Gnamma Pit Growth: Insights from Wind Tunnel Experiments and Numerical Modeling

Abstract:

Gnamma pit is an Australian aboriginal term for weathering pit, and its formation is controlled by a mix of mineral decay and eolian processes. Prior literature suggests that two processes limit pit growth: decay along grain boundaries sufficient to allow mineral detachment; and eolian events sufficient to deflate accumulate minerals. However, prior literature contains little empirical data on the nature of these processes. Our research focuses on developing a better understanding of wind thresholds that deflate particles from pits. A set of wind-tunnel tests with a range of weathering pit shapes and grus particle sizes explored the wind threshold needed to deflate particles in different situations. An empirical equation expresses the way to estimate wind the speed threshold via pit depth and particle size. With this equation, the threshold for evacuating particles in the pit can be estimated by measuring the pit depth and smallest particles in the weathering pit, that indicates that the wind speed would not exceed this value when this pit is still active. We also developed a computational fluid dynamics model to investigate the distribution of wall shear stress. Ultimately, there exists some potential to utilize our refined understanding of gnamma pits as an indicator of paleo-wind intensity in pit locations where the accumulated sediment can be dated (e.g., by OSL) and such filled pits excavated to understand the paleo-wind conditions that would have once allowed the growth of such a pit.