Quantifying the Form‐Flow‐Saltation Dynamics of Aeolian Sand Ripples

Ripples are the most fundamental and ubiquitous aeolian bedforms formed on sandy surfaces, but their small size and fast response times make them inherently difficult to measure. However, these attributes also make ripples excellent flow indicators, and they have been used extensively in planetary locations for this purpose. Here, we use terrestrial laser scanning to measure ripple morphometry and celerity coincidently, as well as saltation height above rippled surfaces. We find that although ripple height and wavelength respond linearly to increased shear velocity, under strong winds ripple celerity exhibits a non‐linear increase. This relationship at high wind speeds is also reflected in the response of aerodynamic roughness and saltation dynamics, with a greater maximum saltation height present over ripple lee slopes. Importantly, when using ripple patterns as indicators of flow conditions, celerity or height should be used in preference to wavelength as their dynamics respond faster to changing wind speed. In planetary and stratigraphic settings where measuring celerity is not possible, wavelength should be considered as indicative of consistent wind conditions rather than the full range of sand transporting wind speeds.

💡 Research Summary

The paper presents a comprehensive field investigation of aeolian sand ripples using terrestrial laser scanning (TLS) combined with high‑resolution wind and particle measurements. Four experiments were conducted on a flat, sand‑covered surface within the Medano Creek area of the Great Sand Dunes National Park (Colorado, USA) during 2022‑2023. TLS (Leica P20/P50) captured 280 scans of a 1 m² ripple field at a spatial resolution of 2 mm, allowing the researchers to extract detailed cross‑sectional profiles parallel to the wind direction. Ripple height (hᵣ) and wavelength (lᵣ) were derived from zero‑upcrossing methods, while ripple celerity (cᵣ) was obtained via cross‑covariance of successive scans, ensuring that migration distances remained smaller than a wavelength.

Wind shear velocity (u*) was measured with a 3‑D sonic anemometer positioned 0.24 m above the surface, and saltation dynamics were recorded using a Sensit‑piezoelectric counter (0.014 m above the surface) together with two optical gate sensors at 0.02 m and 0.05 m. Saltation height (zₛ) was calculated directly from TLS‑derived surface elevations, providing a more accurate estimate than traditional exponential‑fit approaches.

Key findings include: (1) Ripple height and wavelength increase linearly with shear velocity, at rates of roughly 0.03 mm (m s⁻¹)⁻¹ and 0.12 mm (m s⁻¹)⁻¹ respectively. (2) At higher wind speeds (u* > 0.45 m s⁻¹), ripple celerity exhibits a pronounced non‑linear (quadratic) increase, deviating from the linear trends reported in many laboratory studies. (3) Maximum saltation height is significantly larger on the lee side of ripples—on average 1.8 times the height measured on the stoss side—indicating that particle collisions and re‑ejection are concentrated on the lee slope. (4) Aerodynamic roughness (z₀) is strongly influenced by the combined effect of ripple height and the saltation cloud; under strong winds, the saltation layer dominates z₀, producing values that exceed predictions from both the Bagnold roughness law and Charnock‑type formulations.

The authors argue that, because ripple celerity and height respond more rapidly to changes in wind speed than wavelength, these two metrics should be preferred when using ripples as flow indicators. Wavelength, being relatively stable, is better suited as a proxy for long‑term, consistent wind conditions—particularly valuable for planetary applications (e.g., Mars) where direct wind measurements are unavailable. The study also highlights the utility of TLS combined with synchronized particle sensors for capturing multi‑scale form‑flow‑transport interactions in the field, a capability previously limited to laboratory or low‑resolution field methods.

Future work is proposed to extend the methodology across a broader range of grain sizes, sediment supply conditions, and dune morphologies (including coastal and desert dunes), and to integrate the observations with numerical models of grain‑impact dynamics. By doing so, the authors aim to refine predictive frameworks for sediment flux based on ripple metrics and to improve the interpretation of aeolian bedforms in both terrestrial and extraterrestrial environments.