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https://environmentagency.blog.gov.uk/2021/12/23/floods-and-dredging/

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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27 comments

  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

    Reply
    • 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.

      Reply
      • 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.

        Reply
        • Replies to Robert Chambers>

          Comment by Dick Tristram Tooze posted on

          Agreed, SJ and Robert: see my contribution (of Jan 5 below) on the obvious dredging procedure.
          One objection the "experts" cite is: where do we put all that wonderful alluvial silt? The farmers among you might know - it might even be worth money!
          I've never understood why the Government charges a tax for "land fill". Even the rocks excavated by dredging (perhaps even making our rivers DEEPER than Nature did) are useful. People who live on our eroding coasts would be grateful for it to save their homes!

          Reply
  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?

    Reply
    • Replies to Peter Bates>

      Comment by Robert Chambers posted on

      Entirely agree, see my blog below

      Reply
    • Replies to Peter Bates>

      Comment by David Moulson posted on

      Completely agree. The river beds are now higher than the surrounding land in places, a disaster in the making. As the silt builds and the flow slows the deposition of silt increases. The temporary barriers will need to be higher every year. If the rivers had greater capacity the water would be held in them until there had been time for it to flow away. It would be better than holding the water in people's homes, which is what happens now. Does no-one have a conscience?

      Reply
  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
    FH-S

    Reply
    • 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?

      Best,

      Anastasia

      Reply
  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
    Fiona

    Reply
  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.

    Reply
  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.

    Reply
  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?

    Reply
  8. Comment by Andy Barnett-Picking posted on

    Why not get a petition rolling to force the UK government to mandate the clearing of the rivers to increase capacity. Like Robert I understand that the EU made it difficult for land owners and water authorities to dredge the rivers. We're out of the EU now so should be able to control our own destiny.

    It's common sense really. A river is a drain and if it becomes clogged or its capacity is restricted its will overspill onto the flood plain.

    One property of water is of course you can't stop it.build a wall or defence and it will go round it or over it. Maintain or increase the capacity of the drain is the proper solution.

    It's the 21st century and yet we can't seem to manage this ?

    Reply
  9. Comment by Dick Tristram Tooze posted on

    It's refreshing to see that all (or most?) of the above comments tell from experience that resuming dredging is the answer!
    It can't be more expensive than building unsightly barriers within vulnerable populated areas.
    The greater flow will obviously keep water tables lower throughout the year so that this nonsense about "nowhere for heavy rain to go" will be seen in the light of why rivers were created by Nature in the first place!
    The only valid excuse the Envoronment Agency gives (above) is that people down-river might be flooded. Well start dredging from the esturies then, and work your way up-river!
    Sorry to suggest the blooming obvious.

    Reply
  10. Comment by T Cropley posted on

    The reason there is flooding on our rivers is because the environment agency do not dredge rivers because they like to keep money in their bank not bothered about farmland and peoples homes

    Reply
  11. Comment by Robert Loomes posted on

    Stamford in Lincolnshire has a town bridge (1847) specifically designed for a dredged river with barge traffic.

    The wide basin above the bridge was a place for barges to turn. It is also where the fast running Welland slows right down and dumps its silt,

    The river was lazily allowed to silt up and barge traffic became impossible in 1867. Only 13 years later Stamford had a catastrophic flood.

    Since then the town, and later EA, have always dredged regularly. Until 2011. 13 years later we have just had the worst flood in my lifetime. Crackers.

    It is said dredging is not good for wildlife. But our whole town was built for river traffic and regular dredging.

    Reply
  12. Comment by Dick Tristram Tooze posted on

    The EA claims that much "desilting" only lasts a short time so must be repeated (imposing a high, regular cost).
    They must be in a quandary.
    The obvious way to clear the silt at NO COST is to fully dredge each river (from its estuary upwards) so that the more rapid flow sweeps it all out to sea.
    But environmentalists have pushed the opposite idea that flow should be SLOWED so that there is more time for high rainfall to drain gradually, even advocating swampy areas at the higher levels to "store" some of the water for later!
    Robert is latching on to this latter method by suggesting using the rivers themselves as storage reservoirs as he must expect only partial dredging – none downstream.
    But he does demonstrate that the EA has made the wrong choice! Couldn't they try the obvious method on just one river to test the principle?

    Reply
    • Replies to Dick Tristram Tooze>

      Comment by Lo posted on

      Good morrow Dick Tristram Tooze, I am replying to you on behalf of your concerns. I am entirely gutted by the effects that not dredging has had the community. Care to discuss more?

