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Unexploded ordnances and the future of European dredging

2022-09-20 Tamara Parkin
Extreme weather and droughts are uncovering remnants of war. What measures should dredging be taking to avoid delays to projects posed by unexploded ordnances?

20092022 // uxo-campaign-fehmarnbelt-tunnel-project-2.jpg (407 K)

Photo Credit: Deep BV

Record high temperatures in Europe over the summer resulted in unprecedented droughts which unveiled warships and unexploded ordnances (UXO) that have laid dormant for decades. With extreme weather conditions set to continue, and worsen, in the years to come, dredging may have to contend with challenges posed by lower water levels and more instances of resurfaced UXOs creating costly delays to projects. 

In August, the Danube fell to one of its lowest levels in almost a century, exposing more than 20 sunken German warships from World War Two near Prahovo, a river port town in Serbia. Whilst a submerged 450kg World War Two bomb was discovered in the area of the River Po and subsequently detonated. 

Dredging operations are no strangers to unexploded ordnances. In February of 2021 a Belgium registered Hopper Dredger from the Netherlands resurfaced some UXOs when completing work off the east coast of England. Work was halted to mitigate risks and the UXO's were identified and recorded. 

As climate change continues to affect water levels, droughts will become increasingly more common with the increase in resurfacing of war remnants more likely. To help prevent project delays or any potential major incidents from occurring, these remnants of the past need to be accounted for during preparation and risk assessments of dredging projects, particularly in Europe on routes used during World War Two. . 

The dredging industry can, however, take lessons from the methods and technologies used in previous projects that have contended with remnants of war. 

One such project is the ongoing  Fehmarn Belt Tunnel project, which began in 2020.  The project is located between the islands of Fehmarn in Germany and Lolland in Denmark. This area was frequented by cargo ships and military fleets during World War Two due to it being one of the two shipping channels that connect the Baltic with the North Sea. 

Due to the location of the tunnel, construction is the responsibility of two contractor consortia, Fehmarn Belt Contractors (FBC) and Fehmarn Link Contractors (FLC). The tunnel will then be equipped by the contractor consortium Femern Service Contractors (FSC). 

The geophysical site investigation was conducted by Deep BV, a survey company who specialise in hydrography, geophysics, and oceanography, determined whether UXO’s were along the tunnel route and assessed the mitigation measures that would need to be taken to reduce risks. 

Through the use of electromagnetic sensors, the site area was found to contain three specific World War Two remnants that were deemed to be a major threat to dredging operations. These were 500lbs air-dropped munitions, depth charges, and ground mines of which depth charges were initially regarded as the minimum threat item (an item that poses the lowest threat to dredging operations). A minimum threat item is determined by whichever item possesses the smallest electromagnetic response when survey operations are conducted. The minimum threat item changed during surveying operations, which uncovered 6 inch high explosive (HE) artillery shells subsequently changing discrimination parameters. Discrimination parameters refers to the framework in which objects are classified as either relevant or irrelevant items; distinguishing what items are threats to the dredging campaign. Within the Fehmarn Belt UXO campaign the identifying markers were ‘size’ and ‘decay’.  

The discrimination parameters to identify the risk level items posed to dredging operations were size and decay, and  were used to compare objects against the previously identified smallest UXO. Through this identification system the operation combed through key locations considered to be high risk areas to identify any objects which may or may not pose a threat to dredging operations. 

Out of 1514 objects shown as anomalies on the electromagnetic survey, 850 were identified as too small to require excavation and relocation. According to the project leads, this analysis of objects enabled a significant reduction in necessary excavation works; an effective reduction rate of 55%. This method resulted in the identification and clearance campaign being shortened by approximately 26 days.  

Once the survey was complete, an ROV equipped with an electromagnetic induction (EMI) sensor SubTEM-ROV was used to clear the area. The system can be housed in a towed vessel or on a remotely operated vehicle. The clearance campaign found 23 UXO or UXO related items. These consisted of 18 identified artillery shells or projectiles, 3 German depth charges, 1 British ground mine and 1 nose section of an air-dropped bomb. They were all successfully removed and the project leads said that the excavation process did not introduce any added risks to the dredging campaign.

When constructed, the Fehmarn Belt tunnel will be the world's longest road and rail tunnel under water, connecting Rødbyhavn in Denmark with Puttgarden in Germany. The tunnel length will be 18km and the tunnel depth at its deepest point will be more than 40m below sea level. Dredging operations are expected to be completed in 2024, and due to the survey and ROV work, FBC was able to avoid potential dangerous or costly delays to this major project, which will be complete in 2029. 

As climate change continues to drive more extreme weather and droughts, dredging had better get used to taking a more cautious approach to risks that lie beneath the surface. 

More information about the Fehmarn Belt project tackling war threats is available to read through the WodCon XXIII paper, which is available here: https://www.dredging.org/resources/ceda-publications-online/conference-proceedings/abstract/1162 

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