Critical Reflections on the Coherent Flow Structures Paradigm in Aeolian Geomorphology

Equilibrium concepts are widespread in aeolian geomorphology, especially those incorporating time-averaged parameterizations of fluid flow and sediment transport. The main objective of this chapter is to reflect critically on whether a new paradigmbased on coherent flow structures (CFS) is emerging as a viable alternative to the traditional steady-state perspective on aeolian processes. A survey of recent research addressing the existence and nature of CFS in aeolian systems indicates that: (i) there is as yet no convincing evidence in support of the classic bursting process or of any discrete flow structure with a characteristic topology (e.g. hairpin vortex) for the case of near-surface boundary layer flow over flat sandy surfaces (with or without saltation); (ii) there are instances in which near-surface vortices of varying geometry are found in association with small obstacles (e.g. logs, pebbles, vegetation clumps) on sandy surfaces, but it is not always apparent what the pattern of erosion or deposition in the vicinity of the obstacle will be, given that there are additional controls that involve surface hardness and sediment delivery from upwind; (iii) flow over aeolian dunes is generally well understood and very similar to flow over fluvial dunes with characteristic zonation, including flow acceleration and streamline compression on the stoss slope and a recirculation eddy and wake region in the lee; and (iv) the importance of these dune-related flow zones to sediment transport response is more reliably understood through examination of a range of turbulence parameters (e.g. Reynolds stress, turbulent kinetic energy) in addition to the time-averaged flow quantities alone. In order to advance our understanding of CFS in aeolian geomorphology, it will be essential to develop better technologies for high-frequency monitoring of wind and sediment flux, to focus attention on sediment transport events with specific topologies (e.g. streamers) rather than attempt to capture flow events and hope that there is an accompanying transport signal, to move beyond simple quadrant analysis and adopt analytical methods that elucidate the time-history of event signatures, and to reformulate existing time-averaged parameterizations of sediment flux to incorporate event-based information.