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https://environmentagency.blog.gov.uk/2020/02/21/floods-and-dredging/

Floods and dredging

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

A guest blog from the Chartered Institution of Water and Environmental Management's (CIWEM) Director of Policy, Alastair Chisholm.

Floods and dredging

When major floods occur, there is a commonly heard call to "dredge the rivers!" as a solution. But is it?

Dredging the Somerset Levels

In the winter of 2013-14, and the preceding winter, the Somerset Levels experienced serious flooding as a result of successive winter storms bringing extreme levels of rainfall to the region time and again.

Large areas of the Levels were inundated as river embankments overtopped. It took weeks, stretching into months for the water to drain away, leading to calls for far more extensive pumping and dredging to get rid of the water more quickly.

Ultimately the Environment Agency dredged the rivers Parrett and Tone and the Somerset Rivers Authority undertakes frequent maintenance dredging to maintain larger channels. The Levels have not experienced such serious flooding since.

Research at the time indicated that given that succession of storms, dredged rivers in Somerset wouldn’t have prevented the floods. Any additional capacity and conveyance created would have been far outweighed by the amount of water that fell and flowed down the catchment. What dredged channels would have done – and will do in future – is reduce the duration that flood water sat on farmland causing considerable damage to crops and homes.

So, is it the answer elsewhere?

No. Most flooding has occurred in areas where rivers flow considerably faster than in low-lying areas like the Levels. Here such rivers suffer far less from sedimentation and often self-scour with the energy of the water mobilising sediment and preventing excessive siltation.

Expanding the channel in such places would be referred to more as channel engineering and would commonly be associated with the construction of hard defences through towns or cities.

Conveying water away more quickly in this manner is far from risk-free. Faster flows place greater strain on structures such as bridges or defences downstream and can be highly dangerous as we’ve tragically seen this winter. It speeds more water towards downstream communities even faster, potentially putting them at greater risk.

Much of how we’ve managed our landscape over recent decades – centuries even – has contributed to this rapid water conveyance and flood risk.

Allied to more frequent and intense rain events associated with climate change, more intensive agriculture characterised by compacted soils and fewer hedgerows, growing villages, towns and cities with their associated hard surfaces – which rainfall runs off rather than soaking into – are all collectively ushering rainwater into river systems more quickly.

There it flows, quickly downstream, in a flood peak. The faster water arrives in the main river, the steeper, and potentially higher that peak.

Slow the flow

That’s why flood risk managers are now looking to take every opportunity to ‘slow the flow’. This essentially means undoing the effects of the land management actions taken in the past.

It can be done through engineering artificial storage areas with quite hard structures, or working with nature to make maximum use of the highly effective ways in which it can hold back water.

In urban areas, sustainable drainage systems store water, usually in surface features such as ponds, channels, raingardens, green roofs or tree pits, or promote infiltration into the soil via soakaways. This is instead of directing rainwater straight into surface drains leading straight to rivers.

More extensive tree cover can slow the rate at which rain hits the ground, then once it reaches the ground their roots help it to soak in and as water flows down hillsides, fallen trees in streams or woody dams can store water in ponded areas. Wildlife such as beavers can help create these features.

Other habitats, such as upland peat bogs are also excellent at locking up carbon and similarly can store large amounts of rainwater high up in the catchment. They need to be restored and protected.

Even with these interventions deployed extensively, it is likely that we’ll still experience more serious and frequent storms with risk of flooding. More investment in new hard defences, alongside maintenance of existing ones, will always be part of the solution.

Yet still there will be ‘residual’ flood risk. Improving the flood resilience of more individual properties, using flood barriers and doors and more resilient construction techniques should be used more widely.

The exact combination of interventions is most appropriate in any given location will vary. In limited cases, dredging could be part of that picture, but it is a long, long way from the silver bullet that some suggest.

For a more in-depth discussion of this complex picture, read Floods and Dredging – A Reality Check.

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10 comments

  1. Comment by Ian Moodie posted on

    Agree with much of this blog by Alastair. It is true that dredging is not a silver bullet, or that it would help everywhere and we must be led by the science in our response to managing flooding and our changing climate. But there are parts of our lowland catchments where we do need to consider & manage channel conveyance for both people & environment as part of our flood resilience toolkit.

