Wave Resolved and Averaged Numerical Modelling of UXO Migration
Peter Traykovski | Woods Hole Oceanographic Institution
One of the critical areas for underwater munition detection and remediation is very shallow water less than 5 meters (m) deep. The very shallow water emphasis comes from the fact that munitions are most likely to be encountered by the general public in depths that are suitable for wading, swimming and scuba diving, with potential encounters more likely in the shallowest water. In many environments there will be occasional to frequent breaking waves in depths from 0.5 m to 3 m. In this region Unexploded Ordnances (UXO) may either bury, remain proud on the seafloor, or become re-exposed. In environments with energetic forcing or steep slopes the UXO may migrate. Numerical modelling of environmental conditions and UXO response is a powerful tool to assess the relative likelihood of these processes. The overall objective of this project is to improve models for hydrodynamic forcing of UXO migration and burial in shallow water surf-zone environments. This work will address the following questions:
- Does the balance between onshore forcing by wave asymmetry and skewness and offshore forcing by return flows control mobile UXO location in the surf-zone?
- Are the wave asymmetry, skewness and return flow processes in the surf zone represented by phase averaged models adequately to reproduce observed UXO migration events? Or do the parameterizations of these processes for UXO mobility modeling need to be improved?
- Can probabilistic modeling of UXO mobility using the balance between onshore forcing by wave asymmetry and skewness and offshore forcing by return flows be improved?
This project will use both wave-resolving models that explicitly compute non-linear wave transformations and wave-averaged models that parametrize nonlinear transformations in terms of wave asymmetry and skewness. The approach will investigate the balance between onshore forcing by wave asymmetry and skewness and offshore forcing by return flows. The hydrodynamic output from both sets of models will be used to force the equations of motion for UXO objects. The advantage of the wave-averaged models is two-fold. First, wave-averaged models have a much lower computational expense allowing rapid modelling of larger operational areas. Second, wave-averaged models can span a wider parameter space and are useful for developing ensemble results for inclusion in statistical models of munitions behavior. In contrast, the wave resolving models are computationally expensive and typically restricted to O (1 km) domains but may have more accurate descriptions of wave asymmetry and skewness for the purposes of UXO mobility prediction. Incorporating wave-resolving models will fill the gap in scales and processes between a wave-averaged modeling approach appropriate for operations ( MR-2733) and a computational fluid dynamics approach ( MR-2732).
This project will provide improved understanding of UXO mobility in water depths of less than 5 m. This work will aid management of UXO by improving understanding of the potential for UXO migration during the time between wide area surveys, detailed surveys, and removal of UXO by divers. The model will improve site managers’ capabilities to prioritize sites where UXO are most likely to move into public or populated areas. The approach is designed to merge seamlessly and inform existing modeling efforts for UXO mobility and burial.