The Critical Role of the Boundary Layer Thickness for the Initiation of Aeolian Sediment Transport

Estimation of aerodynamic entrainment in developing wind-blown sand flow

Understanding aerodynamic entrainment, a critical process in wind-blown sand dynamics, remains challenging due to the difficulty of isolating it from other mechanisms, such as impact entrainment. Aerodynamic entrainment initiates the movement of surface particles, influencing large-scale processes like sediment transport and dune formation. Previous studies focused on average aerodynamic shear stress to estimate entrainment, but the role of impulse events, which cause significant shear stress fluctuations, remains under-explored. We used 12 hot-film shear sensors to measure the spatiotemporal distribution of aerodynamic shear stress during wind-blown sand flow development. We identified impulse events exceeding the entrainment threshold and analyzed their intensity, classifying particle movement as rocking, rolling, or saltation. Results indicate that after a 2-m fetch, sediment mass flux stabilizes, with aerodynamic shear stress decreasing to 78% of the entrainment threshold. We identified key trends, including the stabilization of rocking events beyond x = 4.5 m and a significant decrease in saltation frequency, indicating fully developed wind-blown sand flow. Impulse characteristics stabilize at a greater distance (4.5 m) than sediment transport (2 m) because turbulent airflow evolves more slowly. Our findings show that impulse events significantly influence aerodynamic entrainment. These insights enhance understanding of sediment transport dynamics and improve modeling of sand dune movement.

Aeolian Ripple Migration and Associated Creep Transport Rates

Wind-formed ripples are distinctive features of many sandy aeolian environments, and their development and migration are basic responses to sand transport via saltation. Using data from the literature and from original field experiments, we presented empirical models linking dimensionless migration rates, urgd (ur is the ripple migration speed, g is the gravity acceleration, and d is the grain diameter) with dimensionless shear velocity, u*/u*t (u* is shear velocity and u*t is fluid threshold shear velocity). Data from previous studies provided 34 usable cases from four wind tunnel experiments and 93 cases from two field experiments. Original data comprising 68 cases were obtained from sites in Ceará, Brazil (26) and California, USA (42), using combinations of sonic anemometry, sand traps, photogrammetry, and laser distance sensors and particle counters. The results supported earlier findings of distinctively different relationships between urgd and u*/u*t for wind tunnel and field data. With our data, we could also estimate the contribution of creep transport associated with ripple migration to total transport rates. We calculated ripple-creep transport for 1 ≤ u*/u*t ≤ 2.5 and found that this accounted for about 3.6% (standard deviation = 2.3%) of total transport.