My research focuses on investigating the dynamics of sediment transport in fluvial systems and how grain- and bedform-scale processes influence the geomorphic evolution of fluvial systems as a whole. I use an integrated approach involving a range of analogue, numeric, and field techniques to capture the dynamics of these systems at multiple spatial and temporal scales. Bridging the gaps between sediment transport dynamics observed in flumes, modern rivers, and numerical experiments is central to my work and enables the integration of these data with sedimentary structures preserved in the rock record. Although grain- to bedform-scale sediment transport has been studied for over a century, many aspects of the fundamental dynamics remain poorly understood and poorly constrained. Better understanding of sediment transport at this scale enables a more sophisticated understanding of river dynamics, landscape evolution, riparian management, and fluvial engineering.
Spatiotemporal Bedload Transport Patterns over Bedforms
Collaborators: Dr. Mark Schmeeckle (Arizona State University)
Currently under review at JGR: Earth Surface
Despite a rich history of studies investigating transport over bedforms and dunes in rivers, the spatiotemporal patterns of transport over bedforms and their link to complex bedform geometry remains poorly understood. Previous experiments assessing the effects of flow separation on downstream fluid turbulent structures and bedload transport suggest that localized, intermittent, high-magnitude transport events, called permeable splat events, play an important role in both downstream and cross-stream bedload transport near flow reattachment. Here, we report results from a set of flume experiments that assess the combined effects of flow separation/reattachment and flow reacceleration up the stoss side of the bedform. The flume was lined with 17 two-dimensional, concrete bedforms that had a 2 cm high crest and were 30 cm long. A high-speed camera observed bedload (d50=0.05 cm) transport along the entirety of the bedform at 250 f/sec. Grain trajectories, grain velocities, and grain transport direction were acquired from bedload images using semi-automated particle tracking techniques. Downstream and vertical fluid velocity was measured 3 mm above the bed using Laser Doppler Velocitmetry (LDV) at 15 distances along bedform profile.
As observed in the experiments of Leary and Schmeeckle (2017), mean downstream fluid velocity increases nonlinearly with increasing distance along the bedform (i.e. trough to crest). Observed bedload transport, however, increases linearly with increasing distance along the bedform with an exception at the crest of the bedform, where both mean downstream fluid velocity and bedload transport decrease substantially. If laterally consistent, this linear increase in bedload transport along the stoss side of the bedform is necessary for bedforms to retain their two-dimensional shape while translating downstream, but how do bedforms attain this pattern of bedload transport and when does it fail? Bedload transport time-series and manual particle tracking data show a zone of high-magnitude cross-stream transport near flow reattachment, suggesting that permeable splat events also play an important role in the region just downstream of flow-reattachment. A simple Exner Equation applied at the sub-bedform scale combined with our observations of particle motion, suggests two potential mechanisms that drive the transition from two-dimensional to three-dimensional bedform geometries: (1) the occurrence of splat events near flow reattachment and (2) localized, nonlinear increases in bedload transport rates along the stoss side of the bedform. These two processes may be genetically linked, and we suggest that (1) could drive (2).
The Importance of Splat Events on the Spatiotemporal Pattern of Bedload Transport over Bedforms: Laboratory Experiments Downstream of a Backward-Facing Step
Published at JGR: Earth Surface: Leary_Schmeeckle_2017
Collaborators: Dr. Mark Schmeeckle (Arizona State University)
Project Summary: Despite numerous experimental and numerical studies investigating transport over bedforms in rivers, the spatiotemporal details of the pattern of transport over bedforms remain largely unknown. Here we report turbulence-resolving, simultaneous measurements of bedload motion and near-bed fluid velocity downstream of a backward facing step in a laboratory flume. Two synchronized high-speed video cameras simultaneously observed bedload motion and the motion of neutrally buoyant particles in a laser light sheet 6 mm above the bed at 250 frames/s downstream of a 3.8 cm backward-facing step. Particle imaging velocimetry algorithms were applied to the laser sheet images to obtain two-dimensional field of two-dimensional vectors while manual particle tracking techniques were applied to the video images of the bed. As expected, the experiments exhibit a strong positive correlation between sediment flux and near-bed fluid velocity. Experimentally observed sediment transport is compared to sediment transport modeled as a function of boundary shear stress using a Meyer-Peter Müller type equation. Modeled sediment transport underestimates observed sediment transport near flow reattachment. Localized, intermittent, high-magnitude transport events are more apparent near flow reattachment than farther downstream. These events are composed of downstream and cross-stream sediment transport of comparable magnitudes. Transport pattern and fluid velocity data are consistent with the existence permeable “splat events”, wherein a volume of fluid moves toward and impinges on the bed. The substantial effects of splat events on transport over bedforms cannot be modeled using simple bedload transport equations and must be included in future models of bedform evolution.
