Dredging
Trawling through YouTube recently we came across a fascinating video showing a huge dredger being used to widen a navigation channel in Southampton’s Western Docks.
Dredging is essential to maintain the viability of shipping channels, as sediment is constantly being washed downstream and filling channels and harbours. Associated British Ports (ABP) required the navigation channel to be widened by up to 46 m over a stretch of 2,300 m and for it to be commenced on December 1st 2023 and completed by 15th March 2024. These time constraints were necessary to minimise disturbance to wildlife, for example sea trout and migrating wild salmon.
Boskalis backhoe dredger 'Magnor'.Photograph taken from television
The work was carried out by Boskalis using trailer suction dredgers and the enormous backhoe dredger ‘Magnor’. Boskalis is a Dutch company with 100 years of experience in dredging across the world. With more than 500 vessels and floating equipment, it has conducted work in 90 countries and employs 11,000 workers.
Boskalis backhoe dredger 'Magnor'.
Photograph taken from television
‘Magnor’ is the largest backhoe dredger in the world. Its bucket can hold 33 cubic metres of material. More detailed technical information can be found here.
Boskalis backhoe dredger 'Magnor'.
Photograph taken from television
Before the dredging work could start, sediment had to be tested to discover the concentration of chemicals in it. Below specified levels, the dredged material could be dumped at sea in locations licensed by the Marine Management Organisation. Above those levels, the material would have to go to landfill.
The work has now been completed and all the dredged material has been dumped south of the Nab Tower in the Solent.
The Nab Tower, Solent
Image courtesy Wikimedia Commons
A professor of biology at the University of Portsmouth, Alex Ford, expressed concerns that the dredging would have deleterious effects on marine snails. TBT(tributyltin) used to be a component of antifouling paint, and was in use globally for 40 years, but was banned in 2008, following the discovery that it was extremely toxic to marine life, causing reproductive anomalies. In snails it caused females to grow male appendages and become infertile.
Marine snailThe professor said, “Since its ban in 2008, the incidence of imposex snails (females with a penis) has dropped considerably around our coasts but still persists in a few areas in the UK where there had been heavy shipping and ongoing dredging.
“Unfortunately, the concentrations of TBT in sediments do not
need to be high to cause sexual abnormalities in marine snails.”