Re-evaluating the Interactions Between Sand Transport, Wind, and Ripples to Improve Aeolian Sediment Transport Prediction

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Description
Sediment transport by atmospheric flows shapes landscapes on Earth and other planets. Improving the ability to quantify and predict sand transport by windblown (aeolian) processes has important implications for managing erosion, land degradation, desertification, dust emissions, air quality, and other…

Sediment transport by atmospheric flows shapes landscapes on Earth and other planets. Improving the ability to quantify and predict sand transport by windblown (aeolian) processes has important implications for managing erosion, land degradation, desertification, dust emissions, air quality, and other climate change hazards and risks. Despite progress since Bagnold's seminal works in the 1930s, the most frequently used aeolian sand transport equations show discrepancies between predicted and observed transport rates upwards of 300%. Differences of this magnitude strongly support re-examining how fundamental physical aeolian processes are expressed in predictive equations. Wind tunnel experiments using a Particle Imaging Velocimetry/Particle Tracking Velocimetry (PIV/PTV) system with a high-speed camera and high-powered laser were conducted to visualize fluid motions and sand particle trajectories to provide simultaneous measurements of wind flow and sand transport to re-examine the fundamental physical relationships between flow dynamics, sediment motions, and bedform development. The first experiment of this dissertation focuses on the characteristics of near-surface sand transport in the saltation cloud. From PTV particle trajectories, mean particle velocities appear independent of freestream wind speed, while velocity distribution characteristics (such as modality) and particle concentration intermittency vary with increasing sand transport. Particle trajectories from rippled bed runs show evidence of local slope influence on near-bed particle vectors. The second experiment used manual sand grain tracking to quantify particle-bed splash interactions. Results highlight that common rebound and ejecta functions do not sufficiently represent aeolian saltation splash events. Data indicate a shadowing effect of ripples, suggesting feedback between the saltation cloud, splash events, and bedform migration. The third experiment used dual PIV/PTV analysis to quantify fluid-particle interactions and compare sand concentrations with fluid stresses and turbulence characteristics through the saltation cloud. Results show that increased saltation leads to the disappearance of the constant fluid stress region, changes in aerodynamic roughness length, and increases in turbulence intensities. Leveraging technology advancements and multiple analysis methods, these results provide new, detailed information on the relationships between flow dynamics, sediment motions, and the presence of ripple bedforms. These novel empirical data illustrate some needed corrections to the theoretical and numerical frameworks for quantifying aeolian sand transport.
Date Created
2023
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Comprehensive Sea-Level Rise Vulnerability Assessment for the Beach-Dune Barrier Systems of the Eureka Littoral Cell, Humboldt County, California

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Description
Increasing rates of sea-level rise (SLR) pose a major threat to coastal communities around the world. Evidence of these impacts is found in increased rates of extreme weather, erosion, coastal flooding, high water levels and wave height, altered geomorphology, and…

Increasing rates of sea-level rise (SLR) pose a major threat to coastal communities around the world. Evidence of these impacts is found in increased rates of extreme weather, erosion, coastal flooding, high water levels and wave height, altered geomorphology, and more. Coastal dunes act as a buffer for neighboring ecosystems and protect inland communities from increased rates of SLR. The Eureka Littoral Cell (ELC) in Humboldt County, California, which extends from Trinidad Head in the north to Cape Mendocino in the south, experiences extreme wave conditions and higher rates of SLR in comparison to the rest of the Pacific Northwest. This study focuses on assessing the vulnerability of the outer-barrier system of the ELC to SLR and complements previous vulnerability assessments of the inner Humboldt Bay. The study area was partitioned into thirteen (13) representative study reaches based on shoreline change rates and geomorphology. Twenty-two (22) environmental and socio-economic variables were identified to characterize the broader human-environmental connections and exposures that define coastal vulnerability beyond basic physical forcing and exposures. The study first compiled and examined a range of physical, biological, hazardous, socio-cultural, and infrastructure attributes of the outer barrier region of the study site for their inherent vulnerabilities. Second, individual vulnerability scores, based on geographic attributes of each variable, were determined by modifying existing methodologies (e.g., USGS), spanning variable data ranges, and/or with feedback from local representatives and a research advisory team. Aggregations of individual variables were used to provide variable category groupings (e.g., physical, biological, hazards, socio-cultural, and infrastructure). Finally, aggregated values were normalized on a one-to-ten scale to determine two sub-categories of vulnerability (environmental, socio-economic) and an overall comprehensive vulnerability for each study reach. The resulting vulnerability assessments identify which reaches are likely to experience low, moderate, and high levels of vulnerability and, based on variable and sub-grouping values, what factors contribute to this vulnerability. As such, this study addresses the significance of including both environmental and socio-economic variables to examine and characterize vulnerability to SLR and it is anticipated that the results will help inform future adaptation and resilience planning in the region.
Date Created
2023
Agent

