The presence of unexploded ordnance (UXO), or munitions, in nearshore and other underwater environments is a worldwide concern due to the past military activities. UXOs have been found in the surf zone and on populated beaches, posing a risk for the public. Behavior of munitions located on the sea bed is poorly understood. The goal of this project was to design and fabricate instrumented surrogate munitions for use in large-scale laboratory and field experiments to investigate UXO burial and migration under a range of forcing conditions. The project team sought to understand the processes that lead to UXO migration and what processes might lead to a munition being located on the beach face.
Instrumented surrogate munitions were designed and fabricated to match physical properties for a variety of common munitions. Munitions instrumentation, when internal volume capacity allowed, consisted of internal motion units for estimating migration and orientation, pressure transducers for estimating water depth and wave arrival, and passive photocells for estimating UXO burial. Extensive large-scale laboratory and field experiments were conducted at the Littoral Wafare Environment (LWE) outdoor wave flume (LWE1; 1:16 foreshore slope, LWE2 1:10 slope), Aberdeen, MD and at the NASA Wallops Flight Facility Wallops Island, VA respectively. In situ measurements of fluid velocity, water depth and sediment concentration were made at multiple cross-shore locations. Imaging systems were used for redundant measurement of fluid processes and to track munitions when visible. Munitions positions and beach morphology were collected using a precision Global Positioning System. Dimensionless forcing parameters related to munitions processes were estimated from the acquired data and used to investigate relationships with the observed munitions migration and burial.
Migration distances for proud munitions initiated in the swash zone were greatest for the steeper foreshore slope study. Under similar forcing conditions, lower density munitions migrated farther offshore than higher 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 two munition diameters. Moderate correlation was observed between burial (r2 = 0.45) or migration (r2 = 0.37) with dimensionless parameters (such as Shields parameter, Keulegan-Carpenter number, and object mobility number), attributed to the high variability in munition response even under similar offshore forcing conditions. Forty-four percent of the field observations showed only burial and no migration. Migration in the field occurred with preferred directions being seaward (71%). Maximum onshore and offshore migrations were 10.3 and 17.9 m, respectively. Munition burial was also influenced by far-field processes with maximum burial between surveys of up to 10 munition diameters.
Data on munitions migration and burial and associated environmental conditions are perhaps the first extensive measurements in prototype scale swash zones. This study provides information on burial and migration characteristics under a wide range of forcing and munitions types (caliber and density) and attempts to relate those processes to commonly used dimensionless numbers. These data and relationships are needed to validate probabilistic models of munitions migration and burial near the shoreline. This study additionally provided robust design characteristics for a variety of munition surrogates, the use of a variety of commercial internal sensor for munitions attitude, and a new sensor array for estimating munition burial.