Laboratory investigation of shape and initial orientation effects on surf zone object migration

Discarded objects like munitions in marine environments pose public safety risks. The behavior of various density objects deployed at four cross-shore positions in the surf zone of a large-scale 120 m x 5 m x 5 m wave flume were observed under different forcing conditions. Net migration was predominantly directed offshore, with approximately 70 % offshore migration observed near the outer surf zone. Density, shape, and initial orientation were identified as important to object behavior, with density acting as the dominant driver in 67 % of the object pairing scenarios. The influence of shape and initial orientation on net migration decreases as near-bed forcing increases from the outer to the inner surf zone. Near the outer surf zone, tapered objects migrated longer distances in 89 % of the pairings than the more symmetric cylinders. Near-instantaneous observations provided by internal inertial motion units further suggest that density impacted object migration distance, migration duration, and motion initiation. The migration distance and duration of a less-dense object were up to ∼15 and ∼10 times that of a denser object in the inner surf zone. Faster migrations were observed on moderate slopes than flat areas, underscoring the role of local bed slope. The observations corroborate prior findings on the influence of density while challenging some shape-dependent migration patterns. The investigation offers detailed insights into the roles of shape and initial orientation on object net migration, and the near-instantaneous response of objects to wave action in the surf zone.