Continuous Monitoring of Mobility, Burial and Re-Exposure of Underwater Munitions in Energetic Near-Shore Environments
Dr. Peter Traykovski | Woods Hole Oceanographic Institution
The development of predictive models and measurement techniques for the mobility, burial, and re-exposure of munitions is essential to planning remediation efforts. In sandy, energetic, nearshore environments, the migration, burial, and re-exposure processes all have the potential to be active depending on munition properties and forcing parameters. The goal of this work, MR-2319, was to develop technology for surveying small-scale seafloor morphology and bathymetry, determining the location and state of burial of active Unexploded Ordnance (UXO) surrogates with imbedded acoustic transponders, and conducting field measurements that span the relevant parameter space at the transition from burial to mobility. To further the understanding of UXO burial and mobility processes, a range of different size and density active UXO surrogates were deployed in energetic surf zone and tidal shoal environments. These deployments and surveys filled gaps in the knowledge in parts of parameter space conducive to high munition mobility (energetic conditions, rapidly changing bathymetry, and a range of UXO densities) that have not been adequately sampled by previous field efforts.
The analysis and field measurement program was guided by a parameterized force balance model for the mobility and burial of UXO in sandy sediments. Based on this force balance, surrogate UXO with relative densities significantly above the density of water, but both above and below the bulk density of water-saturated sand, were chosen to ensure that some objects would be mobile and others would bury. The measurements included in situ continuous monitoring of processes via seafloor frame-mounted rotary sidescan sonars and water velocity sensors, an Ultra-Short Base Line (USBL) acoustic tracking system, Global Positioning System (GPS) buoy-based tracking, and manual passive buoy-based tracking. Measurements were conducted both on Wasque Shoals, near the Muskeget tidal channel between Martha’s Vineyard and Nantucket, MA, and at the Long Point, Martha’s Vineyard surf zone site with weaker tidal currents. Despite both sites having similar wave forcing conditions, the large dunes at the tidally forced site severely constrained UXO migration. Surrogate UXO with relative densities less than water-saturated sand migrated a maximum distance of 14 m, as the objects migrated into the troughs of the large dunes (100 m wavelength, 3 m height). At the surf zone site, UXO surrogates with relative densities less than water-saturated sand migrated 100 to 150 m onshore, came to rest, and subsequently buried 40 m from the beach. Objects with relative densities that were significantly higher than water-saturated sand tended to bury at their deployment location. No objects migrated into the nearshore swash zone or onto the beach as waves breaking offshore decreased the nearbed wave orbital velocity within 50 m of the beach. Predictions of mobility, based on parameterized force balances with a constant initial burial of 10% or 30% of the object diameter, could not predict the measured transition from burial to mobility as a function of wave orbital velocity and UXO relative density. Calculations based on 10% initial burial incorrectly predicted that all objects in the measured data set would be mobile, and calculations based on 30% initial burial incorrectly predicted that all objects in the measured data set would be stationary and buried. Time-dependent calculations that account for the time required for burial of an object were able to successfully predict the initiation of mobility consistent with the observations. In these calculations, the timescale for burial is set by the ratio of the burial scour pit cross-sectional area relative to the sediment transport rate. If the waves increase quickly enough that an object does not have time to bury before the threshold for mobility is reached, it will migrate. If the waves increase slowly, these calculations predict that even low-density objects will bury.
The measurements and analysis of UXO surrogate mobility in this study span a range of conditions and object parameters in which high migration rates are possible. UXO surrogates with density above that of seawater but both below and above that of water-saturated sand, combined with energetic wave and current forcing, resulted in low migration distances in environments with bathymetric constraints and large migration distances in environments that were not bathymetrically constrained. In addition to wave forcing, the density of the objects relative to the density of water-saturated sand appeared to play an important role in determining the threshold for mobility. Objects near to or less dense than water-saturated sand tended to migrate, and denser objects tended to bury.
Measurements of UXO mobility were conducted at a tidal shoals site that was also exposed to open ocean waves and a surf zone site that had weaker tidal currents. The bathymetry at the tidal shoals site was dominated by large migrating sand dunes with wavelengths of hundreds of meters and heights of 2 to 4 m. Despite energetic wave conditions, similar to those that caused large migration distances at the surf zone site, migration distances at the tidal shoals site were limited to a maximum of 14 m as the objects moved into the troughs of the large dunes. At the surf zone site, objects migrated from the outer surf zone 150 m across the surf zone to within 40 m of the beach, but no objects reached the beach. This is most likely due to the reduction of nearbed wave orbital velocities in the nearshore due to wave breaking further offshore.
Predictions of mobility based on parameterized force balances with a constant initial burial of 10% or 30% of the object diameter could not predict the measured transition from burial to mobility as a function of wave orbital velocity and UXO relative density. Calculations based on 10% initial burial incorrectly predicted that all objects in the measured data set would be mobile, and calculations based on 30% initial burial incorrectly predicted that all objects in the measured data set would be stationary. Time-dependent calculations, which account for the time required to bury an object, could successfully predict mobility transitions consistent with the observations. Although the time-dependent calculations are also able to predict the burial of objects, that is a separate study ( MR-2320) conducted in more energetic conditions and higher relative densities, which would have been predicted to be mobile by calculations based on 10% to 30% initial burial. In these calculations, the timescale for burial is set by the ratio of the scour pit for the cross-sectional burial area relative to the sediment transport rate. If the waves increase quickly enough that an object does not have time to bury before the threshold for mobility is reached, it will migrate. If the waves increase slowly, these calculations predict that even low-density objects will bury. This time dependence of the relative roles of processes causing burial or mobility has important implications for both deterministic and statistical models for UXO mobility and burial.
In terms of object tracking methods, in energetic conditions, the GPS buoys were limited to use with only large objects due to the drag from the 10-cm diameter buoy required to float the GPS and batteries. The USBL tracking array and transponder system could track objects at the initial stages of mobility and could determine object locations before and after migration. Tracking was limited during periods of high mobility due to acoustic attenuation from bubbles. With the current design of the USBL transponder, this system was also limited to object diameters of 10 cm or larger. Future work should investigate the use of miniature low-power commercially available pingers for object localization systems. An additional important future technology development would be measurements of hydrodynamic forcing that moves with the mobile objects as conditions measured by fixed sensors are often quite different from conditions at the mobile objects in surf zone environments with high spatial variability.
The measurements conducted in this study provide a unique data set on munitions burial, re-exposure, and migration in energetic nearshore conditions where the potential for significant migration is high. We now have knowledge on the parameters required for moderate distance (100 m) migration that was not previously available. This study provides data on the behavior of a range of different density objects in energetic conditions with mobile nearshore bathymetry to better span parameter space for the development of deterministic predictive models. These deterministic models can be used as input to statistical models for operation over longer time periods and larger spatial domains with greater uncertainty in forcing conditions. An important conclusion from this study is that the time history of the forcing can determine the relative roles of processes causing burial or mobility, which needs to be accounted for in deterministic and statistical models. The technology development of USBL tracking provides a means for tracking objects in energetic conditions that can be used in future studies of munition mobility and burial.