After the terrible flooding on the Somerset Levels in 2014 the Somerset Levels Flood Action Group successfully campaigned for dredging to recommence on the waterways of the River Parrett and Tone.
Dredging on the Somerset Levels has made a "real difference" in preventing flooding, a campaign group has claimed. Last year, the Environment Agency dredged five miles (8 km) of the rivers Parrett and Tone at a cost of £6m. As a result, the river Parrett levels in Langport this winter dropped much faster than in previous years, the Levels and Moors Taskforce said.
But dredging is not always seen as the answer.
Dredging rivers is not a simple answer to solve Cumbria's flooding problems, the county's top Environment Agency representative has warned. Keith Ashcroft, area manager for Cumbria and Lancashire, said dredging was often put forward as a solution but it was a "complicated" issue. Mr Ashcroft told BBC Cumbria the organisation conducted annual surveys to assess which areas would benefit from dredging and that in "certain circumstances" the process has a positive impact.
He said: "On slow-moving, relatively flat rivers that can make a significant difference. What we have in Cumbria are very high energy, steep rivers so it's a different situation. "We've seen huge amounts of material being moved around on the river system and have been very active in Kendal and Keswick and other places removing gravel so it will maintain the capacity of the channel. "The volumes of water are immense. Carlisle had 1,700 cubic metres of water coming through a second at peak. It was a record for England. "You'd have to move or dredge an immense amount of material.
Carlisle Flood Action Recommendation
Alan Cook (C. Eng., M.I.Mar.EST , plus MCIWEM ) CFLAG Projects Officer spent time working with NW Water between 1990 & 2010. He believes that there are areas of the Lower Eden which should be dredged in order to improve the flow into the Solway Firth. The overall aim is to slow the flow nearer to the source of the river but aim to speed up the flow where it discharges. We also have historical evidence from a widow of the engineer responsible for the management of the River Eden and associated rivers in the North West from 1948 to 1976. She remembers annual flood prevention works such as the construction of Gravel Traps which were dug by excavators, generally around bridges, at intervals along fast-flowing mountain streams to catch rocks and silt carried down swollen rivers during winter time, and cleared annually. She remembers walking along the River Eden around the Memorial Bridge in Carlisle a year or two ago, and saw that a great river had become a meandering weed-blocked stream, Rickerby Park resembled a water meadow.
Sediment Management taken from https://www.sepa.org.uk/media/151049/wat-sg-26.pdf
- Water carries sediment downstream gradually over time, particularly during high flows and floods, like a ‘jerky conveyor belt’.
- The natural size and shape of a river is balanced and has evolved over time to accommodate the amount of water and sediment moving through the system.
- River sediments and the movement of these sediments form habitats such as pools, riffles, bars and islands that are important for plants and animals.
- Sediment removal can lead to the loss of or damage to habitats, plants and animals.
- Sediment removal can disturb the natural ‘balance’ of a river and can cause serious problems with river stability (eg excessive erosion).
- Dredging rivers can increase flood risk downstream
Increased deposition after rare events
Rare flood events with very high flows can transport more sediment than more frequent lower flow flood events, leading to a sudden accumulation of sediment. Other events such as landslides can also lead to an over supply of sediment to the river system, leading to a sudden accumulation of sediment. These events are natural occurrences and, where possible, the river should be allowed to adjust to the change. However SEPA recognises that there are instances where the space to allow natural processes isn’t available, in which case sediment removal or similar works may be necessary. Your local SEPA office can be contacted for advice. See Section 6.4 on small scale sediment removal for localised problems