Landscape Evolution of the Hawaiian Islands

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Description
Rivers in steep mountainous landscapes control how, where, and when signals of base-level fall are transmitted to the surrounding topography. In doing so rivers play an important role in determining landscape evolution in response to external controls of tectonics and…

Rivers in steep mountainous landscapes control how, where, and when signals of base-level fall are transmitted to the surrounding topography. In doing so rivers play an important role in determining landscape evolution in response to external controls of tectonics and climate. However, tectonics and climate often covary and understanding how they influence landscape evolution remains a significant challenge. The Hawaiian Islands, where tectonics are minimized but climate signals are amplified, provide an opportunity to better understand how signals of climate are recorded by landscapes. Focusing on the Hawaiian Islands, I examine (1) how variability in rock mass properties and thresholds in sediment mobility determine where waterfalls form or stall along the Nāpali coast of Kauaʻi, (2) I then extend these findings to other volcanoes to test if observed physical limits in flood size, climate, and volcano gradient can determine where waterfalls form, and (3) I explore how thresholds in river incision below waterfalls limit information about the influence of climate on river incision rates. Findings from this analysis show that waterfalls form or stall where the maximum unit stream power is at or below a critical unit stream power for bedrock river incision. Climate appears to have little effect in determining where these conditions are met but where waterfalls stall or form does record information about discharge-area scaling for global maximum observed floods. Below waterfalls the maximum incision depth for rivers on the island of Kauaʻi (which formed ~ 4-5 million years ago) is approximately proportional to the inverse square root of mean annual rainfall. Though maximum river incision depths for some of the younger volcanoes do not exhibit the same dependency on mean annual rainfall rates they are comparable to the maximum incision depths observed on Kauaʻi even though they are a quarter to one-tenth the age of Kauaʻi. Importantly, these patterns of incision can be explained by thresholds in sediment mobility as recorded by river longitudinal profiles and indicate that the Hawaiian Islands are dominated by threshold conditions where signals of climate are recorded in the topography through controls on incision depth but not incision rates.
Date Created
2022
Agent

Disturbance signatures and sediment characteristics in aeolian dune landscapes

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Description

Human-environment interactions in aeolian (windblown) systems has focused research on<br/>human’s role in causing and aiding recovery from natural and anthropogenic disturbance. There<br/>is room for improvement in understanding the best methods and considerations for manual<br/>coastal foredune restoration. Furthermore, the extent to…

Human-environment interactions in aeolian (windblown) systems has focused research on<br/>human’s role in causing and aiding recovery from natural and anthropogenic disturbance. There<br/>is room for improvement in understanding the best methods and considerations for manual<br/>coastal foredune restoration. Furthermore, the extent to which humans play a role in changing the<br/>shape and surface textures of quartz sand grains is poorly understood. The goal of this thesis is<br/>two-fold: 1) quantify the geomorphic effectiveness of a multi-year manually rebuilt foredune and<br/>2) compare the shapes and microtextures on disturbed and undisturbed quartz sand grains. For<br/>the rebuilt foredune, uncrewed aerial systems (UAS) were used to survey the site, collecting<br/>photos to create digital surface models (DSMs). These DSMs were compared at discrete<br/>moments in time to create a sediment budget. Water levels and cross-shore modeling is also<br/>considered to predict the decadal evolution of the site. In the two years since rebuilding, the<br/>foredune has been stable, but not geomorphically resilient. Modeling shows landward foredune<br/>retreat and beach widening. For the quartz grains, t-testing of shape characteristics showed that<br/>there may be differences in the mean circularity between grains from off-highway vehicle and<br/>non-riding areas. Quartz grains from a variety of coastal and inland dunes were imaged using a<br/>scanning electron microscopy to search for evidence of anthropogenically-induced<br/>microtextures. On grains from Oceano Dunes in California, encouraging textures like parallel<br/>striations, grain fracturing, and linear conchoidal fractures provide exploratory evidence of<br/>anthropogenic microtextures. More focused research is recommended to confirm this exploratory<br/>work.

