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 near-shore environments, migration, burial and re-exposure processes all have potential to be active depending on the munition properties and forcing parameters. The goal of this work was to develop technology for rapid response surveys of seafloor morphology, bathymetry, location and state of burial of surrogate unexploded ordnance (UXO) (with imbedded acoustic sources) and conduct field measurements that span the relevant parameter space at the transition from burial to initial mobility and to migration. In order to further the understanding of UXO burial and mobility processes, realistic size and density active UXO surrogates will be deployed in the surf-zone immediately before extremely energetic wave events. This rapid response mode of deployment surveying will fill gaps in our knowledge in parts of parameter space (very energetic conditions, rapidly changing bathymetry and realistic UXO densities) that have not been adequately sampled by previous field efforts.
The analysis is guided by a parameterized dynamic force balance model for the mobility and burial of UXO in sandy sediments. The parameterizations of the roles of relative object density and hydrodynamic drag due to wave and current forcing on both stationary and mobile objects will be examined with previous measurements and a new set of measurements in the surf zone during very energetic conditions. Conducting measurements of UXO mobility and burial in regions of the surf zone with mobile bathymetry during energetic events is very challenging as the bubbles make the water column acoustically opaque and the migrating sand bars tend to bury sensors and UXO. In addition, surrogate UXO deployed in moderately energetic conditions will often bury before a subsequent very energetic event which would have caused mobility. To account for this, the project team deployed surrogate UXO in a rapid response mode immediately before a large event. The location of the UXO and surf-zone bathymetry was surveyed using a recently developed small autonomous surf capable vessel with a post-processed kinetic global positioning system (GPS)/Echosounder bathymetry sensor and ultra-short base line (USBL) receiving array for tracking surrogate UXO with active acoustic sources (“pingers”). A new tracking methodology was developed that relies on the Doppler shift of the received signal and the GPS measured vessel motion to localize the pingers. This avoids the use of expensive USBL technology. The use of commercially available pingers will allow this methodology to be easily extended to a larger community for future projects. The surrogate UXO contained a new generation of low power/long endurance internal sensors to monitor waves, currents, orientation and state of burial.
The measurements conducted in this study provided a unique data set on munitions burial and migration in more energetic nearshore conditions than previous studies where the potential for significant migration is high. In particular, this study documented that hydrodynamic sand sediment transport convergence associated with sand bar dynamics (e.g. the balance between offshore directed undertow and onshore directed wave skewness and acceleration) can result in little UXO migration even in very energetic surf zone conditions.
The parameterized dynamic force balance model was used to successfully hindcast UXO migration using hydrodynamic forcing from the simulating waves till shore nearshore wave and flow model. These deterministic models have potential to be used as input to statistical models for operation over longer time periods and larger spatial domains with greater uncertainty in forcing conditions.