Cutting the cost

Maintenance dredging is Harwich Haven Authority’s biggest single expenditure. Credit: Westminster Dredging Co
Maintenance dredging is Harwich Haven Authority’s biggest single expenditure. Credit: Westminster Dredging Co
Stockholm is one of the first ports in the world to try out freeze dredging
Stockholm is one of the first ports in the world to try out freeze dredging

A necessity for many ports, dredging can be managed to reduce costs and environmental impact, finds Felicity Landon

Maintenance dredging is Harwich Haven Authority’s biggest single expenditure; to ensure clear access to the ports of Felixstowe, Harwich and Ipswich, it costs the authority about £7m a year to remove up to 3m cubic metres of silt from Harwich harbour and estuary every year.

However, one important change could save HHA £500,000, or more, every year. The authority is looking to move its licensed dredging disposal site five miles closer to shore, something which it says could also reduce its carbon output by 1,000 tonnes annually. “We are looking to keep our maintenance dredging sustainable, in all aspects,” says harbour engineer John Brien.

HHA carries out maintenance dredging approximately every ten weeks, with each dredge lasting between ten days and a fortnight. But because of the distance to the current disposal site at Inner Gabbard, the majority of the time is spent getting rid of the dredged material rather than actually dredging it.

The contractor’s trailer hopper dredger generally has a full load after working for 30 minutes in the harbour. But then there’s a three-and-a-half-hour round trip to Inner Gabbard to get rid of the stuff before the dredging operations can begin again.

A UK disposal site must be licensed by the Marine Management Organisation (MMO), and seeking permission for a new site is a complex process. The MMO procedure for analysing, designating and licensing new disposal sites involves a series of studies, reports and consultations; bodies such as Natural England and the Environment Agency, alongside interested groups such as fishermen, are involved in the discussions. The earliest that HHA would be likely to get the go-ahead is mid 2013.


New site

To identify potential new sites on behalf of the authority, consultant HR Wallingford considered areas with water depth of nine metres or more; other activities were then taken into consideration, including anchorages, wind farms, gravel extraction sites, fisheries, shipping channels, habitats and marine ecology. All of these ‘no-go’ areas were overlaid onto a detailed map, and the result showed a number of ‘green’ areas from which a location could be chosen.  

At present, the dredger’s shuttling back and forth between harbour and disposal site helps generate approximately 6,000 tonnes of carbon dioxide a year – the highest CO2 output of all HHA’s operations, by a huge factor.

But quite apart from the carbon footprint, the authority’s early studies showed that for every mile the disposal site was brought closer, £100,000 a year could be saved.

“Moving closer in is logical – if the MMO doesn’t approve this [particular site], they might let us move it somewhere else which is closer to the shore,” says Mr Brien. “For us, any distance nearer is good.”

The Port of London Authority is looking to reduce maintenance dredging costs when its new mooring maintenance vessel (MMV) is delivered in 2014.

The vessel, being built by Manor Marine in Portland, will replace the Crossness and Hookness. It will be equipped with two cranes and carry out salvage, maintenance and other works within the River Thames and the estuary. “But the most important thing of all is that it will have dredging capability, enabling us to undertake maintenance dredging in-house,” says Geoff Buckby, the PLA’s marine services manager. “The plough is being built at Manor Marine alongside the MMV.”

Adding the dredging element is a very cost-effective step and was part of the business case put forward for this major investment, he adds.


Going cold

In Sweden, the Port of Stockholm took another approach. It contracted FriGeo to carry out freeze dredging, a newly developed process, as part of a major quay extension project at Frihamnen.

Stockholm is one of the first ports in the world to try out this method, which offers both cost and environmental benefits, says Anne Wallinder, environmental engineer at Ports of Stockholm.

“The main reason that freeze dredging was used in this project was the specifications in the [court] permit for Värtahamnen-Frihamnen,” she says.

Using this method enabled the port to separate the upper, more contaminated layer from the underlying purer material – and also to dredge without clouding the water, says Ms Wallinder. Although there have been a few logistical complications in the project, due to the pioneering nature of the technology, she believes that freeze dredging is a method that has a great potential to grow.


All year round

Looking back on the work so far, the project could have been arranged and planned a little better, she says. However: “The upside was that we didn’t need any arrangements on the pier to take care of any contaminated leaching water. All the material that was removed by freeze dredging could be transported to the recycling site in solid form. We also could see that when releasing the blocks from the bottom, the water didn’t become muddy, i.e. no turbidity. That opens up a window to use the method anytime during the year.”

The freeze dredging equipment was established at the Frihamns pier on January 3 and the following day FriGeo started to establish the geotextile curtains. By January 6, the first freeze dredging plates were being lowered.

“They finished their work with freeze dredging on March 26,” says Ms Wallinder. “In all, 1,855 cu m of sediments was removed from the bottom of the harbour basin by freeze dredging, taking the depth down from 9 metres to 10.5 metres.”

The Frihamnen project is extending the quay by 80 metres to make the area more accessible for cruise traffic. Work was halted in April, when the cruise season began, and resumed in the autumn; the quay is due to be completed by May 2013.


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