There remains a recognized need for better quantitative understanding of the impact of coastal environments on munitions ad explosives of concern (MEC) mobility and burial. Recent studies have investigated the mobility and burial of MEC in sandy coastal areas, leading to a more comprehensive understanding of munitions mobility and burial in non-cohesive sediments. A significant data gap was identified regarding MEC in shallow, cohesive-sediment environments, where munitions phenomenology is less clear. The goal of this study was to address this data gap through observations of MEC mobility and burial in cohesive-sediment environment and to compare findings to observations and models for munitions in non-cohesive sediments. The primary objectives of this study were to: 1) monitor the mobility and behavior of sensor-integrated surrogate munitions in muddy environments using a high-accuracy acoustic positioning system, 2) observe surrogate munition response to hydrodynamic forcing through in situ hydrodynamics and sonar imagery, and 3) monitor morphological site changes through repetitive geophysical site surveying and benthic sediment sampling.
A suite of custom designed and manufactured surrogate munitions were deployed at two cohesive sediment sites in the lower Delaware Bay estuary from 2017 - 2019. Surrogates were monitored for changes in mobility and burial using a Vemco (Innovasea) VPS acoustic tracking system and inertial motion units. Time-series acoustic imagery and hydrodynamic sensors were also deployed to characterize MEC response to varied hydrodynamic conditions. A series of repetitive surveys were conducted using a hull-mounted, high-resolution phase-measuring bathymetric sonar and autonomous underwater vehicle equipped with a magnetometer. In total, four deployments were conducted between 2017 and 2019, spanning 226 days.
The effects of six separate extra-tropical cyclones were recorded during the deployments, leading to wide-spread sediment transport. Burial was the predominant result across all four deployments, ranging from 50% to greater than 100% burial of surrogates depending on sediment composition and near-bed forcing. No large-scale horizontal mobility of surrogates were observed, although minor motion was recorded during peak-near bed velocities of several storm events. Models developed for munition mobility and burial in non-cohesive sediments were adapted for cohesive sediments and compared to field observations. MEC burial is linked to scour burial in mixed-cohesives with higher sand fraction and plastic deformation and bed fluidization in soft muds. Sediment bearing capacity is shown to be an important predictor for surrogate burial, and a simple model is introduced to predict burial due to bearing capacity. Shallow subsurface geotechnical properties influencing surrogate burial are linked to surficial sediment distributions via seabed classification and discussed in light of site management.
The results from this study will inform key parameters for MEC behavior in storms and muddy environments for integration into the existing expert system modeling of MEC burial and mobility (e.g. Underwater Munitions Expert System, Rennie, 2017; SERDP Munitions Mobility and Modeling Workshop, Dec. 8-9, 2015). The observations will expand the knowledge into an understudied but abundant environment with respect to munitions contamination, and aid in understanding historical and present conditions at formerly-used defense sites containing cohesive sediments.