Part 3a of SRA End of Year Report 2016-17: Key Projects

Contents – quick links

Dredging
Sowy / King’s Sedgemoor Drain Enhancement
Bridgwater Tidal Barrier
Taunton Strategic Flood Alleviation Improvements Scheme

Dredging

1) 750m of Pioneer Dredging on the River Parrett

One of the earliest achievements of 2016 was the pioneer dredging of the first 750m of the Parrett downstream from Northmoor Pumping Station towards the M5 and Bridgwater. This was a complex job. Due to the width of the river, narrow banks, poor access, and the dangers of high-voltage overhead cables, most of the dredging (600m) was done using a pontoon-mounted excavator on the river (see below).

As dredged material could not be disposed of directly on to the existing banks, it was placed in barges, transported by tug to Westonzoyland Pumping Station, unloaded into tractor and trailer, then incorporated into adjoining farmland as a soil conditioner. A shorter length (150m) of dredging was undertaken from the bank next to the road leading to Moorland.

The work was given the go-ahead by the Board of the SRA in October 2015. Money came from the SRA’s funding for 2015-16, and from Heart of the SW Local Enterprise Partnership’s Growth Deal fund. Following the Board’s vote, the Environment Agency – on behalf of the SRA – awarded a ‘design and build’ contract to Galliford Try, Black & Veatch and Land & Water. These companies worked on the 8km dredge of the Parrett and Tone in 2014. The Environment Agency oversaw and managed the contract. One notably successful aspect of the 750m dredge was the way it followed on seamlessly from SRA-funded maintenance dredging of 2.2km of the Parrett, upstream of Northmoor Pumping Station, that was carried out for the SRA by the Parrett IDB with contractors WM Longreach before Easter 2016.

Generally, the 750m dredge only removed silt from one side of the river. The aim was to achieve a cross-sectional area of around 70m2, as with the 8km pioneer dredge and the 2.2km maintenance dredge. Contractors worked around existing hard flood defences so as not to reduce their stability, and avoided environmentally sensitive areas and protected species.

More than 13,000m3 of silt were removed between April – August 2016. The 750m dredge was delivered on time and well within the original budget of £2.14m.

2) Dredging strategy, including trials, silt monitoring & dredging the River Brue

When the Board of the SRA approved the 750m pioneer dredge, members also agreed that more cost-effective dredging techniques and sites should be investigated. A Dredging Strategy Project Board was then set up, and consultants from HR Wallingford produced a ‘Dredging Opportunities Report’, which recommended:

a) Further detailed modelling of the River Parrett upstream of Burrowbridge to Oath and the River Parrett downstream of Northmoor towards the M5

b) Review of proposed dredging locations on the River Brue

c) Trials of potential agitation dredging techniques

d) Monitoring (channel shape and form)

The Dredging Strategy Project Board also asked that:

e) Further work on environmental assessment and benefit assessment should be carried out before the strategy could be considered complete.

Achieved: a) Additional River Parrett modelling sensitivity tests have been undertaken.

c) Initial ‘Dredging trials’ undertaken. Three weeks of dredging trials in November – December 2016 were carried out for the SRA by the Parrett IDB between Westonzoyland Pumping Station and Burrowbridge. The trials carried on from where 2.2km of maintenance dredging was finished in 2015-16 by the Parrett IDB for the SRA.

The techniques tested were water injection dredging and agitation dredging: both aimed to lift silt from the bed of the channel and move it away from critical locations using natural forces in the river. Timing was crucial: work had to be done when an outgoing tide could carry silt away. While contractors Van Oord have successfully used these hydrodynamic techniques in marine locations all over the world, these SRA-funded trials were the first time that these two techniques have been tried out in the UK on a tidal river in conjunction with a sophisticated and long-term monitoring programme, so as to fully understand where silt goes and how effectively it is dispersed.

