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Floods and dredging: explaining the Environment Agency’s approach

Posted by: , Posted on: - Categories: Flood, Flood Planning

two vehicles dredging a river

What is dredging and desilting?

Dredging and desilting are methods that we use to remove an accumulation of silt material, such as fine gravels or soils that have been washed into rivers from surrounding land in the catchment, from the bottom of rivers to temporarily increase the flow.

Desilting is usually undertaken more frequently to remove recent deposits of silt to the bed level of a river. We use dredging to describe removing a range of materials or for making the channel larger by removing the bed and channel side material. It is undertaken less frequently and is a bigger exercise.

In the UK, dredging and desilting have historically been carried out for a broad range of reasons: to drain land, maintain flows to mills, abstract sand or gravel for construction or to improve navigation, in addition to preventing flooding. In the past this has sometimes come with unintended consequences including increasing flooding downstream.

Dredging and desilting are not as effective at reducing flood risk as other options, are often expensive and can be harmful to the environment.  Therefore the level of dredging and desilting has decreased in the UK over recent decades.

Does dredging and desilting prevent flooding?

When used with other flood risk management measures as part of a catchment based approach dredging and desilting can be effective and justified.   In the majority of cases, they are not the most efficient or sustainable ways of reducing flood risk and may actually increase flood risk to downstream communities. .

Natural processes in many rivers means silt will return and accumulate in the same places very quickly, sometimes only weeks after dredging and desilting is carried out, therefore any increase in channel capacity will be short-lived. This is particularly evident in tidal rivers with each tide bringing in new accumulations of silt.

Why doesn’t the Environment Agency do more dredging?

Dredging and desilting are an important part of our river maintenance programme, which is why we spent around £5 million on these activities in 2019/20. This is in addition to the approximately £40 million we spend on other works in channels such as weed clearance and blockage removal to keep them flowing.

We consider each location carefully and dredge where we know it will make a difference to reduce flood risk. We assess each situation individually to understand the effectiveness, sustainability, environmental impact and value for money that dredging and desilting will provide.

Where we conclude that dredging and desilting is economically viable, will not harm the environment and will reduce flood risk, then we will undertake it

What is the impact on the environment?

Dredging and desilting can have serious and long lasting negative impacts on the environment. For example, it can damage or destroy fish spawning grounds and make river banks unstable. Silt can become suspended in the water, lowering oxygen levels, potentially releasing harmful chemicals that may be present. This, in turn, impacts on wildlife, and water quality downstream. The silt that has been removed from rivers can be difficult to dispose of, particularly where it is contaminated due to the historic industrial activity on the lower catchments of our rivers.

Before we undertake dredging and desilting activities we make sure the work will not have any negative impacts on the environment, water quality or flood risk elsewhere in the catchment. We also design and undertake the work in a way that improves the river habitat or if that is not possible, minimise any impact as far as we can.

How does the Environment Agency reduce flood risk in places where it doesn’t dredge?

Taking action across entire river catchments is  a much more effective and efficient way to protect communities and increase their resilience to flooding.

The most visible measures include flood walls, embankments, and demountable and temporary barriers, which help to contain water within river channels. Our strong track record on delivering new defences means that 314,000 homes are better protected since 2015, and the delivery of a record £5.2 billion investment in around 2,000 new schemes across the country will see a further 336,000 properties better protected over this current funding period.

Communities are also protected by other flood risk measures such as storing water upstream and slowing the flow through natural flood management measures such as leaky dams. We create flood storage areas to temporarily store flood water on land that will more readily recover from flooding (such as parkland) to prevent flood water moving downstream too quickly and flooding communities.

Working with partners, we also carry out activity in the upper catchments of rivers, restoring peatlands, planting trees, and building leaky dams, all of which contribute to slowing the flow of water into communities further downstream. In urban areas, we work with local authorities and other partners to promote sustainable drainage systems, which can include the creation of ponds and green spaces to help soak up rainwater, rather than it flowing quickly into drains and rivers.

Across the country we work closely with other Risk Management Authorities such as local authorities and Internal Drainage Boards to manage flood risk. However, even with these measures and the Environment Agency’s strong track record, we cannot prevent flooding all of the time. That’s why we need people to know their risk.  So we encourage everyone to check online if they are at risk of flooding, sign up for flood warnings, and, if they are at risk, know what to do when flooding hits.

