SRA End of Year Report 2017-18: Main River Dredging
After the floods of 2013-14, the Environment Agency spent £6million on the pioneer dredging of 8km of the Parrett and Tone. Since then, Somerset Rivers Authority has funded more pioneer dredging (750m downstream of Northmoor Pumping Station in 2016) and three rounds of maintenance dredging between Burrowbridge and Westonzoyland Pumping Station (in late 2015-early 2016, autumn 2016, and December 2017). Maintenance dredging has been targeted at sections of the Parrett that were pioneer-dredged in 2014, where silt has built up most again along this tidal river. In autumn 2016, trials of water injection dredging techniques were carried out as part of the SRA’s development of a Dredging Strategy. The Strategy is establishing the most cost-effective methods, times and places for removing silt.
Silt monitoring is now carried out in spring and autumn to inform the SRA’s maintenance dredging programme. Works include topographic section surveys of the river channel and bathymetric surveys along the river length.
Arrangements are being made for the pioneering development and installation of flux monitoring equipment that measures silt movement in rivers in near real-time. Fixed location sensors will be fitted at New Bridge on the River Tone and at Oath Lock and Somerset Bridge on the River Parrett.
A note on Main Rivers and Public Sector Co-operation Agreements
Main Rivers are usually larger streams and rivers, but some are smaller watercourses of local significance. In England, Defra (the Department for Environment, Food & Rural Affairs) decides which watercourses are Main Rivers. The Environment Agency has powers to work on main rivers to manage flood risk. However, the Agency may enter into a Public Sector Cooperation Agreement (PSCA) with another risk management authority to enable works.
So, for e.g., if a PSCA is agreed with Somerset Drainage Boards Consortium (SDBC), SDBC can apply for environmental permits, seek the agreement of landowners and procure works on Main River, for the SRA. SDBC comprises the Axe Brue and the Parrett Internal Drainage Boards (IDBs).
Main River Dredging works listed in this section have been done on behalf of the SRA by the Environment Agency (using its powers) or the IDBs (via a PSCA).
Maintenance Dredging: Achieved: A Public Sector Co-operation Agreement (PSCA)* between the Parrett IDB and the Environment Agency allowed the IDB to let a water injection dredging contract to Van Oord UK, following the successful trials of this method in autumn 2016. Text continues after photo of Van Oord’s water injection dredging vessel Borr.
Water injection dredging was carried out day and night over 10 consecutive high tides during one week in early December 2017.
The works cleared around 32,000m3 of silt from a 5km length of the River Parrett downstream from Burrowbridge. The volume of silt shifted in one week was more than got taken out over a period of four months using conventional maintenance dredging methods in late 2015-early 2016.
A further tide was used for a trial run on heavily vegetated and consolidated silt berms not dredged in recent years, to test the effectiveness of water injection dredging for pioneer dredging. Results showed the method could be successful, but refinements would be needed.
Works also included bathymetric and topographic channel surveys to monitor the success of the dredge, along with environmental monitoring to assess its impact on water chemistry, fish and invertebrates.
Channel surveys and environmental monitoring activities are ongoing to ensure works have been effective and environmentally acceptable, and to inform the need for future dredging.
Pioneer dredging – Parrett: In July 2017 the Board of the SRA made a commitment to dredge the River Parrett upstream from Burrowbridge to Oath, as soon as a legally compliant and affordable scheme can be found.
Being achieved: A multi-faceted high level plan was drawn up, a project manager appointed and preparatory work got under way.
- Channel surveys to establish current channel profiles
- Habitat and species surveys, timed to cover key species
- Evaluation and optimisation of initial design, using computer hydraulic monitoring and information from surveys and investigations
- Review of draft contract documents
- Development of procurement arrangements
When all data is obtained, final contract documents will be issued for tender, with a view to works being done during autumn 2018.
Pioneer dredging – Brue: An initial technical partners’ meeting has been held for members of relevant stakeholder groups to identify different sensitivities and opportunities and agree an approach to flood and water level management in the Brue catchment. Full stakeholder consultation is now proposed.
Strategic Mitigation: Many projects funded by the SRA involve complex issues. An important part of the SRA’s purpose as a partnership is to bring people together in ways that seek to resolve those issues, so that delivering schemes becomes easier and cheaper.
In July 2017, the SRA Board agreed to fund the development of a Strategic Approach to Mitigation. Mitigation means works done to offset any unavoidably negative effects of projects. First, so as not to break the law (eg Habitat Regulations). Second, because Board members know that many people in Somerset (and beyond) love the county for its world-class landscapes and wildlife. So it makes sense to combine the best possible ways of reducing flood risks and protecting the environment.
Under the Habitats Regulations process, Flood Risk Management projects in Somerset need to be assessed both individually and in combination with other projects. Any environmentally damaging effects of projects need to be addressed through mitigating actions and in some cases, this process has been very time-consuming. The SRA Board agreed that more streamlined and generic ways of dealing with complex and demanding situations should be developed. Strategic Mitigation work is therefore being led for the SRA by Natural England, which has successfully been involved with more than 40 other similar approaches across the country.
The main aims of Strategic Mitigation in Somerset are to reduce costs and risks, to enable flood risk management schemes in the county to go ahead, and to secure a wide range of environmental benefits.
The work is expected to help SRA-funded schemes including River Parrett pioneer and maintenance dredging, River Brue pioneer dredging, Sowy/King’s Sedgemoor Drain Enhancement, Bridgwater Tidal Barrier and improvements to pumping stations.
