Observation of munitions migration and burial in the swash and breaker zones

Abstract

Behavior of underwater unexploded ordnance located on the sea bed is poorly understood. Different types of munitions were tested in the swash and breaker zones of a large-scale flume containing medium sand on two initial foreshore slopes (1:16 and 1:10) with regular and irregular wave forcing (Hs = 0.3–1.1 m; Tp = 2.6–9 s). Migration and burial were measured via pre- and post-run surveys and internal munition sensors. Water depths, flow velocities, wave runup and beach morphology changes were also measured. Munition sensors consist of an inertial motion unit for roll and heading and optical photocells for burial. Munition migration distances in the swash zone were greatest for the steeper slope and lower density munitions. Munition migration mimicked wave runup with net offshore transport. Breaker zone munitions experienced episodes of onshore- and offshore-directed motion without preferred direction. Denser munitions were observed to bury deeper, up to 2 munition diameters. Moderate correlation was observed between burial (r2 = 0.48) or migration (r2 = 0.37) with dimensionless parameters (such as Shields parameter, Keulegan-Carpenter number, object mobility number, munition density and Iribarren number), attributed to the high variability in munition response even under similar offshore forcing conditions.