Date Created
2021-05
Agent

Anthropocene in the geomorphology of the Sonoran Desert

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Description
Human endeavors move 7x more volume of earth than the world’s rivers accelerating the removal of Earth’s soil surface. Measuring anthropogenic acceleration of soil erosion requires knowledge of natural rates through the study of 10Be, but same-watershed comparisons between anthropogenically-accelerated…

Human endeavors move 7x more volume of earth than the world’s rivers accelerating the removal of Earth’s soil surface. Measuring anthropogenic acceleration of soil erosion requires knowledge of natural rates through the study of 10Be, but same-watershed comparisons between anthropogenically-accelerated and natural erosion rates do not exist for urbanizing watersheds. Here I show that urban sprawl from 1989 to 2013 accelerated soil erosion between 1.3x and 15x above natural rates for different urbanizing watersheds in the metropolitan Phoenix region, Sonoran Desert, USA, and that statistical modeling a century of urban sprawl indicates an acceleration of only 2.7x for the Phoenix region. Based on studies of urbanization’s erosive effects, and studies comparing other land-use changes to natural erosion rates, we expected a greater degree of urban acceleration. Given that continued urban expansion will add a new city of a million every five days until 2050, given the potential importance of urban soils for absorbing anthropogenically-released carbon, and given the role of urban-sourced pollution, quantifying urbanization’s acceleration of natural erosion in other urban settings could reveal important regional patterns. For example, a comparison of urban watersheds to nearby non-urban watersheds suggests that the Phoenix case study is on the low-end of the urban acceleration factor. This new insight into the urban acceleration of soil erosion in metropolitan Phoenix can help reduce the acute risk of flooding for many rapidly urbanizing desert cities around the globe. To reduce this risk, properly engineered Flood Control Structures must account for sediment accumulation as well as flood waters. While the Phoenix area used regional data from non-urban, non-desert watersheds to generate sediment yield rates, this research presents a new analysis of empirical data for the Phoenix metropolitan region, where two regression models provide estimates of a more realistic sediment accumulation for arid regions and also urbanization of a desert cities. The new model can be used to predict the realistic sediment accumulation for helping provide data where few data exists in parts of arid Africa, southwest Asia, and India.
Date Created
2019
Agent

The Role of Velocity, Pressure, and Bed Stress Fluctuations in Bed Load Transport Over Bed Forms: Numerical Simulation Downstream of a Backward-Facing Step

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Description

Bed load transport over ripples and dunes in rivers exhibits strong spatial and temporal variability due to the complex turbulence field caused by flow separation at bedform crests. A turbulence-resolving flow model downstream of a backward-facing step, coupled with a…

Bed load transport over ripples and dunes in rivers exhibits strong spatial and temporal variability due to the complex turbulence field caused by flow separation at bedform crests. A turbulence-resolving flow model downstream of a backward-facing step, coupled with a model integrating the equations of motion of individual sand grains, is used to investigate the physical interaction between bed load motion and turbulence downstream of separated flow. Large bed load transport events are found to correspond to low-frequency positive pressure fluctuations. Episodic penetration of fluid into the bed increases the bed stress and moves grains. Fluid penetration events are larger in magnitude near the point of reattachment than farther downstream. Models of bed load transport over ripples and dunes must incorporate the effects of these penetration events of high stress and sediment flux.

