

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

Best, James L

Doctoral Committee Chair(s)

Best, James L

Committee Member(s)

• Parker, Gary
• Anders, Alison M
• Garcia, Marcelo H
• van Dijk, Thaiënne

Department of Study

Geology

Discipline

Geology

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• dunes
• rivers
• morphology
• big rivers
• morphodynamics
• alluvial channels

Abstract

Sand dunes, formed in water or air, are generated by the action of fluid moving over a mobile sediment surface, and possess a morphology that exerts feedbacks to influence flow and sediment transport. The depositional products of such bedforms may provide clues to the movement of water and wind across ancient, otherwise unknown, landscapes. Key to leveraging these records of ancient environments is an understanding of the links between flow, sediment transport, and bedform morphodynamics. Such an understanding can aid both how we can use modern bedforms to interpret the conditions of modern and ancient Earth and planetary surfaces, as well as how contemporary environments are managed. Despite decades of research concerning alluvial bedforms, we still lack a complete understanding of how their morphological complexities link to the controlling mechanisms of dune formation and kinematics. Recent work suggests several key mechanisms and processes control the formation of low-angle and complex alluvial dune shapes: dune superimposition, sediment suspension, bedform three-dimensionality, and liquefied avalanche flows generated on the dune leeside in deep flows. This thesis investigates the morphology and behavior of sand dunes in a wide array of alluvial environments and laboratory flows to provide insights into their evolution, shape, movement, and likely depositional products. This thesis details development of a new automated bedform analysis method to interrogate large bathymetric datasets, the Bedform Analysis Method for Bathymetric Information (BAMBI). BAMBI represents a step forward in identifying dune leeside shape and complexity by utilizing slope and aspect maps to identify lee and stoss slopes, and the bounding crest and trough points. Using an objective bedform scale threshold, BAMBI successfully separates large dunes from smaller, often superimposed, dunes and this functionality results in statistically significant comparisons with existing methods. The data analyses permitted by BAMBI reveal that in big rivers, dunes have complex shapes with low-angle leesides, and heights (H) that are much smaller, with respect to flow depth (Y; H/Y often equals ~0.1), than previous research has indicated (H/Y ~ 0.3). This has important implications for how we recognize the products of alluvial dunes in the ancient sedimentary record. Specifically, the presence of low-angle cross-stratification may be difficult to identify, or be incorrectly interpreted as flat-bed laminae, resulting in incorrect paleohydraulic reconstructions. Distributions of leeside angles for dunes within all the big rivers examined herein reveal that dunes have mean leeside angles of 10° and their maximum leeside slopes are often ~15°, which is less than the angle-of-repose leeside of 20-30° assumed in the majority of past research. These lower angle dunes will not exhibit permanent flow separation, thus yielding a lower flow roughness as compared to dunes assumed to possess permanent flow separation in the dune leeside. Analysis of data from laboratory experiments and big rivers also demonstrates that low-angle leesides are generated in much shallower flows, and high-angle dunes are formed in deep flows, demonstrating that the formative mechanisms for low-angle dunes are not related to flow depth. This finding unequivocally shows that liquefied leeside avalanches are not a controlling mechanism for the formation of low-angle dunes. Instead, the balance between the influence of bedform superimposition and sediment suspension is highlighted as two key mechanisms that influence the formation of low-angle dunes. In laboratory experiments, dunes formed in suspension-dominated conditions, (when the suspension number, or ratio of bed shear velocity and particle fall velocity, reaches unity) possess lower leeside angles compared to those formed in bedload-dominated conditions. Additionally, where bedload transport dominates and sediment suspension is likely of lesser importance, preliminary analysis shows that bedform superimposition can result in the lowering of dune leeside angle. Because dunes generated under bedload-dominated conditions likely have a higher potential for larger scale, and permanent, leeside flow separation, this highlights the need to incorporate the differences in flow separation in flow resistance predictions from dunes. This may be accomplished by taking the transport stage into account and applying a reduction factor for lower angle dunes. The similarity between low-angle dune leesides formed in big rivers and those found in suspension-dominated laboratory flows highlights that current flow resistance predictions in big rivers also require at least 10% reduction. Changes in dune shape and leeside angle are also examined through different types of floods in the River Waal, Netherlands, to constrain possible dune hysteresis, where changes in bedform dimensions may lag behind changes in flow discharge. The responses of dune height, wavelength, and leeside angle are compared across a simple flood and a flood series, and reveal dune hysteresis in dune height and leeside angle occurs in the northern side of the River Waal channel. Throughout both flood types, the mean leeside angle is low and less than 15°. In the simple flood, quantification of superimposed dune abundance suggests superimposition to be negligible. Instead, dunes grow in height and leeside angle during the rising stage. On the falling stage, these dunes continue to grow during flow deceleration, which produces a decrease in dune migration and an increase in deposition in the trough that lowers the dune leeside and eventually the dune height. The present study of dune morphology within a range of alluvial channels provides new insights into the dynamics of these ubiquitous bedforms, and highlights how their geometry needs to be incorporated into existing paradigms of dune morphology within contemporary channels, and interpretations of dune cross-stratification within the sedimentary record.

Graduation Semester

2021-05

Type of Resource

Thesis

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Copyright 2021 Julia Cisneros

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