The study of underwater unexploded ordnances (UXO) mobility and burial in underwater environment is of paramount importance due to the large number of sites potentially contaminated with munitions. This project, called MINELAB, aimed at modeling the fate of munitions displaced over or buried into the heterogenous sedimentary bed of a micro-tidal estuarine site, subjected to both fluvial and marine forcing. The objective was twofold (Figure 1): (1) to directly observe the combined effect of river flow, water waves and tide on the mobility and burial of UXO; (2) to predict the behavior of UXO subjected to the combined action of river discharge and waves in a micro-tidal environment characterized by mixed sediments.

Figure 1. MINELAB workflow.

Some preliminary Delft3D numerical modeling was conducted to better characterize the hydro-morphodynamics of the test site, reducing uncertainties and risk associated with the design of the field experiments. These were performed at different locations in the estuarine area, to assess the role of different actions (e.g., river flow, waves of various nature) and soil material on the UXO mobility and burial. Results of such field experience, together with the data collected by an integrated monitoring system operating in the study area, were used to contribute to the validation of a single tool that combines the modeling of both hydro-morphodynamics and object mobility, with the aim to reproduce the deployed UXO motion.

Results of field tests showed that UXO deployed in areas characterized by different soil material behave differently. Once placed on the muddy riverbed, the UXO almost instantaneously got buried and did not move. Just the smallest and least dense UXO showed some migration and resurfaced at times. On the other hand, the UXO remained proud on the sandy seabed, until a wave storm produced a significant morphological seabed evolution, likely fostered by fluidization of the sandy bed, that caused their complete burial. The largest observed migration distances were of the order of 7 m. The Underwater Munitions Expert System (UnMES) simulation, aiming at reproducing the UXO behavior during the field test performed at sea, showed a migration distance in the range 0 m to 5 m, in agreement with the observations. Predicted burial depths were, instead, smaller (50% to 75% of the UXO diameter) than those observed in the field (over 100% of the UXO diameter).

Results of the project provided information on UXO mobility and burial in an estuarine environment, characterized by multiple forcing actions and mixed sediments. On the one hand, such characteristics make the site different from other sites usually studied in the literature, thus interesting; on the other hand, they make the analysis very complex. Field observations provided a baseline data set for verification and calibration of the model set up to predict munitions mobility and burial, based on the combination of Delft3D and UnMES models. Due to (1) the limited duration of both the experimental campaign and project and (2) the measurement procedures and techniques tested for the first time within this project (some fairly innovative in the overall), just few data were produced to feed UnMES. A longer field test using more instrumentation to characterize the study area and more precise and consolidated measurement procedures would provide valuable results for the validation of UnMES.