SRA Annual Report 2019-20: Maintenance dredging and silt monitoring
Contents – quick links
Background
Activity in 2019-20
Silt monitoring
Background
After the devastating floods of 2013-14, the Environment Agency spent £6million on pioneer-dredging 8km (5 miles) of the Rivers Parrett and Tone. In 2016, Somerset Rivers Authority (SRA) funded a further pioneer dredge of 750 metres of the Parrett downstream of Northmoor Pumping Station. Pioneer dredging is the name given to the dredging of areas that have not been dredged for several years, where banks need to be re-profiled.
The pioneer dredges of 2014 and 2015 removed 248,500m3 of silt. They put the two rivers’ capacity to carry water back close to what it was in the 1960s, when – in response to the big floods that hit Taunton and Somerset in 1960 – the channels of the Parrett and Tone were made bigger and the River Sowy was created. The 2014-2015 dredges – combined with Environment Agency investment in temporary pumps and pumping facilities – significantly reduced flood risks to people, properties, roads and land.
To preserve this achievement, since 2015 the SRA has funded annual programmes of maintenance dredging and silt monitoring. Without targeted maintenance dredging, silt would re-accumulate, flood risks would increase, and expensive pioneer dredging would eventually have to be done again. Silt monitoring shows precisely where silt is accumulating and enables dredging to be targeted most effectively.
Maintenance dredging and silt monitoring on the Parrett are 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 to make sure that activities comply with numerous legal and environmental requirements.
Maintenance dredging in the winter of 2015-16 used conventional techniques, with excavators working from river banks and a floating pontoon.
In autumn 2016 the SRA funded a trial of water injection dredging (WID) techniques using international specialists Van Oord and their vessel Borr.
The success of this trial prompted a more extensive trial of WID in 2017, which was again effective. In late November 2018 a five- year contract for maintenance dredging along the River Parrett was let to Van Oord and more work was carried out soon after in December.
How water injection dredging works on the River Parrett
Read more about how water injection dredging works on the River Parrett.
Activity in 2019-20
In previous years, water injection dredging has been done before Christmas, but this time round it was done over two weeks in January 2020. This was to avoid clashing with pioneer dredging between Stathe and Burrowbridge in late 2019.
Big tides and heavy rain meant that water levels in the Parrett were high, and conditions for water injection dredging were very good. In total, around 25,500m3 of sands, silts and clays were removed from the bed of the Parrett and dispersed through natural processes, from Burrowbridge down past Northmoor Pumping Station. That is 4,500m3 more than got shifted over four months in the winter of 2015-16 using conventional methods. (In 2015-16, there were up to six excavators working from the banks, and an excavator worked for four weeks on a floating pontoon with a tug boat and hopper barges).
Silt monitoring
Silt monitoring is carried out every spring and autumn along the Rivers 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 in height.
Some very advanced technology is used. For example, flux monitoring equipment at New Bridge on the River Tone and at Oath Lock and Somerset Bridge on the River Parrett can measure silt movement in rivers in near real-time.
The SRA’s short-term aim is to identify places where sediment builds up – how sediment builds up – and what type of sediment 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. This quest is being greatly helped by academics leading scientific studies of water injection dredging on the River Parrett.