Tom Ashley


Hi! I’m a geomorphologist interested in the landscape-scale outcomes of particle-scale sediment transport processes. I’m a postdoc in the Fluid and Sediment Dynamics Lab at Virginia Tech.


Recent Stuff

InterCOH 2021 Poster

My Research

All life on Earth exists near the interface between its solid surface and a thin layer of flowing fluids. Sediment transport processes mediate the evolution of this interface, giving rise to dynamic landscapes that shape society and stratigraphic architectures that record Earth’s history. The aim of my research is to promote effective stewardship of our physical environment by enhancing our understanding of fluid-driven sediment transport systems. I am interested in processes that span a wide range of scales, from the motion of individual sediment particles to the continent-scale organization of river systems driven by climate, tectonics, and lithology. My work is focused on unifying descriptions of transport and morphology using insight gained from laboratory experiments, field work, mathematical models, and statistical analysis. I am motivated by issues related to (a) responsible management and engineering, and (b) quantitative interpretation of surface processes and environmental conditions from landform morphology and sedimentary strata.

I am particularly interested in the following topics:

Sediment transport mechanics: Fluid flowing over a granular bed exerts a shear force that can mobilize sediment particles. Each particle motion causes a small decrease in bed elevation at its origin and a small increase in bed elevation at its terminus. Over geologic timescales, these small changes in bed elevation erode mountains and fill sedimentary basins. They are also responsible for the evolution of landforms like dunes, rivers, and deltas. Developing simplified models for grain-scale transport processes is an important first step towards understanding the morphological outcomes of sediment erosion and deposition at larger scales.

Tracer particle motion
Cohesive sediment boundary exchange
Bedload transport regimes that govern bedform initiation

Self-organized landforms: Feedbacks between flow, sediment transport, and topography often produce striking patterns like ripples, dunes, and braided or meandering river channels. I design targeted experiments and field campaigns to understand how these emergent, self-organized landforms shape Earth’s surface and sedimentary record.

Bedforms in rivers
Granular mechanics of bedform evolution in a laboratory flume

Large-scale organization of transport systems: In the broadest sense, climate, tectonics, and lithology provide the boundary conditions for sediment transport and geomorphology. However, Earth’s surface is dominated by self-organized landforms. As a result, feedbacks between flow, sediment transport, and topography are as important as the large-scale boundary conditions in determining topography and preserved stratigraphic architecture. My research investigates how the mechanics of sediment transport and self-organized landform evolution influence continent-scale patterns of mass transfer and geomorphic change.

A laboratory analogue for continent-scale, source-to-sink fluviodeltaic systems

Publications

Ashley, T. & Strom, K. (2022) Does the threshold of sediment motion constrain the width of an incising laboratory river? Submitted to Geophysical Research Letters. https://doi.org/10.1002/essoar.10511526.1

Cohen, S., Syvitski, J., Ashley, T., Lammers, R., Fekete, B., & Li, H. (2021) Spatial trends and drivers of bedload and suspended sediment fluxes in global rivers. Water Resources Research. doi.org/10.1029/2021WR031583

Ashley, T., Naqshband, S., & McElroy, B. (2021) Lower-stage plane bed topography is an outcome of rarefied, intermittent sediment transport JGR: Earth Surface https://doi.org/10.1029/2020JF005754 No Access?

Herz-Thysen, R., Miller, Q., Rother, G., Kaszuba, J. , Ashley, T. , & Littrell, K. (2021) Nanoscale interfacial smoothing and dissolution during unconventional reservoir stimulation: Implications for hydrocarbon mobilization and transport. ACS Applied Materials & Interfaces. https://doi.org/10.1021/acsami.0c22524

Ashley, T., Mahon, R., Naqshband, S., Leary, K., & McElroy, B. (2020) Probability distributions of particle hop distance and travel time over equilibrium mobile bedforms. JGR: Earth Surface.  http://dx.doi.org/10.1029/2020JF005647. No Access?

Ashley, T., Buscombe, D., Grams, P., Kaplinsky, M., & McElroy, B. (2020) Estimating bedload flux from suspended load and water discharge in sand bed rivers. Water Resources Research. https://doi.org/10.1029/2019WR025883. No Access?

Ashley, T. (2020) Granular Morphodynamics of Fluvial Bedforms. (PhD Dissertation) Download


tcashley@vt.edu