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seabed: from cannons and cannonballs to anchors and ship parts. On top of all that,
large numbers of German sea mines were placed in the Solent strait during the
Second World War and approximately 40,000 bombs were dropped. The entire area
had to be inspected and cleared to prevent risks and delays in the dredging work.
We were working with colleagues from Boskalis Hirdes, which specializes in the
detection and clearance of UXO.”
ADVANCED SURVEYS
The inspection of the port began in November 2015. About 800 objects were
detected using traditional survey techniques such as side-scan sonar and
magnetometer survey. A crane vessel then brought the objects to the surface.
“We, however, also unintentionally retrieved a German sea mine with 700 kilograms
of explosives,” said Van den Bosch. “It was made of aluminum and so it escaped
detection by our equipment. Sea mines can still explode and we therefore switched to
an advanced survey technique known as Sub-Tem that is also used to detect non-
ferrous metals. This technology was developed for use on land, but we adapted it for
use underwater. It worked perfectly. More than 12,000 different objects – weighing in
total more than 1,100 tons – were removed from the seabed.”
RISKS
“Given the risks, our NINA safety program played a leading role on this project,”
Van den Bosch continued. “Portsmouth is a busy port with a lot of leisure shipping.
To keep the public at bay, the Smit Stour was put on patrol. We worked with a team
of certified UXO divers who didn’t start to work until we were certain they were not
exposed to any danger.” A light-sensitive underwater camera was fitted to the grab of
a pontoon’s crane to obtain a clear picture of the objects on the seabed. A pump was
also fitted to the grab that was used like a vacuum cleaner to carefully remove the
sand from around obstructions, which could be a torpedo, bomb or sea mine. That
process was monitored with the camera. “The UXO divers came into action only if
we couldn’t make a clear assessment of the object on the seabed with the camera,”
Van den Bosch added. “If the divers believed that the object was indeed UXO, then it
was removed and disposed of by the Royal Navy’s own UXO divers. We also found
all sorts of other objects that could disrupt the dredging work, such as anchors and
big bundles of twisted steel cables.”
LAYER BY LAYER
The dredging work was done layer by layer. “The Sub-Tem surveys allowed us to
inspect the seabed to a depth of two meters,” Van den Bosch explained. “We
repeatedly conducted intensive investigations of the area to make sure we didn’t miss
anything. Then we removed the objects we found and dredged out that layer of soil
before investigating the next layer. It was a time-consuming approach but the number
of objects found dropped as we removed more material and then things started to
speed up.” More than 80 Boskalis colleagues were at work in Portsmouth, which is
now ready for the arrival of the aircraft carriers in 2017.
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Dredging work by the trailing suction hopper
dredger Shoalway in the port of Portsmouth.