ABSTRACT: Sand, Wind, and Water: Recent insights into aeolian transport across beaches.

CGRG Bibliography of Canadian Geomorphology
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Since the work of R. Bagnold beginning in the early 1900s, aeolian geomorphologists have been trapped within a theoretical-conceptual paradigm that links sediment flux explicitly to specific attributes of the wind field, usually wind speed or shear velocity raised to the third power. The assumptions under-pinning this approach--most notably, spatial uniformity, temporal steadiness, and general equilibrium--turn out to be rather restrictive when applied to field situations, leading to unsatisfactory predictive outcomes. Improvements have been reported when account is taken of such additional factors as grain size mixtures, surface roughness, topographic slope, surface moisture content, surface crusting, and the influence of vegetation. The tendency over the past decades has been toward even more reductionist investigations into the detailed mechanics of sediment transport with a focus on either the wind field (e.g., unsteadiness, coherent structures), the particles in transport (e.g., particle spin, ballistic trajectories, grain-grain impacts, cratering), or the fluid-sediment interactions (e.g., momentum exchange). Results from controlled wind tunnel experiments and from computational fluid dynamical algorithms have been particularly critical to advancing our understanding of sediment transport mechanics. Nevertheless, our ability to predict sediment flux in ordinary field situations remains surprisingly crude.Coastal aeolian geomorphologists are slowly coming to the realization that the equilibrium approaches to sediment transport prediction embraced in the past century are largely inadequate when applied to natural beaches. It is now anticipated that predictions will not align well with measurements of sediment flux at any single location, and the predominant interest lies in understanding spatial and temporal trends in transport as well as the range of factors that precondition them. Using the results from several field studies on beaches, a selected range of these complex interactions will be introduced and discussed. In particular, the 'fetch effect' will be used as a conceptual framework to assess where, on a beach, the equilibrium transport conditions predicted by the formulations of Bagnold and others are least (most) likely to occur. The development of internal boundary layers across beaches will be shown to be of relevance to understanding downwind trends in sediment flux.Aeolian geomorphology is still far from a 'scientific revolution' in the sense described by Kuhn. But it is clear that the dominant paradigm has been under siege for a long time, that it is being proven to be increasingly untenable, and that it is experiencing serious challenges from those with alternative frames of reference. As an indication of the advancing state of our science, this can only be a positive development.

Bibliography of Canadian Geomorphology