Bedform Kinematics: Field Investigations
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.
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.