      Reply
      • Replies to Lo>

        Comment by Dick Tristram Tooze posted on

        Certainly Lo.
        When discussed in Parliament I am less than calm (Ted!) as no-one seems to mention DREDGING, bar one MP who used the word at the end of a list of solutions.
        Gary Alan's Darent might be an exemplar river to test the principle.
        Another (Man Made!) drain dug at great expense in the 1950s, the "Cut-Off Channel" intended to help farmers in western Norfolk and Suffolk by taking water from their drainage diches down to Denver Sluice on the Great Ouse, was apparently REVERSED to flow to I know not where, but must have been an early example of this erroneous storage concept!
        (Btw, I was referring to Robert Chambers' idea, not Robert Loomes'.)

        Reply
    • Replies to Dick Tristram Tooze>

      Comment by Philip Phillpotts posted on

      Just wondering Dick, you say "The obvious way to clear the silt at NO COST is to fully dredge each river (from its estuary upwards) so that the more rapid flow sweeps it all out to sea." If you're fully dredging the river from the estuary, at a cost of £1M per mile how is that "NO COST"? You also seem to not understand how siltation works. It's a constant process from water flowing over, and eroding, rocks and such. If you waved a magic wand and cleared all the silt out of all the rivers instantly, it would just build back up again. I'm afraid I'm with the EA on this one. Dredging is not a good use of tax payers' money. It's a sticking plaster, it's not treating the root cause of the problem.

      Reply
  13. Comment by H. Williams posted on

    Perhaps we should be trying to tackle the main issue here, which is where the silt is coming from. Massive amounts of soil are washed into watercourses because of unsustainable, intensive farming practices, like leaving arable fields with bare soil over winter. Maybe if we can tackle that, and keep soil on the fields rather than in the river, we won't even need to have a discussion about whether rivers should be dredged or not. Deepening and dredging rivers is not the answer.

    Reply
  14. Comment by Paul Millard posted on

    Saltfleet Haven has not been dredged since over 50 years by the boat club. That let the water drain away and stopped the risk of flooding to homes. Now bird watching is put before people's lives. The sea ward side of the river is just abandoned to silt up and access water cannot drain. Letting good land flood so they can shirk responsibility is a bad move.Canals were dug to stop flooding not for a few southerners to watch birds.

    Reply
    • Replies to Paul Millard>

      Comment by Jesse posted on

      Hello Paul, I think you'll find canals were dug to transport goods and materials, in the first instance. Any use as a flood defence was an afterthought.

      Reply
  15. Comment by David Lloyd posted on

    A schoolchild will know all of this – because it is obvious. So does everyone in Holland. A deeper wider river will carry more water away faster. A shallow, narrow river will carry less water.

    If we took the Thames (say) and halved its width and depth from the Thames estuary to its source in Kemble, it would carry one quarter of the volume of water and subsequently flood MORE often. BAD.

    Instead if we doubled the width and depth of the Thames, from the estuary to the source, it would carry 4 times the volume of water and flood LESS often. GOOD.

    It is obvious that the size of every single, non engineered river in the UK is ARBITRARY. It is defined by historic rainfall and erosion.
    But in these times of increasing rainfall, rivers are not permitted to widen and deepen, because there are bridge arches, houses, roads, fields, future housing developments etc, etc IN THE WAY.
    Also we humans have ruined the climate so rapidly, nature would not establish wider deeper rivers overnight and would take centuries (probably).
    So we need to get involved and fix the problem.

    If we humans effectively widen and deepen rivers (there are lots of ways to do this) then the extra capacity NEEDED in the UK will be available and we will see much less flooding. It will cost MONEY. The Dutch spend lots of MONEY to keep their country – which is below sea level – virtually flood free. Their amazing Water Control Boards actually know what they are doing.

    We in the UK need to solve this simple problem instead of just issuing weather warnings (which are pointless and stupid).

    This process will disrupt some wildlife. But, historic flooding in Holland effectively created the Zuiderzee. So nature can also disrupt wildlife. Wildlife can adapt.

    See the LA River for a great example of river engineering.
    We in the UK need bigger (wider and deeper) rivers all over the country. We need them yesterday.

    Reply
  16. Comment by PHILLIP posted on

    Remember George Osborne Conservative Chancellor of David Cameron's government. He promised " A BONFIRE OF USELESS QUANGOS " A pity he hadn't started with the ENVIRONMENT AGENCY , But alas the fire never got going !! .

    Reply

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