Characterizing Mass-Flux Profiles in Aeolian Saltation Systems

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Windblown particles reshape many of the Earth's landscapes; these are known as aeolian processes and certain fundamental aspects are still unknown. In fact, predicting aeolian transport rates is a keystone in the geomorphological quest to develop deterministic models for the modification of erodible surfaces by wind (erosion, accretion, and development of bed forms) and for the creation and evolution of coastal and arid dune systems. Understanding of the dynamics of wind-blown sand systems has improved dramatically over the last three decades. There remains, however, considerable dissatisfaction with the prediction of sand transport rates using basic environmental information, such as wind speed and mean sand grain size, and in relation to particular aeolian systems. In particular, there does not exist agreement as to the characteristic distribution of saltating grains above a sand surface for even the most basic transport systems. This is important because the distribution of moving grains is a fundamental control on the total rate of sand transport. This project is designed to elucidate the singular characteristics of the vertical distribution of wind-driven, saltating sand grains in a natural environment. In particular, the investigators will assess the applicability of the Rouse concentration profile model to aeolian saltation by collecting and analyzing a high-quality field data set. The Rouse concentration profile normalizes vertical particle distributions according to discernable characteristics of grain size and wind shear velocity, and its utility has been demonstrated for hydrodynamic systems. The investigators hypothesize that the vertical distribution of aeolian saltation corresponds with the Rouse profile model; that the Rouse parameter corrects some of the variability between mass-flux profiles obtained under different environmental conditions; and that the characteristics of mass-flux profiles measured in the field will be statistically distinct from those measured in wind tunnel experiments. They approach this issue through the design of an innovative, spatially and temporally detailed experiment that will measure the vertical structures of the near-surface wind and saltation fields. They have designed a new saltation impact-sensor to be used with ruggedized thermal anemometers, specially designed for deployment within the saltation layer. Characterizing the mass-flux profile will have theoretical and applied implications. A physically-based characterization of the profile should improve the predictive capacities of aeolian transport models. Further, well-known scaling constraints associated with the derivation of transport relationships from wind tunnel studies mandate that prototype-scaled models should be based on prototype data. This study will provide one such basis. As a result, the results will have the potential to resolve a longstanding debate about the flux-profile distribution, to improve transport models, and, ultimately to enhance the ability to plan for and against the environmental impacts of blowing sand, including dune building or aeolian abrasion. This benefits the communities of geomorphologists, geographers, geologists, engineers, physicists and mathematicians, and planners concerned with the various aspects of blowing sand. The project will also develop international research collaborations.