One of the critical areas for underwater munition detection and remediation is very shallow water less than 5 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 of 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 the work was to improve models for hydrodynamic forcing of UXO migration and burial in shallow water surf-zone environments. This work addressed 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?
• What is the role of morphology (e.g., migration of sandbars) in UXO burial and migration and can this be captured by phase resolving models?

The research used both a wave-resolving model (SWASH) that explicitly computes non-linear wave transformations and a wave-averaged model (XBeach) that parametrizes nonlinear transformations in terms of wave asymmetry and skewness. The output of these nearshore hydrodynamic models is input into a parameterized UXO burial and mobility post-processing module based on a previously developed time-dependent burial model. The approach investigated the balance between onshore forcing by wave asymmetry and skewness and offshore forcing by return flows as described by an interpolation function calculated from the parameterized equations of motion for a cylinder. The advantage of the wave-averaged models is three-fold. First, wave-averaged models have a much lower computational expense allowing rapid modelling of larger operational areas. Second, due to the lower computation expense longer runs which capture coastal morphodynamic change are possible. Third, wave-averaged models can span a wider parameter space and are useful for developing ensemble results for inclusion in statistical models of munitions behavior.

When forced with steady waves both phase-averaged and resolved models show similar results of low density (S = UXO density / Water density < 2.5) UXO migration toward the outer edge of the surf zone where offshore returns flow balance onshore forcing due to skewness and acceleration in the absence of burial by migrating sandbars which were only modelled with the phase averaged model. The location of the balance point can be calculated by a power law relation with the breaker index (Wave Height divided by Water Depth) for inclusion in probabilistic models for UXO migration. The time dependent model typically predicts burial of higher density (S > 3.5) UXO. In the transitional regime (2.5 > S > 3.5) the threshold for mobility depends on the rate of wave energy increase. If a proud UXO is suddenly forced with large waves it is more likely to migrate than a UXO that is forced with moderate waves for prolonged periods allowing scour burial. When forced with time series of wave conditions from two different field deployments the modelling system produced output of deep burial by offshore migrating sand bars in one case and onshore migration of low density UXO in the other, consistent with measurements.

The models can 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 models can improve site managers’ capabilities to prioritize sites where UXO are most likely to move into public or populated areas.