Dynamics of sediment storage and release on aeolian dune slip faces: A field study in Jericoacoara, Brazil

摘要

Sediment transport on the lee sides of aeolian dunes involves a combination of grain-fall deposition on the upper portion of the slip face until a critical angle is exceeded, transport of a portion of those sediments down the slip face by grain flows and, finally, deposition at an angle of repose. We measured the mean critical and repose angles and the rate of slip-face avalanching using terrestrial laser scanning (TLS) on two barchans of different size in Jericoacoara, Brazil. Wind speeds and sand fluxes were measured simultaneously at the dune crests. We found that the mean critical and repose angles decrease with increasing wind speed. We attribute this to turbulent shear stresses, the magnitude of which are predicted using 3D Large-Eddy Simulation (LES) modeling, that episodically act down the slip face (i.e. in the direction of gravity) to trigger grain flows at lower angles than with gravity stresses alone. The rate of avalanching is a maximum in the morning at our study site and coincides with the maximum rate of increase in wind speed and not with the maximum rate of sediment supply to the slip face. We developed and tested a new predictive model for the rate of avalanching that includes both sediment flux delivered to the slip face and the derivative of the critical angle with time. Our data also suggest that the mean critical angle varies inversely with slip-face height. These results have important implications for aeolian dune evolution, interpretations of aeolian stratigraphy, and granular mechanics.