Dredging The Parrett: New Era Advances

Dredging the Parrett: new era advances

New techniques for dredging the River Parrett are being tested this November and December, as Somerset Rivers Authority seeks cheaper and better ways of maintaining tidal rivers.

Three weeks of trials are being carried out between Westonzoyland Pumping Station and Burrowbridge to see how effective water injection dredging and agitation dredging can be at getting silt moving down to Bridgwater Bay and preserving the River Parrett’s capacity.

Cllr John Osman, SRA chairman, said: “This is the first time that such trials have been done on the Parrett in conjunction with a long-term programme of detailed monitoring.

“Potentially, new methods offer Somerset big benefits: done at the right times, in the right places, they could be much cheaper, more effective, and better for the environment, local residents and farmers.”

The trials are being delivered for the SRA by the Parrett Internal Drainage Board with contractors Van Oord.

New dredging techniques aim to keep silt moving out to sea

The techniques being trialled for Somerset Rivers Authority are water injection dredging and agitation dredging: two ways of getting and keeping silt moving. Work must be done when an outgoing tide can carry silt away.

The vessel being used on the Parrett is called Borr (a name from Norse myth meaning ‘son’).

A diagrammatic side view of the Van Oord vessel Borr

After being delivered to Dunball Wharf in halves, then re-assembled, Borr motored upstream through Bridgwater on Saturday night, 12 November (pictured below).

The Van Oord vessel travelling up the Parrett through Bridgwater, on the way to carry out dredging trials for Somerset Rivers Authority

Three weeks of trials started on Tuesday, 15 November, in conjunction with detailed monitoring before, during and after. Along the trials section, there is 150% more silt than there was in April. In places, four times as much.

Work therefore carries on from where 2.2km of maintenance dredging was finished earlier this year by the Parrett IDB for the SRA. (That dredging, and these trials, both maintain the 8km pioneer-dredged in 2014).

How does water injection dredging work?

In the picture above (not taken on the Parrett!), the injection bar, spraying water, is raised so you can see it. Nozzles pump out a high volume of water. The plan on the Parrett is to lower the bar just above the river bed. Surveying has created a very precise profile of the river – and this profile can be updated in near real-time. As the vessel passes, it re-measures. A screen on board shows the relative positions of bed and injection point, so it’s possible to see what effect work is having, and to raise and lower the bar, and vary the pressure and volume.

River water is pumped through the injection bar. It’s aimed at the soft silts in the bed of the Parrett, so they become super-saturated and more separated, in a turbulent fluid lower layer of their own, slightly different to the surrounding water. Van Oord call this a “density current” (see image below): it is, effectively, a different material to the water around it, so it behaves partly independently. In this state, on an outgoing tide, it should travel: the hope is that it will carry down right back out into Bridgwater Bay, and disperse naturally.

Image showing principle of water injection dredging

Agitation dredging (pictured above at Tayport in Scotland) uses a Farrell attachment, 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 that silt is given much more time to carry before it drops down again and settles.

The trials aim to get a better understanding of where silt goes and the best ways to keep it moving. Many different variables will be measured, particularly turbidity (how cloudy water is – and when and where it settles again).

Five potential benefits of the new techniques

Cheaper – potentially much cheaper.

More efficient – letting water carry silt away should be mechanically more efficient than digging it out and hauling it away with excavators, barges, tractors and trailers. If natural processes take silt out to sea, this will reduce costs.

Better for the environment – bankside flora and fauna do not have their habitat disturbed other than at the mud level.

Better for residents – less traffic coming and going, no road closures.

Better for farmers – not spreading silt over fields means land does not have to be taken out of commission until vegetation re-grows.

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