Date Created
2015-02-09
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Experimental study of the role of grain size in erosion of bedrock channels by abrasion

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Description
The morphology of mountainous areas is strongly influenced by stream bed incision rates, but most studies of landscape evolution consider erosion at basin scales or larger. The research here attempts to understand the smaller-scale mechanics of erosion on exposed bedrock…

The morphology of mountainous areas is strongly influenced by stream bed incision rates, but most studies of landscape evolution consider erosion at basin scales or larger. The research here attempts to understand the smaller-scale mechanics of erosion on exposed bedrock channels in the conceptual framework of an established saltation-abrasion model by Sklar and Dietrich [2004]. The recirculating flume used in this experiment allows independent control of bed slope, water discharge rate, sediment flux, and sediment grain size – all factors often bundled together in simple models of river incision and typically cross-correlated in natural settings. This study investigates the mechanics of erosion on exposed bedrock channels caused by abrasion of transported particles. Of particular interest are saltating particles, as well as sediment near the threshold between saltation and suspension - sediment vigorously transported but with significant interaction with the bed. The size of these erosive tools are varied over an order of magnitude in mean grain diameter, including a sand of D¬50 = 0.56 mm, and three gravel sizes of 3.39, 4.63, and 5.88 mm. Special consideration was taken to prevent any flow conditions that created a persistent alluvial cover. The erodible concrete substrate is fully exposed at all times during experiments reported here. Rates of erosion into the concrete substrate (a bedrock proxy) were measured by comparing topographic data before and after each experimental run, made possible by a precision laser mounted on a high speed computer-controlled cart. The experimental flume was able to produce flow discharge as high as 75 liters per second, sediment fluxes (of many varieties) up to 215 grams per second, and bed slopes up to 10%. I find a general positive correlation is found between erosion rate and bed slope, shear stress, grain size, and sediment flux.
Date Created
2016
Agent

The Sentinel-Arlington Volcanic Field, Arizona

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Description
ABSTRACT

The Sentinel-Arlington Volcanic Field (SAVF) is the Sentinel Plains lava field and associated volcanic edifices of late Cenozoic alkali olivine basaltic lava flows and minor tephra deposits near the Gila Bend and Painted Rock Mountains, 65 km-100km southwest of Phoenix,…

ABSTRACT

The Sentinel-Arlington Volcanic Field (SAVF) is the Sentinel Plains lava field and associated volcanic edifices of late Cenozoic alkali olivine basaltic lava flows and minor tephra deposits near the Gila Bend and Painted Rock Mountains, 65 km-100km southwest of Phoenix, Arizona. The SAVF covers ~600 km2 and consists of 21+ volcanic centers, primarily low shield volcanoes ranging from 4-6 km in diameter and 30-200 m in height. The SAVF represents plains-style volcanism, an emplacement style and effusion rate intermediate between flood volcanism and large shield-building volcanism. Because of these characteristics, SAVF is a good analogue to small-volume effusive volcanic centers on Mars, such as those seen the southern flank of Pavonis Mons and in the Tempe Terra region of Mars. The eruptive history of the volcanic field is established through detailed geologic map supplemented by geochemical, paleomagnetic, and geochronological analysis.

Paleomagnetic analyses were completed on 473 oriented core samples from 58 sites. Mean inclination and declination directions were calculated from 8-12 samples at each site. Fifty sites revealed well-grouped natural remanent magnetization vectors after applying alternating field demagnetization. Thirty-nine sites had reversed polarity, eleven had normal polarity. Fifteen unique paleosecular variation inclination and declination directions were identified, six were represented by more than one site with resultant vectors that correlated within a 95% confidence interval. Four reversed sites were radiometrically dated to the Matuyama Chron with ages ranging from 1.08 ± 0.15 Ma to 2.37 ± 0.02 Ma; and one normal polarity site was dated to the Olduvai normal excursion at 1.91 ± 0.59 Ma. Paleomagnetic correlations within a 95% confidence interval were used to extrapolate radiogenic dates. Results reveal 3-5 eruptive stages over ~1.5 Ma in the early Pleistocene and that the SAVF dammed and possibly diverted the lower Gila River multiple times. Preliminary modeling of the median clast size of the terrace deposits suggests a maximum discharge of ~11300 cms (~400,000 cfs) was necessary to transport observed sediment load, which is larger than the historically recorded discharge of the modern Gila River.
Date Created
2015
Agent

