Formation and organization of aeolian bedforms

Abstract

Aeolian bedforms represent some of the most intricate land forms on the planet encompassing a variety of scales from wind ripples on the order of centimeters to desert sand seas comprising thousands of square kilometers. Hypotheses focusing on the formation and organization of these bedforms have been nearly as complex and varied as the dunes themselves. Three distinct topics were chosen for the focus of this study: (1) the formation and organization of wind ripples; (2) morphology and sampling effects from measurement of dune lee side airflow; and (3) the formation of star dunes based on wind tunnel studies of small scale star ripples. Each of these topics comprises a chapter of this thesis, which will be published individually at a later date. Wind ripple formation has been a topic of discussion for nearly a century. For this study ripples were formed in a wind tunnel and recorded using time-lapse photography. Data reveal that ripples form in a pattern that closely matches the formation of computer simulated bedforms using only basic rules of sand transport, but neglecting any sort of aerodynamic control of bedform morphology. These findings support the hypothesis that aeolian bedforms are self-organizing and that no predisposed template of formation exists within the fluid flow. Aeolian dune lee side airflow comprises a complex subject of study. The airflow is the result of a complex interaction between the wind and the sand surface. Using a new technique of wind velocity data collection, a more accurate picture of the lee side airflow morphology is now available. This data set provides the most comprehensive look at this environment to date. Of particular interest are the data outlining the Internal Boundary Layer, which is closest to the sediment surface and controls sediment transport. The data collected imply that no point of dune nucleation, caused by a drop in shear velocity, exists in the interdune corridor, rather, the shear increases downwind until another dune is reached. Conclusions based on the data support the theory of self organization when applied to desert dunes. Star dunes are the largest and most complex single bedforms found on Earth. Due to their size and long development time, observations of star dune formation are scant. Using a rotating table mounted in a wind tunnel, a series of complex wind patterns were simulated. Small scale star ripples were formed and the process recorded using time-lapse photography. Based on these findings at least two distinct types of star dunes are possible in nature. Because of the relationship between the wind regime and the bedforms produced, a causal link between aerodynamic control and bedform formation is dubious. More likely, however, is the self organization theory which has proven useful in all scales of bedforms examined in this study