Seafloor Burial of Surrogate Unexploded Ordnance by Wave-Induced Sediment Instability

Burial of munitions and other unexploded ordnance (UXO) on the seafloor complicates the detection and remediation of associated threats in coastal areas. We observed up to 0.6-m burial of surrogate UXO placed near the 8-m isobath during a series of winter storm events from late January to early March 2015 in Duck, NC, USA. We used time series from a pressure sensor located roughly 1 m above the bed during the experiment to apply an existing analytical solution for dynamic poroelastic wave-sediment interaction. The analytical solution yielded the spatial and temporal distributions of wave-induced pore pressures and effective stresses, which were used to evaluate five different criteria for wave-induced sediment instability. Four of the criteria have been applied previously considering different deterministic or stochastic wave types, whereas the fifth is novel and may be regarded as a generalization of the Sleath condition for plug flow. Using the estimated maximum depths of sediment instability (or bed failure) as predictors for observed surrogate UXO burial depths, we concluded that criteria based on vertical effective stress and the vertical distribution of pore pressures were most accurate. We believe that our data and the accompanying analysis are unique in the literature, where field observations of wave pressures were combined with a dynamic wave-seafloor interaction model to make predictions of sediment failure depths that were validated against actual observations of surrogate UXO burial depths.