The trials produced promising results. Water injection dredging is capable of moving large volumes of material from the bed of the channel very quickly indeed. Agitation dredging is slower, but may be more precise. Overall, the potential benefits of using new hydrodynamic dredging techniques are:

  • cheaper
  • more efficient
  • better for the environment
  • less trouble for residents
  • less impact on farmers

d) The SRA has been working to identify places where silt builds up – how silt builds up – and what type of silt it is – so that maintenance work can be better targeted and dredging activities optimised. HR Wallingford’s Dredging Opportunities Report recommended the use of contemporary survey technology. The shape and form of the dredged Parrett and Tone have been measured regularly since the 8km pioneer dredge was completed in 2014, and an excellent data set is being recorded and used to identify trends. As HR Wallingford recommended, Single Beam and Multi Beam Bathymetric Survey of the channel bed, combined with Laser Scanning of the banks, were trialled with positive results.

e) A full Environmental Impact Assessment (including Water Framework Directive and Habitat Regulations Assessments) was completed for the dredging trials. Water quality was continuously monitored throughout the trials and river habitats and geomorphological features surveyed and monitored before, during and after. Only transient minor environmental effects were recorded. No significant damaging impacts were observed. Work on environmental assessment and benefit assessment continues.

Not yet achieved: b) Brue dredging review. The SRA’s original intention was to dredge a 4km stretch of the River Brue between North Drain Pumping Station and Westhay, to achieve better conveyance of water. However, HR Wallingford recommended further investigations and modelling, as their research suggested this might not be the best place to dredge. IDB engineers and their advisors reviewed the Environment Agency’s River Brue Hydraulic Model to determine if it could be used for dredging investigations, to the level of accuracy required. Discussions about improving the model are yet to conclude. Brue work has also suffered from limited IDB resources and a greater initial focus on the Parrett and Tone.

3) Maintenance dredging on the River Parrett

Achieved: After the dredging trials, the Parrett IDB asked the SRA if works could go on for another two weeks, upstream to Burrowbridge itself, in a wider range of conditions, so as to maintain sections of the 8km that was pioneer-dredged in 2014. The Parrett IDB were encouraged by early analyses of data, and observations of silt movements along the river.

Costs fell well within budget. In total, more than 8,000m3 of silt was moved.

Sowy / King’s Sedgemoor Drain (KSD) enhancement

A key part of Somerset’s 20 Year Flood Action Plan, a series of actions designed to improve the entire River Sowy/KSD system while balancing a range of interests. This key project will reduce flood risk over a wide geographical area, maintain access, increase resilience, respect the local environment, and promote business confidence.

In 2016-17, the SRA and its partners followed up on works done after the floods of 2013-14. For several weeks in early 2014, Beer Wall on the A372 near Othery was impassable because of flooding. Somerset County Council repaired the road, raised it by 60cm, then – underneath it, in 2015 – installed four massive culverts.

Achieved: 1) Beer Wall. The Environment Agency, working with contractors Skanska for the SRA, increased the capacity of the Sowy/KSD system by creating two new river channels under the A372. These branch off from the Sowy and Langacre, pass under Beer Wall through the new culverts, then reconnect downstream. The channels and culverts more than double the amount of water that can flow underneath the road. (Below: Work on new channel at Beer Wall. Photo by Charlie Granger).

Work also included installing concrete piled walls, supporting piles and a base slab framework for two new tilting weirs. Dutch engineers were commissioned for the specialist job of fabricating and installing the tilting weirs in the two new channels, to get greater control of upstream water levels. The new structures can be controlled remotely by the Environment Agency. (Below: Beer Wall, Summer 2016. Photo by Hendrik Robinson of Arcadis).

Better river gauging equipment and new CCTV cameras have also been installed to improve flood and water level management, and maintain appropriate environmental conditions within an area of international importance for wildlife (chiefly, migratory birds). Other features include innovative, combined passes for eels and otters, and platforms which allow disabled people to go fishing.

The SRA used Growth Deal money from Heart of the SW Local Enterprise Partnership.

2) Chedzoy Flap. The Environment Agency, for the SRA, improved Chedzoy Flap, which controls the confluence of the Penzoy system (including Chedzoy New Cut) and the KSD. The new structure prevents water entering the Penzoy river system from the KSD during normal operation, so better protecting farmland around Chedzoy and Andersea.

