Trials of different dredging methods on the River Parrett produced promising initial results for Somerset Rivers Authority (SRA).
Tests of water injection dredging and agitation dredging, in combination with sophisticated monitoring, were carried out for the SRA by the Parrett Internal Drainage Board and contractors Van Oord over three weeks in November and early December, on sections of river between Westonzoyland Pumping Station and Burrowbridge.
Encouraged by early analyses of data, and observations of silt movements along the river, the Parrett IDB asked the SRA if the tests could be continued for a further two weeks, upstream to Burrowbridge itself, in a wider range of conditions.
Cllr John Osman, Chairman of Somerset Rivers Authority, said: “We agreed to the IDB’s 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. So 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.”
In water injection dredging, the idea is that river water is pumped through an injection bar positioned just above the bed of the Parrett and aimed at soft silts so they become super-saturated and travel, in a layer of their own, on an outgoing tide, where they disperse into the natural system.
Iain Sturdy, Chief Engineer at the Parrett IDB, said: “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.”
Agitation dredging uses an articulated arm with a cutting device, which rotates, and 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. The idea here is, again, that silt is carried out by the tide before it disperses.
Mr Sturdy commented: “Agitation dredging is slower, although covering 90m of river in a day is still pretty good and this method may be more precise. It may, for example, be better at removing silt from the side of the channel, that’s been deposited in what we call berms. We’ve also been finding out more about how we can better manage berms, by using side-jets in a variant method of water injection dredging.
“All in all, we’ve gathered a lot of data and it will take time to assemble it into a form that can be shared and understood.
“We have also gained significant useful experience in planning future maintenance dredging.”
Jasper Blaauw, operations manager with Van Oord, said the company had invested more than 25 years of research and development into the dredging vessel Borr: “It has been a very delicate process, building upon many years of experience using water injection dredging and agitation dredging techniques throughout the world.”
He went on: “We welcomed this opportunity to work on the River Parrett, where dealing with combined tidal and fluvial flows, whilst preserving the flood banks, offered a particular challenge.
“It has been a great experience working with the IDB team, to fully understand the site constraints, recognise the importance of maintaining the channel and monitoring the effects of the work. Now that we are more familiar with the site specifics, and the people we’re working with, we can seek to further optimise our process, as there is much more to achieve.”
The cost of continuing trials for a further two weeks fell well within the budget allowed for this work. In total, the tests moved more than 10,000m3 of silt.
Cllr Osman said: “I went out to see the trials for myself and I know that the Parrett IDB and the contractors Van Oord worked very hard. I’d like to thank them for all their efforts so far. It’s important work and I look forward to seeing more results.”