Practical applications and limitations of bedform tracking to determine bedload transport rates in sandy rivers
Collaborators: Daniel Buscombe (NAU/USGS), David Dean (USGS), Dave Topping (USGS), Paul Grams (USGS), and Matt Kaplinksi (NAU)
The ability to accurately quantify bedload transport in fluvial systems is critically important to calibrating landscape evolution models, managing riparian zones, managing dams and other fluvial infrastructure, and assessing the impacts of climate change on fluvial environments. Classic bedload transport equations that rely on statistics of the fluid to predict transport are known to be unreliable. Currently, the best methods of measuring bedload transport rely on bedform translation and bedform geometry to calculate bedload flux (Simons et al., 1965). However, most bedload transport estimates based on bedform translation assume an idealized two-dimensional geometry, an assumption that is violated in most natural systems.
My research seeks to compare and improve the accuracy of three data collection methods—multi-beam, single beam, and multiple single beam sonar systems—in calculating bedload transport. Using two sets of repeat multibeam sonar surveys with large spatiotemporal resolution and coverage, we compute bedload using three field techniques for acquiring BEPs: repeat multi-, single-, and multiple single-beam sonar. As as result of systemic variability in dune geometries through time, significant differences in flux arise between repeat multibeam and single beam sonar. Mulitbeam and multiple single beam sonar systems can potentially yield comparable results, but the latter relies on knowledge of bedform geometries and flow that collectively inform optimal beam spacing and sampling rate. Results from these analyses serve to guide design of optimal sampling, and for comparing transport estimates from different sonar configurations.
Assessing the Importance of Cross-Stream Transport in Bedload Flux Estimates from Migrating Dunes: Colorado River, Grand Canyon National Park
Collaborators: Daniel Buscombe (NAU/USGS), Paul Grams (USGS), and Matt Kaplinksi (NAU)
Bedforms are ubiquitous in sand-bedded rivers, and understanding their morphodynamics is key to quantifying bedload transport. As such, mechanistic understanding of the spatiotemporal details of sand transport through and over bedforms is paramount to quantifying total sediment flux in sand-bedded river systems. However, due to the complexity of bedform field geometries and migration in natural settings, our ability to relate migration to bedload flux, and to quantify the relative role of tractive and suspended processes in their dynamics, is incomplete.
Most bedload transport estimates based on bedform translation assume an idealized two-dimensional geometry. However, natural bedforms are typified by highly three-dimensional crescentic, barchanoid, or otherwise irregular planform geometries (Best, 2005). Additionally, bedforms evolve via a combination of translation and deformation. Broadly speaking, the three-dimensionality of bedforms is due to the spatiotemporal distribution of translation and deformation rates. While a mechanistic understanding of the cause of varying rates in translation and deformation remains elusive, bedload estimates will remain only approximate. Fluid-bed interactions are most often invoked as the mechanism responsible for varying rates in deformation. Recent flume and numerical investigations (Leary and Schmeeckle, in revision; Schmeeckle, 2015), however, indicate the potential importance of cross-stream transport, a process previously regarded as a secondary and diffusive (Jerolmack and Mohrig, 2005), to the three-dimensionality of bedforms.
This research seeks to understand and quantify the importance of cross-stream transport in bedform three-dimensionality in a field setting. This work utilizes a high-resolution (0.25 m grid) data set of bedforms migrating in the channel of the Colorado River in Grand Canyon National Park. This data set comprises multi-beam sonar surveys collected at 3 different flow discharges (~283, 566, and 1076 m3/s) along a reach of the Colorado River just upstream of the Diamond Creek USGS gage. Data were collected every ~6 minutes almost continuously for ~12 hours. An existing tool, the Bedform Tracking Tool (van der Mark et al., 2008) enables automated quantification of bedform geometry variably along bedform elevation profiles. Using this tool we extract detailed bedform geometrical data (i.e. bedform height, wavelength) and spatial sediment flux data over a suite of single bedforms at each flow. Coupling this spatially extensive data with a generalized Exner equation, we conduct mass balance calculations that evaluate the possibility, and potential importance, of cross-stream transport in the spatial variability of translation and deformation rates.
Preliminary results suggest that intra-dune cross-stream transport can partially account for changes in the planform shape of dunes and may play an important role in spatially variable translation and deformation rates. Parameterization of cross-stream sediment transport could lead to accounting for ambiguities in bedload flux calculations caused by dune deformation, which in turn could significantly improve overall calculation of bedload and total load sediment transport in sand bedded rivers.