Householders and business owners can also take other action to protect their property, such as installing Property Flood Resilience (PFR) measures. These can include flood doors, airbrick covers, and flood-resistant coatings on walls, helping to either keep water out of properties, or allow a quicker return to normality after flooding.

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  1. Comment by Gary Alan posted on

    Interesting read.
    When will DARTFORD CREEK be dredged?
    very built up tidal mud flats and silt / debris / rubbish

    • Replies to Gary Alan>

      Comment by S.J. Metcalf posted on

      Luckily, I live on a hilltop, but feel so sorry for the people who have been flooded out of their homes. In the past, the farmers dredged the rivers, but that was stopped by some well-meaning university know-all. Or was it stopped by the EU? Will you please tell the public what is the truth.

      • Replies to S.J. Metcalf>

        Comment by Robert Chambers posted on

        I entirely agree and I understand dredging stopped by both theoretical so called 'experts' who will not consult or listen to very knowledgeable and practical Country folk.Thanks and well said. See my blog below.

  2. Comment by Peter Bates posted on

    Surely flooding and high river levels cause a massive impact on wildlife and residents living alongside the rivers of the uk , and at one time river beds were a hell of a lot deeper cause of the huge size of boats and vessels that used our rivers in the past !! So all that build up that has shollowed our river beds needs to be removed to allow this huge amount of water that we can’t cope with over and over again?

    • Replies to Peter Bates>

      Comment by Robert Chambers posted on

      Entirely agree, see my blog below

  3. Comment by Fiona Hampson-Searle posted on

    Peter Bates

    I am able to provide a cost effective and tried and positively environmentally friendly process in order to desilt water bodies and return desilted water back to the source.

    Unfortunately I am stopped at the gate post each time before I can present my method.

    Are you able to assist me?

    Best Regards

    • Replies to Fiona Hampson-Searle>

      Comment by Anastasia Cockerill posted on

      Fiona Hampson - Searle,

      I would be very intersted to hear this process if you'd have some spare time?



  4. Comment by Fiona posted on

    Dear Anastasia Cockerill

    That would be a pleasure. Can you give me some background and your interest in environmental desilting.

    Where are you based. Currently I am based in Holmfirth on an assignment.

    Kind regards

  5. Comment by Conor posted on

    Let’s say I own a piece of land which has a stream running though it and their is silt and plants growing in it what causes my land to flood, am I then not able to dig it out to stop the flooding issue which their wasn’t before the silt and plants were their.

  6. Comment by John Dyer posted on

    Our local river Witham is a canalised between Lincoln and Boston, that’s a 31 miles stretch of river with a lock at Lincoln, Barney and Boston. I’ve been a regular user of the river for the last 17 years. In that time I’ve never seen the river being dredged. The river is now quite shallow, fish spawning sites have been added in places increasing the silting. We now experience a massive build up of weed as soon as air and therefore water temperatures rise. Once again we are seeing dead fish in our river possibly due to a blanket of weed, very low water flow and shallow warm water. We have swans along the river, they also struggle with the thick weed. I realise a deeper river would not increase water flow but it would not warm up so quickly, a greater depth may reduce the weed issue which may be reducing the oxygen and causing so many fish to die. We have fish spawning sites along the river assisting breeding fish, the fish struggle to survive summer months.

  7. Comment by Robert Chambers posted on

    I am very annoyed about the lack of dredging which would significantly increase the holding capacity of rivers back to what they used to be and also slow the flow rate which is an established fact in drainage. (This is why up to 5 houses combined sewers/drains all feed into a 100mm pipe!). All Country folk such as Landowners and Farmers have seen rivers silting up and the banks closing in vastly reducing the holding capacity to such an extent that multi arch bridges are often only flowing through the central arch and we wonder why it overflows! When will the theoretical, so called 'experts' listen to experienced country people? There is a well known saying that 'a bit of practise destroys a lot of theory' and this is a case in point. We know this all started with the EU, Baroness Young in particular, at the Somerset levels which flooded after dredging and pumps were stopped to build a £30m bird sanctuary, however, after the river was then dredged at £1m per mile, the problem was largely solved. The EA say that dredging will damage wildlife but it didn't appear to do years ago as wildlife has allegedly declined since! Surely the first priority is to protect the tax payers who pay their salaries! I recently sent an article to the Derby Telegraph, which was published on November 6th 2023, outlining my concerns. I also sent a copy to my MP and requested it to be forwarded to the EA and any other appropriate Departments but his was rebuffed and I was referred to the above article by the 'experts'! I wonder if they ever get flooded?


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