COLLABORATIVE RESEARCH: Clarifying the ingredients and significance of nonlocal versus local sediment transport on steepland hillslopes

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Hillslopes of varying steepness cover much of Earth's terrestrial surface, and may be entirely mantled with regolith or consist of bare bedrock surfaces, or involve mixtures of these conditions. Because of the central role of hillslope behavior in landscape dynamics, and because of the importance of soils and soil transport in critical zone processes that sustain much of life on Earth, the topic of sediment transport on hillslopes has a particularly rich legacy. Viewed at a human timescale, this topic is central to studies of erosion and erosion control in the agricultural and environmental sciences. Further, this topic is central in geomorphic studies focused on descriptions of transport that can be used in models of hillslope and landscape evolution. This project is aimed at clarifying the factors that contribute to "local" transport, where downslope sediment movement involves small particle motions controlled by local soil and land-surface conditions, versus "nonlocal" transport associated with soil slips, biotic disturbances (e.g., tree-throw), particle ravel following fire, etc., which can involve relatively rapid, long-distance sediment motions. The research will combine theoretical, experimental, and field-based work in the Oregon Coast Range and the Sierra Nevada Mountains to clarify rates of sediment transport and the effects of transport on changing steepland topography. The work is a collaborative research and education effort involving students and faculty from Vanderbilt University and the University of Oregon. It will provide teaching tools deriving from visualizations of experiments and numerical simulations, and it will provide data sets to the science community.Recent work on hillslope sediment transport suggests the need to clarify the ingredients and implications of "nonlocal" versus "local" transport processes and associated formulations of the sediment flux, as expected hillslope behavior under these two conditions is quite different. However, the supporting theory has outpaced experimental and field-based observations needed to inform this theory. This project is aimed at closing that gap, as implications for understanding hillslope evolution are far-reaching. The project will: (1) combine experiments with scaling analyses to clarify ingredients of rarefied particle-surface interactions during their downslope motions, as these influence key processes of sediment disentrainment; (2) explore theoretical formulations of nonlocal transport for two-dimensional topography that is convergent-divergent in planform; (3) combine field observations with modeling work centered on hillslopes in the Oregon Coast Range and moraines in the Sierra Nevada Mountains that were selected to reveal ingredients of nonlocal transport behavior; and (4) formulate descriptions of nonlocal transport with appropriate space-time averaging to take into account the discontinuous (patchy, intermittent) nature of sediment motions on steepland hillslopes, as is necessary for treating the sediment flux as a time differentiable quantity.