Using Laboratory Techniques in the Field to Measure Munitions Mobility in the Underwater Environment
Blake Landry | U.S. Naval Research Laboratory
In this project, researchers seek to provide high-fidelity, laboratory-type measurements in the field to unify prior laboratory and field observations of the incipient motion, burial, and transport of munitions in underwater coastal environments. Through an extensive set of high-resolution in situ experiments, researchers plan to capture a complete picture of the field-scale phenomena involved in the mobility and burial of underwater munitions. The field experiments will facilitate detailed observations (e.g., hydrodynamic forcing, turbulence characteristics, and bed morphodynamics) in the natural environment around unexploded ordnance (UXO). The observations have not been practically possible to achieve in prior field and laboratory experiments and cannot be directly simulated with numerical models due to the high Reynolds number and wide-range of bottom roughness observed in situ. This unification study will provide a more complete and comprehensive view of the environmental conditions necessary to determine the relative importance of hydrodynamic phenomena such as inertial flow effects, turbulent fluctuations, and flow impulses on munitions mobility and burial in the field.
In this study, researchers will deploy the newly developed (at the U.S. Naval Research Laboratory) Imaging System for Littoral Environments (ISLE), which can perform in situ stereo particle image velocimetry (stereo-PIV) of the complex flow field as well as quantify the displacements of both the munitions and the surrounding sediment bed. ISLE will allow for field-scale quantification of the incipient motion and fate of UXO with resolution and fidelity typical of the laboratory. As part of ISLE, various laboratory techniques will be transferred to the field (e.g., optical, electronic, and acoustic sensing methods) to provide high-fidelity data on incipient motion, rotations, and burial of munitions, in addition to monitoring the hydrodynamic, sediment transport characteristics, and bed form morphodynamics. To facilitate long-term deployment (up to one month), ISLE will be cabled to, and operated from, the shore. Repeat dive operations will be conducted to perform in situ maintenance (e.g., to mitigate biofouling) and to position and reposition different types of UXO in the test area for study.
This experimental effort represents the best opportunity to bridge the gap between field and laboratory measurements for munitions mobility and burial. To complete our understanding of the phenomenology of munitions mobility and burial in the natural environment, we need to bring higher fidelity instrumentation from the laboratory into the field. This project will benefit the DoD by identifying the precise conditions in the field that determine the onset of mobility and burial of munitions during storm events. The novel technical approach will make it possible to observe field scale dynamics, not readily reproducible in the laboratory (e.g., boundary layer streaming, Stokes’ drift, and wave spectra variability) while retaining the high spatial and temporal resolution of previous observations only available in the laboratory. For example, hydrodynamic observations in the field are usually very sparse and typically include only point measurements or single vertical profiles of velocity. Additionally, previous velocity measurements in the field are rarely directly adjacent to the surface of munitions. In particular, the transfer of the well-established laboratory technique of stereo-PIV to the field will provide detailed flow measurements adjacent to and around munitions to elucidate the role of near bed turbulent fluctuations on the initiation of munitions mobility and burial critical to calibrate and validate predictive numerical models.