Event detection and conditional averaging in unsteady aeolian systems

Abstract

Traditional models of sediment flux ignore wind unsteadiness, and there are sound empirical and theoretical arguments that implicate this deficiency as being fundamental to explaining their limited predictive abilities. Drawing on concepts and theories from the fluid mechanics literature on turbulent boundary layers, and with the implementation of fast-response instrumentation (hot-film anemometers and continuously-weighing sand traps) this study begins to address a series of inter-related questions regarding the character of wind and transport events. Foremost among these is whether an event-based analysis of wind speed time series bears close resemblance to sediment flux events.The research demonstrates that fluid ‘ejection’ events identified by the Variable-Interval Time Averaging (VITA) method are only crudely associated with those identified using the quadrant-threshold method. Moreover, VITA events show poor correspondence with sediment flux events. The reasons for this are unclear, although it is likely that the character of events in this beach boundary layer differ substantively from the character of structural events typically associated with the sublayer bursting process. Additional unresolved issues include whether the inner/outer layer model applies to the grain-laden/grain-free zones of a saltation system, and whether the structural events evident in velocity time series from wind tunnels and natural beaches are analogous. Event-detection and conditional-averaging techniques hold promise for characterizing the fundamental nature of unsteadiness in aeolian systems.