Complex systems in aeolian geomorphology

Aeolian geomorphology provides a rich ground for investigating Earth surface processes and landforms as complex systems. Sand transport by wind is a classic dissipative process withnon-linear dynamics, while dunefield evolution is a prototypical self-organisation phenomenon.Bothofthesebroadareasofaeoliangeomorphologyarediscussedandanalysedinthecontextofcomplexityandasystems approach.Afeedbackloopanalysisoftheaeolianboundary-layer-flow/sediment-transport/bedforminteractions,basedoncontemporary physicalmodels,revealsthatthesystemisfundamentallyunstable(oratmostmeta-stable)andlikelytoexhibitchaoticbehaviour.Recent field-experimental research on aeolian streamers and spatio-temporal transport patterns, however, indicates that sand transport by wind may be wholly controlled by a self-similar turbulence cascade in the boundary layer flow, and that key aspects of transport event timeseriescanbefullyreproducedfromacombinationof(self-organised)1/fforcing,motionthreshold,andsaltationinertia.Theevolutionof various types of bare-sand dunes and dune field patterns have been simulated successfully with self-organising cellular automata that incorporateonlysimplifiedphysically-basedinteractions(rules).Becauseoftheirundefinedphysicalscale,however,itnotclearwhether they in fact simulate ripples (bedforms) or dunes (landforms), raising fundamental cross-cutting questions regarding the difference between aeolian dunes, impact ripples, and subaqueous (current) ripples and dunes. An extended cellular automaton (CA) model, currently under development, incorporates the effects of vegetation in the aeolian environment and is capable of simulating the development of nebkhas, blow-outs, and parabolic coastal dunes. Preliminary results indicate the potential for establishing phase diagrams and attractor trajectories for vegetated aeolian dunescapes. Progress is limited, however, by a serious lack of appropriate concepts for quantifying meaningful state variables at the landscape scale. State variables currently used in the bare-sand models are far from capturing the rich 3D topography and patterns and are not sufficiently discriminative to distinguish different attractors. The vegetationcomponentintheextendedmodel,andindeedecogeomorphicsystemsingeneral,poseevengraverchallengestoestablishing

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