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UK: River Parrett dredging trials produce promising results

Project Updates // January 4, 2017

Trials of different dredging methods on the River Parrett, carried out by Van Oord working in partnership with the Parrett Internal Drainage Board and the Somerset Rivers Authority, have produced promising initial results.

Tests of water injection dredging and agitation dredging were carried out on sections of the River Parrett near Bridgwater. The tests initially took place over three weeks in November and early December but were then extended for a further two weeks following early analyses of data.

It is hoped that the new techniques may provide cheaper and better ways of maintaining tidal rivers on the Somerset Levels and Moors, and across the country.

In traditional dredging, silt is removed from the riverbed using excavators, either from the riverbank or with cranes mounted on special barges. The silt is then taken away in trucks and either deposited on agricultural land or used to strengthen existing riverbanks.

Water injection dredging and agitation dredging have significantly less impact on local people and the natural environment.

The River Parrett trials started on 14 November, in conjunction with detailed monitoring before, during and after.
Surveying created a very precise profile of the river, which can be updated in near real-time. A screen on board Van Oord’s water injection dredging boat shows the relative positions of bed and injection point, making it possible to see what effect work is having. The injection bar can be raised and lowered, and the pressure and volume varied as required for maximum effectiveness.

River water is pumped through the injection bar towards the soft silts in the riverbed, which become ‘super-saturated’ and more separated.

‘Agitation dredging’ deploys an articulated arm with a cutting device which rotates, along with a suction hose. As the cutting head moves towards its target area, mud and water are vacuumed up, and blown back out into the top of the river, giving silt  much more time to be carried before it drops down again and settles.

Talking to the County Gazette (Somerset), Councillor John Osman, chairman of Somerset Rivers Authority, said: “We agreed to the request for further trials because different methods of dredging potentially offer Somerset big benefits. Work done at the right times, in the right places, could be much cheaper, more effective, and better for the environment, local residents and farmers. Evidence so far suggests that we’re on the right lines – and no detrimental effects have been observed. But we need to keep getting the most useful experience and information that we can. Even though the dredging vessel has now gone, we’ll keep on monitoring throughout the winter. We need to understand not just the immediate effects of different dredging techniques, but what happens along the Parrett in the weeks and months afterwards.”

Iain Sturdy, chief engineer at the Parrett IDB, added: “We have found that water injection dredging is capable of moving large volumes of material from the bed of the channel very quickly indeed.

“Further scouring also occurred after our dredging because of heavy rain at the end of November. If we can turn that kind of event to our advantage, and get natural forces to work for us, that’s potentially a very interesting development and one that we’re going to look out for and monitor.”

There have also been positive results from agitation dredging which again sees silt carried out by the tide before it disperses.

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