Effect of Orientation and Size of Helley‐Smith Sampler on Its Efficiency

Two series of laboratory experiments were conducted to evaluate the influence of mean flow direction and sampler size (7,6 cm aperture vs 3 cm aperture) upon the efficiency of Helley‐Smith samplers in subcritical flows. The formation of flow circulation zones both inside and outside the sampler was found to occur at divergence angles between the sampler and mean flow of as little as 10°. The creation of these recirculation regions, and especially a zone of separated flow inside the sampler nozzle, is mirrored by a decrease in the sediment sampling efficiency. Additionally, Investigation of the performance of small Helley‐Smith samplers reveals that these trap less sediment per unit width than standard size samplers (7,6 cm). This lower sampling efficiency is linked to the lower protrusion of the sampler into the boundary layer.