3) Dunball. Somerset County Council removed obstructive masonry (popularly known as the “lump of concrete”) from beneath Dunball Old Bridge to improve the capacity and flow of water through the final stretch of the KSD; the Environment Agency is considering upstream channel widening works in Autumn 2017 to help smooth flow under both Dunball A38 bridges and so maximise the benefit of removing the “lump”.

4) Parchey & Dunball. Vegetation cleared around Dunball Rail Bridge and Parchey Bridge. Surveying was also done to see if de-silting work would be beneficial. It is planned to undertake de-silting at one or both of these sites in 2017-18.

5) Channel widening. A programme to increase the amount of water that can be evacuated through the Sowy/KSD system, thereby relieving pressures on the Parrett and Tone. Enabling the Sowy system to be used more flexibly also means that upstream and downstream pumping stations can be operated earlier. Increased rates of pumping from moors in a large-scale flood will benefit many places affected by the events of 2013-14 such as Langport, Muchelney, Thorney, Moorland and Fordgate.

It is challenging across 20km2 of an already complex landscape to understand impacts on environmentally sensitive areas (which have legal protection) and on landowners, and to bring different interests together, but much progress has been made in getting a legally compliant scheme that can go out to tender.

Bridgwater Tidal Barrier

Several studies have identified a Bridgwater Tidal Barrier as the best long-term solution for the protection of approximately 10,000 properties and over 600 businesses for the next 100 years or more. Somerset Rivers Authority is using Growth Deal money from the Heart of the South West Local Enterprise Partnership to support the initial development stages of the Barrier project. The aim is for a Barrier to be working by 2024. It is a key element of Somerset’s 20-Year Flood Action Plan, which is overseen and co-ordinated by the SRA. The Bridgwater Tidal Barrier project is being developed by the Environment Agency and Sedgemoor District Council, working with consultants from CH2M.

Being achieved: A site on the River Parrett between Express Park and Chilton Trinity village has been selected as the optimum location for a Bridgwater Tidal Barrier. The Environment Agency and Sedgemoor District Council project team also decided that the best design for the Barrier would be a structure with two vertical lift gates. Two gates give the best flexibility and reliability for operation generally and allow for continued navigation during maintenance. These choices followed two rounds of public consultation in 2016. The site selected is known as Site 5. Judged against Site 4 – which is about halfway between Express Park and Dunball Wharf – Site 5 was found to have seven factors in its favour, as a location, such as the greatest confidence of getting permission for actually building a Barrier, and lower construction and maintenance costs.

Taunton Strategic Flood Alleviation Improvements Scheme (TSFAIS)

Taunton urgently needs strategic flood alleviation improvements. Defences built in the late 1980s came close to being overtopped in 2000 and 2012 and forecasts for climate change suggest a return to the kind of severe flooding seen in 1960 unless action is taken. In 1960 more than 360 houses, shops and businesses were flooded.

TSFAIS is a crucial component of the pioneering Taunton Garden Town initiative. As well as protecting existing properties, it will allow planned development – around 4,350 new homes and nearly 10,000 new jobs – to happen safely.

TSFAIS proposals include a Bradford on Tone flood detention reservoir, improvements to Taunton town centre flood defences, possible works at French and Firepool weirs, and further options for Taunton town centre and Bathpool. The detention reservoir will work by ‘storing’ up to 1.8 million cubic meters of water in the upper catchment of the River Tone, above Taunton, in times of flood, releasing this in a controlled and gradual fashion and reducing peak water levels downstream. The reservoir would only be used during flood events: generally, it would be dry and could be maintained for agriculture. SRA funding is helping the Environment Agency and Taunton Deane Borough Council, as partners, to develop a scheme for which major capital funding bids will continue to be submitted.

Being achieved: Consultants appointed; extensive flood modelling work carried out. A draft Single Options report has been submitted to Taunton Deane Borough Council and the Environment Agency and is being assessed. It provides information on the technical performance of different options and whole-life costs of construction for 1) flood water storage and 2) increasing the height of existing flood defence walls in Taunton town centre. Environmental, site and ground investigations have begun, although lack of access to a key area of land at Bradford on Tone is delaying the flood storage option. Legal advice is being sought about getting the necessary powers to secure access.

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