Military activities over the years have left munitions on land and in underwater environments. Munitions in nearshore environments may pose a risk to humans through recreational and commercial activities. Munitions on or in the sea floor may not remain fixed and their migration and burial characteristics are poorly understood. The goals of this large-scale laboratory study were to investigate munitions burial and migration across the nearshore for a variety of munition shapes and bulk densities.

Surrogate munitions and canonical shapes (cylinders and spheres) of various densities were constructed for conducting the experiments. Several additional munitions were housed with an inertial motion unit for tracking real time attitude. Large-scale laboratory studies were conducted in the Institut National de la Recherche Scientifique wave flume, the largest academic flume in North America. Multiple cases containing multiple trials were undertaken to ascertain the variability in burial and migration in relation to forcing. The beach profile from Mantoloking, NJ, was scaled into the flume using Froude scaling principles. Wave conditions from offshore New Jersey were also scaled for forcing. In situ measurements consisting of water level and nearbed velocity were obtained roughly every 5m across the beach. Over 150 munitions and canonical objects were placed initially at these sensor locations. Following trials within cases, beach morphology and munitions burial and position data were collected.

Over 2200 migration measurements were obtained across the six wave cases tested. For simplicity, migration measurements were binned to “no movement” for object migration less than 0.05 m (assumed measurement accuracy), short distance for migration between 0.05 and 0.5 m, and medium distance for migration greater than 0.5 m. Eighty-five percent (85%) of the measurements indicated no movement or travelled < 0.5 m short distance, meaning that the default condition for objects in this study was to remain “in place”. For the approximately 15% of the objects that did migrate there was variability dependent on cross-shore location. Objects in the swash zone did not display a preference for onshore versus offshore migration. This finding is likely due to some munitions carried landward over the berm and trapped in a local depression. Migration in the surf zone was offshore-dominant likely driven by steeper slopes. Migration in the offshore zone was onshore-dominant likely driven by the near flat bathymetry and skewed waves. Instrumented munitions in the surf zone indicated large offshore migration for less dense munitions (specific gravity = 2.5) and almost no migration for more dense munitions (specific gravity = 4.18). Additionally, the less dense munition maintained an almost constant spanwise orientation relative to the forcing further indicating it was in near constant onshore-offshore motion. Conversely, the more dense munition maintained a near streamwise orientation relative to the forcing commensurate with the less than 0.5 m migration measured. Over 1600 burial observations were recorded. Unfortunately, all efforts to relate burial to forcing proved fruitless. No trends were observed for burial in relation to object bulk density, the Shields parameter, the Keulegan-Carpenter number, or wave statistics. The lack of relationships likely stems from the complex interaction of near-field and far-field burial processes.

The extensive data set produced from this work has been made available through the project's SERDP webpage and is on the SERDP munitions data repository maintained at the University of Delaware. It is being used by researchers at the U.S. Naval Research Laboratory for testing Underwater Munitions Expert System (UnMES) and by researchers at CorvusWorks for testing UXOMob. Overall, the data suggested that low-density munitions, possibly training rounds or those without internal components, could migrate long distances, assuming they were not buried. 

Munitions at or above the density of dry sand would be presumed buried unless morphodynamic conditions (dune, berm, sandbar) caused erosion or feature migration exposing the munitions to hydrodynamic forcing. Thus, the default condition in sandy environments would suggest a low probability of munitions migration. (Project Completion - 2025)