Early-stage aeolian dune development and dynamics on the upper-beach

Early-stage aeolian protodunes, or ephemeral dunes, are ubiquitous features on the upper-beach, acting as requisite precursors to fully developed dunes. Their formation during high energy aeolian sand transport events plays an important role in the beach morphodynamics and sediment budget. However, little is documented about their life cycle from generation to degeneration as well as their interactions with environmental factors. This study takes the opportunity of the advance in high resolution terrestrial laser scanning technique to acquire morphological changes at an appropriate frequency and spatial resolution to characterize ephemeral dune dynamics. A 1.5 days set of hourly laser scans indicates the development of a field of protodunes with a height ranging from 0.15 to 0.42 m concentrated on the upper-beach. They develop in a few hours under oblique alongshore winds above 7 m/s. The results also show that the protodunes migrate relatively fast with an average rate up to 1.1 cm/hour. Spatial and temporal variability of their morphological characteristics occur across the upper-beach. The morphological changes reflect the pattern of the development and behaviour of the protodune field which seem to be self-organized within a complex system controlled by the interactions between the bedforms themselves, environmental factors and aeolian sediment transport. Our findings highlight the importance of quantifying processes at spatio-temporal scales for monitoring protodune morphology.