Here is an updated version of the Environment Agency’s guide to How the Somerset Moors work.
The words, photos and graphics below show how water is moved around the Moors when the River Parrett and the River Tone flood. Rivers, Moors, spillways, sluice gates and pumping stations interact in complex ways.
Download a PDF of the 24-page guide to How the Somerset Moors work (5MB).
Alternatively you can read the entire contents of the leaflet on the page below. A few minor changes have been made to reflect the shift from print to online, for example in some of the photo captions.
Somerset was originally called ‘Sumersata’, which means ‘land of the summer people’. This is because the land was flooded though the winter and so its population only came down from the hills in the summer to graze their flocks. Its low-lying and flat landscape causes regular flooding even today.
The Somerset Levels and Moors consists of ‘The Levels’ (the higher coastal clay belt) and ‘The Moors’ (the low-lying basin into which the rivers overspill and flood). The lower part of the River Parrett flows through the Moors and its three main tributary rivers, the Tone, the Yeo and the Isle also flow through parts of the Moors.
The text and graphics below illustrate how the Parrett and Tone Moors flood and how water is moved around. The Brue and Axe Moors are not dealt with. It’s a complex interaction of embanked rivers, Moors, spillways, sluice gates and pumping stations.
A typical flood on the Somerset Moors
1. It starts to rain.
2. Water drains from the high ground and down the main rivers, which are the Tone, Yeo, Isle and Parrett. Where these rivers cross the Moors they act as “high level carriers” and in many places they are embanked (raised banks higher than surrounding land). At times of high flow their principal function is to take water from the high ground to the sea by gravity discharging into the Severn Estuary.
3. If the embanked rivers become full, they start spilling into the adjacent moors. On the River Tone, the Hook Bridge Spillway starts spilling into Curry and Hay Moors and they fill up together.
Similarly the Rivers Yeo, Isle and Parrett can spill into the Moors upstream of Langport Bridge e.g. Witcombe Bottom, Wet Moor, West Moor and Huish Level.
4. If the Parrett is bank full downstream of Langport, Allermoor and Beazleys Spillway start spilling into the Sowy River. As soon as Beazleys and Allermoor start spilling, we turn off the pumping stations at Long Load, Huish Episcopi, Midelney and Westover.
5. The capacity of the Parrett at Burrowbridge is affected by the combined flow of the Parrett and the Tone whose confluence is just upstream of the bridge.
6. When Beazleys and Allermoor stop spilling, we can open up Monksleaze Clyce and send flood water down the Sowy River along the King’s Sedgemoor Drain and into the Parrett at Dunball, downstream of Bridgwater. This allows us the capacity in the system to resume pumping at the stations upstream of Langport (Long Load, Huish Episcopi, Midelney and Westover).
7. As Curry and Hay Moors fill, they in turn start spilling into Northmoor and Saltmoor via the Lyng Cutting Spillway and then Athelney Spillway and then ultimately Baltmoor Wall.
8. As Northmoor and Saltmoor start filling, properties in Moorland and Fordgate are at risk of flooding. We then increase the pumping capacity of Saltmoor and Northmoor pumping stations with additional temporary pumps. If rainfall continues to impact the area, Bridgwater is at risk from rising flood water in Northmoor spilling northwards between the canal and the banks of the River Parrett.
9. If Hookbridge Spillway on the Tone stops running, we can start Currymoor pumping station to lower the levels in Curry and Hay moors which will stop spilling into Northmoor and Saltmoor.
1. If it continues to rain for a prolonged period of time, Northmoor and Saltmoor will continue to fill.
2. To allow the level in the Tone to drop, we need to lower the level of the River Parrett. To lower the level in the River Parrett, we have to send Parrett water down the Sowy Flood Relief Channel, into the King’s Sedgemoor Drain and out to Dunball. To do this, we need to open Monksleaze Clyce. Additional pumping may be required at Dunball to overcome tide locking.
3. As Northmoor and Saltmoor continue to fill, a barrier bank is required at Huntworth with additional pumping capacity. Water will spill into the Bridgwater Canal and additional pumping capacity is required where the canal enters Bridgwater Docks so that we can discharge this flow into the River Parrett.
Operational Flood Infrastructure
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The text, photographs and graphics above come from Version 4 of the Environment Agency’s guide to How the Somerset Moors work, updated in November 2021 by Edward Mellor, Melvin Wood, Antony Ryan and Adrian Govier.