SRA Annual Report 2020-21: Pioneer dredging and silt monitoring
Contents – quick links
Background
Activity in 2020-21
Silt monitoring
Background
A few months after the devastating floods of 2013-14, the Environment Agency spent £6million on pioneer dredging 8km (5 miles) of the River Tone and the River Parrett, down to Northmoor Pumping Station.
Continuing in 2016, the Environment Agency pioneer dredged the next 750 metres (0.47 miles) of the Parrett downstream of Northmoor Pumping Station. Somerset Rivers Authority (SRA) funded this 2016 work. In 2018, the SRA funded pioneer dredging of the 2.2km (1.4mile) stretch of the Parrett between Beazley’s Spillway at Stathe and its confluence with the River Tone at Burrowbridge.
The pioneer dredges of 2014, 2016 and 2018 removed around 270,500m3 of silt. They put the 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 (find out more about the history of the River Sowy). The three pioneer dredges – combined with Environment Agency investment in temporary pumps and pumping facilities – significantly reduced flood risks to people, properties, roads and land across a large part of the Somerset Levels and Moors.
Over the winter of 2015-16, in December 2017, December 2018, and January 2020 the SRA funded maintenance dredging of the Parrett and Tone.
The aim of maintenance dredging is to prevent silt re-accumulating and flood risks increasing. The SRA also funds twice-yearly silt monitoring. Monitoring shows where silt has been newly deposited, so that dredging can be targeted effectively.
Maintenance dredging and silt monitoring on the Parrett and Tone are carried out for the SRA by the Parrett Internal Drainage Board (IDB). The Parrett 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.
In 2016 consultants from HR Wallingford produced a report on Opportunities for further dredging in Somerset. This recommended the SRA to test water injection dredging (WID) techniques. In autumn 2016, funded by and on behalf of the SRA, the Parrett IDB led a trial of 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 again was effective. In November 2018 a five-year contract for dredging along the River Parrett was let to Van Oord by the Parrett IDB, on behalf of the SRA.
In January 2020, after the completion of maintenance dredging by the Borr, the vessel was used in a short pioneer dredging trial along the Parrett. This was successful, and so plans were made for more Parrett pioneer dredging in January 2021.
River Parrett pioneer dredging down from beyond Northmoor to the M5
Pioneer dredging in January 2021 covered 2.2km (1.37miles) of the River Parrett, downstream of the 750 metres (0.47miles) dredged by the Environment Agency for the SRA in 2016.
January’s dredging was undertaken for the SRA by the Parrett IDB, working closely with the Environment Agency and Natural England, and using Growth Deal funding from the Heart of the South West Local Enterprise Partnership.
Van Oord’s water injection dredging vessel Borr was mobilised at Dunball Wharf on Saturday 16 January, and moved upstream to a temporary compound near Westonzoyland Pumping Station. Dredging began the day after.
High river levels and consistent seaward flows enabled dredging to continue for an average of 10 hours a day, every day for 14 days, mostly in daylight. (A feature on the SRA’s website, from last year’s annual report, explains in detail how water injection works on the River Parrett).
Van Oord’s staff and Parrett IDB officers had to operate within challenging coronavirus restrictions and Somerset Rivers Authority is grateful to them all for their hard work in difficult circumstances.
The Borr was demobilised back at Dunball Wharf on Sunday 31 January and a post-works bathymetric survey was done the next day. Around 22,000m3 of consolidated silt deposits were removed from the 1.37 miles of the Parrett down to the M5 bridge, so making the capacity of this stretch of the river align with the 2016 dredge.
Dredging down to the M5 has had three main benefits. Firstly, it has reduced flood risks for properties in the Northmoor area. Secondly, it has been helping to reduce the risks of agricultural damages, which tend to be worst from spring and summer floods (as seen in 2012). Thirdly, it has created better possibilities for managing flows of water around the Somerset Levels. It will help the SRA and its partners to make further improvements, like those planned for the River Sowy and King’s Sedgemoor Drain in summer 2021. It also complements the pioneer dredging done in 2019 between Stathe and Burrowbridge.
Stathe to Burrowbridge dredging follow-ups
In 2019, 2.2km (1.4miles) of the River Parrett between Beazley’s Spillway at Stathe and the confluence with the River Tone at Burrowbridge were dredged.
This scheme was led for the SRA by the Parrett IDB, working closely with the Environment Agency, Natural England, contractors WM Longreach and local specialist sub-contractors. The SRA used Growth Deal funding from the Heart of the South West Local Enterprise Partnership.
Exceedingly wet weather in November 2019 meant that a few tasks had to be postponed. In 2020-21 these included some final bank restoration and re-seeding, plus mitigation activities such as the planting of old varieties of apple tree, and erecting fencing.
Silt monitoring
The Parrett IDB has continued silt monitoring along the Parrett and Tone to inform the SRA’s dredging programme. The two rivers are divided – for the purposes of data-gathering and analysis – into a series of sections. Each section is 50 metres apart. There are 67 sections of interest on the River Tone, making a total of 3.35 kilometres (2.08 miles), and 243 sections on the River Parrett, totalling 12.15 kilometres (7.55 miles). Surveys are usually carried out twice a year, at the end of summer when silt deposition tends to have reached its annual peak, and at the end of winter when silt levels are low because of natural processes of scouring.
Techniques used 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.
The aim is to build up a useful and detailed picture of seasonal and year-on-year trends. In practice, not every cross- section of the Tone and Parrett is individually scrutinised twice a year. Attention is focused on priority areas, for pioneer dredging or maintenance dredging. For example, the Parrett IDB now has regular, consistent data for the 102 sections (5.1 kilometres or 3.17 miles) where maintenance dredging is targeted.
Flux monitoring is also important. This measures levels of turbidity, which means in very simple terms how clear or cloudy a river is, in other words how much sedimentary material has been washed into a river or stirred up. Knowing more about where sediment comes from and where it is going – or not going – helps the SRA to understand where dredging should take place, to what degree. Flux monitoring has been performed at New Bridge Sluice on the River Tone and at Oath Lock Sluice on the River Parrett. Much of this work has been commissioned by the Parrett IDB, for the SRA, from the local marine environmental scientist Dr Rob Nunny.
The SRA’s long-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 collaborations with scientists.