Experimental study of burial and mobility of unexploded ordnance in the swash zone
Date
2020
Authors
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Publisher
University of Delaware
Abstract
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. Instrumented surrogate munitions were designed and fabricated to match mass properties for a variety of common munitions (81 mm Mortar, BLU-61 Cluster Bomb, M151-70 Hydra Rocket, M107 155 mm). Surrogate sensors include various combinations of inertial motion units (IMUs), pressure transducers, shock recorders, and photocells. Errors between physical characteristics of real and surrogate munitions are below 20%. ☐ Instrumented surrogates were then utilized during laboratory large-scale flume studies undertaken at the Littoral Warfare Environment (LWE) at the Aberdeen Test Center, MD and fieldwork at the NASA Wallops Flight Facility in Wallops Island to investigate their behavior. At LWE, munitions were deployed in the swash and breaker zones of the 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 migration distances in the swash zone were greatest for the steeper slope. The munition with lowest density was most often mobilized. 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. ☐ A field study was conducted at the Wallops Flight Facility located in Wallops Island (VA). Munitions were deployed for two months and subjected to tidal (tidal range 1-1.4 m) and wave forcing conditions (Hs = 0.5 – 3.3 m; Tp = 5 – 15 s) with waves generally near shore-normal. The steepness of the cross-shore beach profile at the beginning of the experiment was 1:8 with maximum vertical morphology changes of 0.2 m. The occurrence of a tropical storm lessened the profile steepness to 1:30 (mildest observed) with vertical profile changes that reached 1.3 m (both deposited and accreted). The munitions deployment area was confined by a U-shape fence that spanned 60 m in the alongshore to prevent munition loss. Surveys of munitions location and burial were collected when munitions were located in the study area. Forty-four percent of the observations showed only burial and no migration. Migration occurred with preferred directions being seaward (71%) and south (62%) for cross-shore and alongshore motion. Maximum onshore and offshore migrations were 10.3 m and 17.9 m, respectively. The maximum alongshore migrations were instead 12.6 m (southern) and 2.1 m (northern). Munition burial was influenced by far-field processes. However, data showed munition scour also plays a role with maximum burial between surveys of 10 diameters. Time series of forces acting on the munition for a single swash event showed drag and friction being the dominant forces related to the munition acceleration force. A calibration of the drag coefficient via force estimates showed values below 0.8. The temporal evolution of the drag coefficient is similar to other observations made in the swash zone.
Description
Keywords
Unexploded ordnance, Munitions, Instrumented surrogates