Do munitions move in mud? Field observations of munition surrogate mobility in cohesive sediment within an energetic estuarine setting.

A significant data gap has been identified regarding the behavior of legacy munitions in formally-used defense sites containing cohesive sediments. In order to address this knowledge gap, we initiated field investigations munitions mobility in cohesive sediment environments. Using a shallow estuarine site in the Delaware Bay, we monitored the mobility and behavior of sensor-integrated surrogate munitions using a high-accuracy acoustic positioning system, while concurrently measuring in situ hydrodynamics and morphology through an instrumented bottom frame. Preliminary results suggest that munitions burial is likely in both soft muds and mixed sandy-muds, despite observing surrogate munitions in several energetic storm events. While the mechanisms for munitions burial in non-cohesive sediments are well established, be it through scour-burial processes or granular sorting, the resistive nature of cohesive sediment to granular erosion reduces the likelihood of granular sorting as a primary mechanism for burial, inhibits scour, and may result in other mechanisms for burial not characteristic of non-cohesive sediments. This study expands the research on munitions mobility and burial in cohesive sediments through a focused field study on the geotechnical properties of cohesive sediments that most contribute to the fate of munitions. Specifically, this study 1) conducted an in-field deployment of instrumented and acoustically-tracked surrogate munitions in a dynamic, cohesive-sediment study site, 2) continuously monitored site hydrodynamics and morphodynamics through in situ instrumentation, coupled with periodic site-wide geophysical surveying in order to inform 3) contemporaneous in-field geotechnical sediment characterization through repetitive sampling and application of a free fall penetrometer. Results indicate a close correlation between observed depth of burial of in situ surrogate munitions and the quasi-static bearing capacity of sediments derived from penetrometer tests.