Summary

Artificial sand and oil agglomerates (aSOAs) of a variety of sizes and shapes were deployed in an oscillatory flow tunnel to (1) test the application of shear-stress formulations to O (1–10-cm)-diameter round and ellipsoidal particles and (2) attain direct observations of the interaction of aSOAs and a movable sand bed, including migrating topographic features such as ripples.

Analysis of the calculated critical shear-stress values on the immobile rough bed shows that using the traditional or modified version of the Shields parameter to account for large-particle exposure in a mixed bed is sufficient to predict the flow conditions required to cause incipient motion for smaller size particles (<2.5 cm). In contrast, larger (2.5–10-cm) round aSOAs moved at exerted shear-stress values less than those predicted, while ellipsoidal aSOAs of the same sizes required higher shear-stress values for incipient motion than those predicted with Shields.

During experiments conducted on a movable sand bed, a variety of aSOA-sand bed interactions were observed, depending on the relative mobility of the aSOAs compared to sand and migrating ripple features. Smaller aSOAs tended to remain unburied due to their higher mobility, while larger aSOAs were more frequently buried and unburied. Even mobile aSOAs were observed to have the potential to be buried by migrating sand ripples if they were caught in small-scale circulation patterns in the trough of a ripple. These results suggest that additional studies are needed to correctly parameterize the size-dependent incipient motion of aSOAs, as well as their interaction with a sandy sea floor.