Somerset Rivers Authority’s proposed enhancement of the River Sowy and King’s Sedgemoor Drain is intended to make the entire system work better. The scheme outlined here – designed by the Environment Agency for the SRA – ties in with numerous improvements such as work at Beer Wall, Chedzoy Flap and Dunball and the dredging of the Parrett and Tone.
The Sowy was conceived as a relief channel to the Parrett after severe flooding in 1960.
Athelney Inn during the flooding of 1960. Athelney was initially evacuated by boat. At least 50,000 acres of Somerset were estimated to be under water.
In 1961, it was proposed to cut a relief channel from Langport to Dunball that could carry 30 cubic metres per second (cumecs) – and widen the KSD. To save money at the time, this was reduced to 17 cumecs. Work started in 1969, and took three years.
A key aim then, as now, was to reduce the risk of flooding – particularly summer flooding – on moors upstream of Langport and some lower roads, moors west of the Parrett, and homes and farms.
Aller Drove Bridge was built so as to allow for the River Sowy system to be made larger in the future.
Though the Sowy was built smaller than first suggested, all associated sluices or bridges were built or modified so they could deal with 30 cumecs and the system could be made larger in future. The SRA is now picking up the possibilities left to us by an earlier generation, with ideas adapted to the 21st century realities of life in this precious part of Somerset.
In December 2014, the Environment Agency shared options for how extra capacity could be created in the Sowy-KSD system. Following a public meeting in Othery Village Hall on 10 December 2014, the EA collated views on which options people preferred.
Through 2015, the EA developed an outline proposal for the scheme, incorporating features that local people deemed to be a priority.
Although there was no clear preference for a single option, there was a slight preference for:
A: Enhanced operation – more water via Monk’s Leaze Clyse
B: KSD simple improvements at Dunball
C: Channel widening
Sowy-KSD Enhancement: Benefits
Considered to have the widest benefit in reducing flood depth and duration compared to all other flood risk projects in Somerset’s 20-Year Flood Action Plan
Greater operational flexibility to move more flood water through the River Sowy/King’s Sedgemoor Drain corridor before and after a flood
Reduces the risk of summer flooding - devastating to farmers and the environment
Extra capacity can be used to:
- Increase flow through the River Sowy without increasing river depth
- Reduce flood depth and duration over the Parrett and Tone moors
- Parrett and Tone levels drop faster = upstream pumping stations switched on earlier
- Shorten duration of overland flooding
- Improve evacuation of flows from the River Cary
Areas gaining flood relief benefit from an enhanced River Sowy system.
Flood water levels near Langport dropped by 0.5m within 2 days when excess Parrett water was diverted to the River Sowy in February 2014, making local roads passable
Impact of an enhanced scheme on the 13/14 winter flood:
- An enhanced scheme would not have prevented the widespread flooding in 2013/14, but would have reduced the impact:
- Reduction in peak moor flood level - Muchelney 600mm, Huish 130mm, West Moor 980mm, West Sedgemoor 950mm
- Reduction in flood duration (approx) - Muchelney 7 days, Huish 7 days, West Moor 7 days, West Sedgemoor >20 days
- Some downstream benefits for Currymoor and Northmoor – these would be greater in a Tone- dominated event
Better control of floodwater with more capacity and flexibility
Retains character of this valuable landscape
Retains the means to manage lower flows in summer (feeding wet fence rhynes)
Improves flood management whilst conserving wildlife and archaeology
The floodplain will continue to hold floodwater for short periods in routine floods
The images below are from early January 2016. Flooding of this scale will still occur routinely once works are complete, enabling farmers to continue claiming environmental stewardship payments.
River Sowy corridor downstream from Beer Wall (view to the north).
River Sowy corridor upstream from Beer Wall (view to the south). Both photos courtesy of Steve Rogers of Skanska.
Importantly, in January 2016 no roads or properties were flooded. Events of this scale are common in winter months.
Other projects being carried out as part of the River Sowy - King's Sedgemoor Drain Enhancement
Beer Wall enhancement (Phase 3)
First stage of works completed December 2015. During New Year flood, the site acted as designed, preventing the road from flooding. Stage 2 of Phase 3 will begin this spring, to be completed this year.
In stage 2, we will create new river channels and install control weirs to give enhanced capacity and operational control at Beer Wall; this will complement the future enhanced Sowy-KSD scheme.
Chedzoy Flap refurbishment
Replacement sluice mechanisms at confluence of Penzoy new cut and KSD. Design will be finalised shortly and works will be completed in spring this year.
KSD improvements: “lump of concrete” removal from under southbound A38 Dunball Old Bridge
Somerset County Council have begun SRA-funded works to reduce this feature (left over from the 1950s) and taper the river channel immediately upstream.
Temporary pump capacity now installed at Dunball. Estimated 15% increase in river volume discharged from KSD at Dunball due to temporary pumping facility and removal of “lump of concrete”. (Assessed for last 6 weeks of winter 13/14 flood event).
This is roughly equivalent to an extra two full Curry Moors being discharged.
The SRA’s Board has decided to approve more work on this scheme, using Growth Deal funding from Heart of the South West Local Enterprise Partnership.
The Environment Agency has been approved as the SRA's delivery partner and companies on the EA's list of 'Design and Build' framework contractors will be asked to tender for the work. It's expected to cost several million pounds; no more precise figure can be given until detailed designs have been drawn up and bids for the work received.
The EA will shortly begin seeking statutory approvals and talking to landowners.
Some works should start in late 2016, depending on permissions being obtained from relevant regulatory bodies. First moves are likely to be made at the lower end of King’s Sedgemoor Drain. The EA expects works to continue until the autumn of 2018.
The design team will include construction experts with knowledge of working in flood plains. The project’s delivery plan will be determined by matters such as:
- handling excavated materials
- traffic management
- potential impacts on land/landowners
- working with plant in isolated locations
- timing of works in environmentally sensitive locations
The tight timetable laid out below will only be achieved if all necessary permissions are granted with minimum challenge.
If more time is required for consultation or re-design, works would be very unlikely to begin before 2017.
Proposed schedule of works for the enhancement of the River Sowy - King's Sedgemoor Drain system