A particularly awkward clean-up of 22,300m3 of contaminated sediments in Falmouth Marina has begun.

Specialist contractor Land & Water Services Ltd has provided a bespoke clean-up strategy and dredging team to the Premier Falmouth Marina. The £1.66 million project involves installing tubular mooring piles and pontoons to remove the contaminated sediment to improve the mooring and operational facilities at the site.

The close proximity of a number of highly protected sites and the inbuilt risks of recovering a highly mobile contaminant in the tidal environment, make the works particularly high profile for the regulators. Land and Water has developed their own site specific waste strategy plan, with environmental protection being offered by combining a number of technologies, developed and operated by the company.

The risk of suspended sediment release was mitigated using a combination of 4 technologies:

 A ‘visor bucket’ attached to the excavator to collect the sediments from the seabed and then trap them to prevent and fluidisation on raising the bucket.

A3D dig control system allowing the operator to finely position the bucket and a telemetry system to track and monitor the excavator.

A ‘moon-pool’ system created around the digging envelope in front of the dredge pontoon. This allows sediment to be collected successfully despite the tidal conditions.

Water monitoring buoys which measure the suspended solids in the water column and alert the operators to any levels that exceed the threshold.

Land & Water will also manage the transfer and recycling of the dredged arising with 4Recycling Ltd. The low level contaminated dredgings will be used as a restoration soil over a number of nearby sites formerly used for china clay extraction.

Read more in the Silt Curtain Case Study

Land & Water Services had to provide a sustainable solution for a difficult and unique dredging challenge at Falmouth Marina in Cornwall.

A bespoke silt curtain was designed & manufactured 94m long by 8.4m deep, ballasted to enclose the purpose built Land & Water pontoon and thus prevent contamination leaking into the River Fowey during the dredging operation. The silt curtain weighing approx 2 tonnes was installed by a 15 men team, including 4 divers, in one day.

The marina site falls within the Fal-Helford area, a SSSI, however the site formerly occupied the Empire & Kings wharves, each more than 200m long, which were constructed in the 1930s. The wharves have now been removed and as they have not dredged since 1938 almost 10,000m3 of highly contaminated material containing heavy metals toxins such as TBT- which experts claim is the most toxic substance ever deliberately introduced into the marina environment- needs to be removed from site.

The silt being unsuitable for disposal at sea, due to the local oyster & mussel beds, is being transported in sealed lorries to the redundant China Clay workings at St Austell where, after being mixed with green waste and sewage, it will be used to landscape the former industrial site without the contamination proving to be an on-going issue. 

 See video of the dredging project.

Silt curtain arriving on site

Silt curtain assembely

Silt curtain assembely

Weighing almost 2 tonnes the silt curtain required plenty of labour and an experienced team of divers to fully install

Land & Water team work hard to make sure the silt curtain is positioned correctly.

The silt curtain is continually installed around the floating booms.

The silt curtain now fully operational and the long reach excavator is ready dredge

The silt curtain now fully operational and the long reach excavator is dredging

The silt curtain fully operational and the long reach excavator is dredging

During the summer months of 2011, over 32,000 tonnes of contaminated sediments was removed by Land & Water services from the harbour area of Falmouth marina, Cornwall

Tributyl Tin (TBT) is an active ingredient in certain antifouling paints used to coat the bottom of ships to prevent sea life such as algae and molluscs attaching themselves to the hull.  However these compounds persist in the water killing sea life and harming the environment.

An EU ban on the presence of TBT-based antifouling’s in EU ports came into effect on 1st January 2008.

Land & Water were contracted to carry out the maintenance dredging at the entrance to A & P Dry Dock No 2 and the adjacent areas of quayside to a specified profile.  The area of dredging was heavily contaminated with TBT and heavy metals resulting in the arising silts having to be stabilised in lagoons on site before being transferred in sealed wagons to Teeside for final disposal at an approved landfill Site. 

Dredging was undertaken using a 40 tonne long reach excavator equipped with a lidded “Eco-Bucket” and mounted on a 12 section Raverstein Pontoon. All works were carried out within a Silt Curtain to prevent the migration of sediments during the works.  The dredged profile was accurately controlled using a Trimble Dig Control System mounted on the excavator.