Indeterminacy in aeolian sediment transport across beaches

Conventional models of aeolian transport relate sediment flux to shear velocity, among several other relevant factors such as grain size, grain density, and air density. Application of these predictive models to natural systems has yielded mixed results. Analysis of data from field experiments designed to provide simultaneous measurements of wind speed and sediment flux across beaches underscores the limitations of these models, especially when surface conditions are variable over space and time. Accurate predictions of aeolian sediment flux may never be universally realized because conventional parameterizations of the problem are indeterminate. Indeterminacy arises because the number of unknowns exceeds, by more than one, the number of available equations, and this analytical constraint is inextricably bound to empirical aspects of the problem. It is not immediately apparent what the relevant fundamental variables are, how they might be linked in a system of equations, nor how the inherent uncertainty in their specification, measurement, and spatio-temporal character might be surmounted. In particular, it is becoming more evident that the indiscriminant use of shear velocity, as a surrogate for a spatially and temporally variable shear-stress field, in sediment-transport equations is problematic and an oversimplification of a complex process-response system. Accepting indeterminacy implies accepting uncertainty, and this suggests developing complementary research strategies that explore and quantify natural-system variability.