Damming ephemeral streams: understanding biogeomorphic shifts and implications to traversed streams due to the Central Arizona Project (CAP) canal, Arizona

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Description
Ephemeral streams in Arizona that are perpendicularly intersected by the Central Arizona Project (CAP) canal have been altered due to partial or complete damming of the stream channel. The dammed upstream channels have experienced decades long cycles of sediment deposition…

Ephemeral streams in Arizona that are perpendicularly intersected by the Central Arizona Project (CAP) canal have been altered due to partial or complete damming of the stream channel. The dammed upstream channels have experienced decades long cycles of sediment deposition and waterlogging during storm events causing the development of "green-up" zones. This dissertation examines the biogeomorphological effects of damming ephemeral streams caused by the CAP canal by investigating: (1) changes in the preexisting spatial cover of riparian vegetation and how these changes are affected by stream geometry; (2) green-up initiation and evolution; and (3) changes in plant species and community level changes. To the author's knowledge, this is the only study that undertakes an interdisciplinary approach to understanding the environmental responses to anthropogenically-altered ephemeral stream channels. The results presented herein show that vegetation along the upstream section increased by an average of 200,872 m2 per kilometer of the CAP canal over a 28 year period. Vegetation growth was compared to channel widths which share a quasi-linear relationship. Remote sensing analysis of Landsat TM images using an object-oriented approach shows that riparian vegetation cover gradually increased over 28 years. Field studies reveal that the increases in vegetation are attributed to the artificial rise in local base-level upstream created by the canal, which causes water to spill laterally onto the desert floor. Vegetation within the green-up zone varies considerably in comparison to pre-canal construction. Changes are most notable in vegetation community shifts and abundance. The wettest section of the green-up zone contains the greatest density of woody plant stems, the greatest vegetation volume, and a high percentage of herbaceous cover. Vegetation within wetter zones changed from a tree-shrub to a predominantly tree-herb assemblage, whereas desert shrubs located in zones with intermediate moisture have developed larger stems. Results from this study lend valuable insight to green-up processes associated with damming ephemeral streams, which can be applied to planning future canal or dam projects in drylands. Also, understanding the development of the green-up zones provide awareness to potentially avoiding flood damage to infrastructure that may be unknowingly constructed within the slow-growing green-up zone.
Date Created
2014
Agent

Desert fluvial terraces and their relationship with basin development in the Sonoran Desert, basin and range: case studies from south-central Arizona

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Description
A fundamental gap in geomorphic scholarship regards fluvial terraces in small desert drainages and those terraces associated with integrating drainages. This dissertation analyzes four field-based case studies within the Sonoran Desert, south-central Arizona, with the overriding purpose of developing a…

A fundamental gap in geomorphic scholarship regards fluvial terraces in small desert drainages and those terraces associated with integrating drainages. This dissertation analyzes four field-based case studies within the Sonoran Desert, south-central Arizona, with the overriding purpose of developing a theory to explain the formative processes and spatial distribution of fluvial terraces in the region. Strath terraces are a common form (Chapters 2, 3, 4) and are created at the expense of bounding pediments that occur on the margins of constraining mountainous drainage boundaries (Chapters 1, 2, 3). Base-level fluctuations of the major drainages cause the formation of new straths at lower elevations. Dramatic pediment adjustment and subsequent regrading follows (Chapter 3), where pediments regrade to strath floodplains. This linkage between pediments and their distal straths is termed the pediment-strath relationship. Stability of the base level of the major drainage leads to lateral migration and straths are carved at the expense of bounding pediments through an erosional asymmetry facilitated by differential rock decay between the channel bank and bed. Fill terraces occur within the Salt River drainage basin as a result of the integration processes that connect formerly endorheic basins (Chapter 4). The topographic, spatial, and sedimentologic relationship of the Stewart Mountain terrace (Chapter 4) points to a different genetic origin than the lower terraces in this basin. The high Stewart Mountain fill terrace records the initial integration of this river. The strath terraces inset below the Stewart Mountain terrace are a result of the pediment-strath relationship. These case studies also reveal that the under-addressed drainage processes of piracy and overflow have significant impacts in the evolution of drainages the lead to both strath and fill terrace formation in this region.
Date Created
2013
Agent
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