Studying unexploded shells on seafloor

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University of Delaware
UD graduate students (from left) Ken Haulsee, Carter Duval and Tim Pilegard deploy an autonomous underwater vehicle equipped with an advanced magnetometer for detecting underwater unexploded ordnance.

Nobody is really sure how many unexploded bombs, mortars and missiles lay on the ocean floor. And until relatively recently, it was also unclear how these weapons behaved in marine environments.

Researchers at the University of Delaware are starting to get a better idea, at least for ordnance in muddy estuarine settings. Three recent rounds of testing in the Delaware Bay suggest unexploded mortar rounds generally stay where they are despite strong storms and tidal currents.

“While there was some limited rocking and rolling, they buried mostly,” Carter Duval, a doctoral student at University of Delaware, said of the inert mortar rounds used as stand-ins for the real weapons. “That was the result we were really interested in.”

Delaware is among the organizations studying the effects of unexploded ordnance in different marine settings. Others, such as Woods Hole Oceanographic Institution and the U.S. Navy Research Laboratory, are performing similar research along wave-breaks and sandy bottoms.

The Delaware team, which includes professor Arthur Trembanis, outfitted replica ordnance ranging from 60 mm mortar rounds to 155 mm shells with sensors to track their movement. The team dropped the inert units into shallow water and monitored them for three 30- to 60-day intervals in 2017 and 2018. Another study period is scheduled for spring 2019.

Weather and sea conditions varied during each of these “deployments,” but the one conducted in spring 2018 coincided with several nor’easter storms. Despite these powerful weather events, the mortars did not move along the seafloor. In case after case, the inert ordnance remained locked in place while sediment accumulated around it. The objects became buried and remained that way during the duration of the deployments.

Researchers outfitted various shells, including this dummy 60 mm mortar round, with tracking gear to study its movement along the seafloor.

University of Delaware

Physics suggests these objects would bury to the point where their tops become level with the seafloor. But Duval said in some cases they became buried much deeper. He theorized the entire seabed could loosen during high-energy events, allowing the ordnance to sink deeper.

“Conversely,” he added, “there is the possibility for sediment to move onto a site on top of the … object.”

This research has numerous purposes. For one, it sheds some light on what happens when unexploded arms land in the water. That information is useful for officials monitoring former military testing or disposal sites, and utilities looking to develop offshore wind sites.

Duval said it’s likely his team’s results will be combined with data collected by other researchers to model the effects of unexploded ordnance over different time frames.

“It’s not just a U.S. problem, it’s a global problem because unexploded ordnance is found throughout the globe as legacy of wars and conflict,” Duval said.

“Because we are increasingly using the shallow shelf for commercial fishing and alternative forms of energy,” he continued, “that is putting people into contact with these munitions. It is people whose livelihoods depend on being able to do work on the ocean.”