Observations of ripple morphodynamics for sand-silt mixtures in full-scale oscillatory flows

Wave-generated ripples strongly influence bed roughness and near-bed sediment transport. Despite the existence of many silt-dominated coastal areas, systematic datasets of ripple development and equilibrium geometry for sand-silt mixtures are limited. This study presents the first full-scale laboratory experiment of ripple morphodynamics in sand-silt beds under skewed oscillatory flows. We used artificial spherical glass beads to isolate grain-size effects. Five beds (one reference pure fine-sand bed and four sand-silt mixtures with 20% and 40% silt by dry weight, prepared using very fine sand and coarse silt fractions) were tested for three skewed oscillatory flows with free-stream velocity u rms = 0.2, 0.3 and 0.4 m/s. The ripple development and equilibrium geometry were measured in high detail using acoustic and optical laser-based profilers. The presence of silts slowed down ripple development for the 0.2 m/s condition, whereas it increased development for the 0.3 m/s cases. 3D equilibrium ripples were found for all mixtures at 0.2 and 0.3 m/s, whereas at 0.4 m/s the bed was in a transitional regime and ripples were very small. Compared to the reference pure sand bed, the sand-silt mixtures produced consistently smaller equilibrium ripples. Ripple dimensions were more sensitive to changes in flow intensity than to increasing silt content (i.e., from 20% to 40%). The effect of silt on ripple geometry is captured through a mobility number based on the effective grain size instead of the median grain size . Existing sand-bed ripple predictors show improved agreement when a -based mobility number was applied. This idealized experimental study provides new insights into ripple morphodynamics in sand-silt environments, supporting improved coastal sediment transport modelling for engineering applications.