SRA End of Year Report 2017-18: Sowy / King’s Sedgemoor Drain (KSD)
Somerset’s 20 Year Flood Action Plan proposed to improve the entire River Sowy/King’s Sedgemoor Drain (KSD) system, while balancing a range of interests. The main aim was to increase the amount of water that could be evacuated through the Sowy/KSD, so as to relieve pressures on the River Parrett and the River Tone, and enable upstream and downstream pumping stations to be operated earlier. This would confer operational flexibility in times of flood and benefit places such as Langport, Muchelney, Thorney, Moorland and Fordgate.
Part of the impetus came from a dramatic experience during the floods of 2013-14.
In February 2014, after the Environment Agency decided to fully open up Monk’s Leaze Clyse sluice, at the head of the River Sowy near Aller, flood water levels fell by 80cm in two days.
The Sowy is a man-made river that branches off from the Parrett at Monk’s Leaze Clyse and after 12 km (7.5miles) joins up with the KSD.
It was conceived after severe floods in 1960 as a relief channel to the Parrett. Construction started nine years later (after a series of delays and difficulties) and took three years. To save money, the river was only built to 57% of the water-carrying capacity originally planned. But structures such as bridges were built full size so as to leave a future generation a range of possibilities for making further improvements.
A lot of important work has been done by SRA partners since 2014, and the emphasis now is on establishing plans for the next three years up to 2021. Plans are proceeding in conjunction with the development of other SRA-funded proposals for pioneer dredging and the Strategic Approach to Mitigation. The Somerset Levels & Moors are a highly complex landscape, heavily protected by environmental designations, and the combined effects of all schemes must be considered – and all schemes must be legally compliant.
Sowy/KSD works funded by the SRA have so far used Growth Deal money from the Heart of the South West Local Enterprise Partnership (HotSWLEP). Future works are expected to use more Growth Deal money, but sums have also been put aside from Local Partners (council tax and IDB contributions).
The Sowy/KSD project is led for the SRA by the Environment Agency, working closely with the Parrett Internal Drainage Board.
Sowy / KSD works in 2017-18
Major improvement works were completed during 2016-17 at Beer Wall on the A372 near Othery and at Chedzoy Flap. Achieved: In 2017-18, bank repairs were required at Beer Wall because high flows eroded newly-cut river channels before grass was established. (Grass roots help to bind soil together). A few concrete cracks were also repaired and some trees moved.
At Dunball, during 2016-17, 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 King’s Sedgemoor Drain. Being achieved: In 2017-18, the Environment Agency has been monitoring river flow to help assess the impact of this work. More channel widening is likely to be required to further smooth flow upstream.
Vegetation was cleared around Dunball Rail Bridge and Parchey Bridge during 2016-17, and surveying was done to see if de-silting work would be beneficial. Being achieved: Designs are being drawn up for bridge clearance work later in 2018.
Finalising a major programme of channel widening for the Sowy/KSD, across 20km2 of an already complex landscape, continues to be challenging. Being achieved: An outline scheme proposal was developed during 2017- 18 to deliver channel widening along the full length of the Sowy/KSD. The Environment Agency used this proposal to assess the constructability and costs. The initial cost estimate was significantly more than is available through the SRA and HotSWLEP. The Environment Agency and the IDB therefore undertook further investigations, to develop a scheme which is more affordable but still delivers the desired objectives of channel enhancements and increased capacity. Work is ongoing, with a revised Sowy/KSD proposal due to be taken to the June 2018 SRA Board meeting for consideration.