Experimental observations of vertical segregation of sands in oscillatory flows

ABSTRACT Measurements assessing the composition of bed sands under oscillatory flows were conducted in the large wave flume (Großer Wellenkanal, GWK) in Hannover, focusing on diverse sand mixtures. The experiments featured velocity‐asymmetric flows with a consistent wave period of 7 s, while considering two distinct wave heights (1 and 1.5 m). The sand mixtures were derived from two well‐sorted grain‐size fractions: a fine (F) sand with a median diameter of 0.21 mm and a coarse (C) sand with a median diameter of 0.58 mm. This study scrutinises the bed composition of three bimodal graded sands, initially composed of 75%, 50% and 25% fine fractions. Sand characteristic analyses were performed using a Laser Diffraction Particle Size Analyser, enabling precise counting and sizing of particles in heterometric sands. Over time, the initially well‐mixed bed composition undergoes alterations, revealing a vertical segregation of mixed sands at the bed layer. Distinct behaviours emerge in the different fractional experiments. For the sand mixture with a higher percentage of coarse sand fraction, a gradual increase in the maximum percentages of coarse sands is observed, particularly in the upper layers (upward coarsening). Conversely, with a higher percentage of fine sands, a discernible trend is noted where finer sands tend to occupy the superficial layers, while coarser sands predominate in the deeper layers (upward fining).