SRA Annual Report 2018-19: Maintenance dredging and silt monitoring (W1)

Towards the end of November 2018, a five-year contract for maintenance dredging along the River Parrett was let to water injection dredging specialists Van Oord. Within days, Van Oord’s vessel Borr was taken to Dunball Wharf near Bridgwater in halves on the back of two lorries. The boat was bolted together and craned into the water. The crew then sailed up the Parrett to a compound near Westonzoyland Pumping Station, and worked many long and carefully-targeted hours during the first week of December.

Big tides and heavy rain meant that water levels in the Parrett were high, and conditions for water injection dredging (WID) were excellent. As powerful jets of water dislodged silt from the bed of the Parrett, big tides carried it away.

There were larger amounts of silt to shift than normal, because last year’s dry summer meant that flows down the Parrett were lower than usual, which in turn allowed more silt to be carried up with the tide. In total, 37,500m3 was removed in just a few days, from Burrowbridge down past Northmoor Pumping Station. That is 16,000 cubic metres more than got shifted over four months in the winter of 2015-16 using conventional methods. (In 2015- 16, there were six excavators working from the banks, plus an excavator working for four weeks on a floating pontoon with a tug boat and hopper barges).

Maintenance dredging on the Parrett is delivered for Somerset Rivers Authority (SRA) by the Parrett Internal Drainage Board (IDB). The IDB acts under a Public Sector Co-operation Agreement with the Environment Agency, and works closely with the Environment Agency and Natural England on making sure that activities comply with a large number of legal requirements.

The River Parrett is the first place in the UK where water injection dredging techniques have been used on a tidal river in combination with a long-term, sophisticated silt monitoring programme.

Through partnership working and ongoing monitoring, the SRA is discovering ways of making maintenance dredging much cheaper, much quicker, and much less disruptive for local residents, farmers, nearby road users and the environment. It is a considerable success.

Silt Monitoring

Silt monitoring is carried out every spring and autumn along  the Parrett and Tone to inform the SRA’s maintenance dredging programme. Works include single beam and multi-beam ‘bathymetric’ (underwater) surveys of the channel bed, and laser scanning of the banks, to measure any changes.

Some very advanced technology is being developed and installed. For example, fixed location sensors have been fitted at New Bridge on the River Tone and at Oath Lock and Somerset Bridge on the River Parrett. This flux monitoring equipment can measure silt movement in rivers in near real-time.

The SRA’s short-term aim is to identify places where silt builds up – how silt builds up – and what type of silt it is – so that maintenance work can be accurately targeted and dredging activities optimised.

The SRA’s longer-term ambition is to get a better understanding than anybody has ever had before of how the tidal River Parrett- River Tone system really works. Such knowledge is extremely valuable.