Being achieved: The project began in late November 2017 with the establishment of a working group of key partners and stakeholders: Natural England, Environment Agency, Somerset County Council, Sedgemoor District Council, Somerset Drainage Boards Consortium, National Farmers Union, Country Land and Business Association (CLA), Farming & Wildlife Advisory Group South West, Royal Society for the Protection of Birds and Somerset Wildlife Trust.
Members agreed to focus first on:
1) Enabling Maintenance Dredging: The plan is to produce – by the end of August 2018 – a 5 Year joint Natural England / Environment Agency advice / protocol and associated consent to enable ongoing maintenance of the recently dredged profiles of the River Parrett and the River Tone. This will mitigate for and minimise direct in-channel impacts, enabling maintenance dredging to progress in a timely and cost-effective manner.
2) Enabling measures that will reduce flooding to be delivered, whilst ensuring greater certainty of A Resilient Wetland at the Heart of Somerset: The SRA Board wants to achieve better management of water during periods of wet weather, and better management of the natural environment (especially maintenance of the water levels necessary to protect the wetland environment). All key stakeholders have agreed that these two aspirations must go hand in hand. They would also like floodplains to be managed in ways that maximise the resilience of vulnerable farmland. This means, for example, supporting the adaptation of farms with land in vulnerable areas. (This ties in with SRA-funded projects on Maintaining the Resilience of Wet Grassland and Levels Land Trust).
Next Steps: The project team is waiting for detailed design and modelling data which describes the ‘in combination’ impacts of proposed dredging and river channel works. This is needed before work can be done with IDB engineers and members to develop an agreement which will provide certainty about the implementation of whatever water level management measures will be required to mitigate for flood risk reduction schemes that could harm legally-protected wetlands.
Preparatory meetings with NFU, CLA and IDB members have indicated support for this approach.
Other de-silting work
The SRA funds de-silting work on watercourses additional to those currently de-silted by the Environment Agency and IDBs, through, for example, the Agency’s existing Wessex De-Silt programme. The aim is to improve the ability of channels to carry water away from flooded and flood-prone areas and deliver it to pumping stations and outfalls more quickly. This reduces the frequency, duration and overall severity of flooding.
De-silting also adds value to watercourses as it reduces the risk of low oxygen levels in channels and of fish dying in small, heavily silted channels. It can reduce how often main rivers need to be weed-cut as part of routine Environment Agency work, which saves money and brings environmental benefits.
Additional de-silting and dredging
Additional de-silting/dredging – Not yet achieved: Sections of Decoy Rhyne, Division Rhyne and James Wear River, all in the Godney area and classifed as main river, were assessed for dredging/de-silting. Inspection showed it would be unsafe to use machines on banks for this work. This is because parts of the banks of all three watercourses are in poor condition, with water overtopping and/or percolating.
The Parrett IDB (acting under a PSCA with the Environment Agency) therefore intends to seek the SRA’s approval for spending money originally given for dredging/de-silting on bank restoration work instead. The argument being this would reduce local flood risks and enable safe access for future dredging/de-silting.
Mark Yeo (local sections): De-silting work at Rooks Bridge was deferred from 2016-17 for environmental reasons. When a site visit was made later in 2017, silt had cleared naturally, so no work was required at that time. Not yet achieved: The Mark Yeo will be inspected during 2018 to establish whether other localised silt problems need to be dealt with.
Wessex de-silt top-up
Wessex De-Silt Top Up: This addition to the Environment Agency’s ongoing Wessex De- Silt programme was approved by the SRA Board in 2016 after five other schemes were withdrawn, moved or underspent – so their funding could be re-allocated. A lot of de-silting work was deferred from late 2016-early 2017 for environmental reasons – in several places because dissolved oxygen levels in the water were too low.
Achieved: 1) In 2016-17, the Environment Agency dredged 3.5km of the Westport Canal in Hambridge. The SRA funded the dredging of the remaining 700m in September 2017.
2) Early in 2017, the Environment Agency cleared 5,000 tonnes of silt from Witcombe Bottom, immediately upstream of Long Load Pumping Station, to enable more efficient pumping. The silt was stockpiled locally for use in building up low banks of the River Yeo. Geo-technical analyses checked the material was right for the job, and with further suitable spoil hauled from Westport Canal, improvements to Long Load flood banks were duly made by contractors Land & Water, for the SRA. Text continues after photo of Long Load bank building work.
Better banks will help to reduce flooding and disruption caused by road closures. Checks to see whether the banks had been affected by settlement of material were made in spring 2018. Some repairs to access routes are expected to be done by summer 2018.
2) Isle Brewers Mill Stream was de-silted by the Environment Agency for the SRA in September 2017. The stream’s banks will be re-seeded, if required, in summer 2018.
3) Huntworth Brook was de-silted by the Environment Agency, for the SRA, in autumn 2017. Huntworth Brook feeds the Screech Owl outfall into the River Parrett, where the SRA has also funded the installation of a powerful new pump (see the Dredging and River Management section of this report). Text continues after photo of Huntwork Brook de-silting.
4) Bridgwater: Hamp Brook and Stockmoor Rhyne were de-silted by the Environment Agency for the SRA in autumn 2017, and ugly masses of dumped rubbish were also removed from Hamp Brook. Along Hamp Brook and Stockmoor Rhyne, the Environment Agency is only usually funded to deliver the annual maintenance activities of weed removal and vegetation management. This year’s extra SRA-funded maintenance means that more water can now get more quickly via Stockmoor Rhyne to Stockmoor Pumping Station for evacuation, while Hamp Brook can now discharge more water more quickly into the Parrett between tidal cycles. In both cases, flood risks are now reduced. The banks of Stockmoor Rhyne will be re-seeded, if required, in summer 2018.