    Sensitive targeted dredging, and indeed controlling aquatic vegetation, when undertaken within a considered package of measures can have positive impacts. It can improve channel conveyance that reduces flood extent/duration & can enhance biodiversity. For instance within slow-flowing watercourses & pumped systems in lowland landscapes modified by man over many centuries, such as The Fens, Somerset Levels, or the Netherlands.

    Silt removal by Black Sluice IDB and Royal Smals Dredging provides a good example from a main river in Lincolnshire where work considered its impact & was carefully monitored - https://www.ada.org.uk/2017/08/video-suction-dredging-work-south-forty-foot-drain/

    Ian Moodie
    Technical Manager
    ADA (Association of Drainage Authorities)

    Reply
  2. Comment by Giles A Crust posted on

    I am Chairman of LMDB, The IDB area containing Wainfleet the town flooded in June this year. All around our area are the banks erected by the Romans and the dykes built to drain the fens by the Dutch. This area contains huge growers of potatoes and vegetables. I am dismayed that on BBC television "inside out" a couple of weeks ago Norman Robinson spokesman for the environment agency was advocating managed retreat and giving Skegness, Mablethorpe, Wainfleet and all villages in between including almost all of the Lincolnshire tourism areas back to the sea. The farms and pack houses will all be lost together with thousands of houses factories and schools. All this at the same time they are spending millions of pounds relocating badgers and paying compensation to farmers in Essex to flood land for water bird reserves, Country file last Sunday. It is about time the people who pay rates and taxes to fund the EA are given at least the same protection as wildlife. For 40 years no work has been done on the main rivers to facilitate IDBs to evacuate their water to protect houses and farms. The answer is to give all the main rivers together with the funds to bring the rivers back to their design standards and allow the EA to look after the wildlife.

    Reply
  3. Comment by S D Peltell posted on

    After the Wainfleet Floods we were told that our drainage system would benefit from schemes other than dredging. We have 2 feet of water growing masses of sedge where we used to have 12 feet in places and free water. The outfall to the sea is silted up due to sand pumping onto Skegness beach.
    But still the EA refuse to commit to a dredge of the whole system.
    The drainage system flows approximately 15km falling 0.5m in that distance and is never going to desilt itself.
    The EA are failing the people of Wainfleet and surrounding areas who are living under constant stress and anxiety. It is our opinion that they do not care and hence are not fit to be in control of our drainage system.
    Stewart FLAG Chair.

    Reply
  4. Comment by Andy Horsley posted on

    "you can't fit a quart in a pint pot"
    ...and dredging won't double the capacity of any watercourse.
    Dear government,
    you seem to be having trouble,
    my consultancy rates are still at 2005 levels with incremental decreases for environmentally /community led projects.
    A.Horsley, S.Lakes

    Reply
  5. Comment by Rob Coleman posted on

    Back quite a few years in the area where i live we had a regular dredging boat up and down the Ouse river,local canals etc called the 'Goole Bite' as to why this was stopped iam unsure,however in my 40 + years ive never seen flooding this bad,surely this has something to do with it? it makes physical sense that if a river is dredged it can hold more water.

    Reply
    • Replies to Rob Coleman>

      Comment by Jay posted on

      Just saw this comment and had to reply on a side issue, I was once messing about on the dredger as a lad in the mid 70's when it was parked up in Goole docks and reaching overboard to get on a long boat painting raft nearby I fell in!! If it wasn't for a beat policeman (remember them) who had decided to follow me after chasing me out of the local coal yard near Goole Railway station then I doubt I'd be here. I'd learnt to swim but the oil in the water and my coat etc. was pulling me under and I couldn't pull myself onto the raft. Fortunately the copper got on the raft and pulled me out by my hood. A good scrubbing in the bath to get the oil off and several whacks awaited me at home...happy days!

      Reply
  6. Comment by Neil Hampshire posted on

    Wainfleet and maybe Fishpool seem to have the same problem as the Somerset Levels. Alistair creates the impression that dredging down in Somerset was initiated by the Environment Agency.

    Reading some of the articles on the internet I think they had to dragged kicking and screaming to dredge the rivers. What did those guys down in Somerset do to get the EA to agree to dredge their rivers?

    Reply
  7. Comment by Ian Cowley posted on

    About time somebody in Government got a ‘grip’ of the Environment Agency. There is no doubt dredging works, but when it is reintroduced - for example Somerset Levels - this is brushed under the carpet and we are told it won’t work anywhere else. What might work is shutting down some of the EA offices, redeploying the capital back into boats and dredging equipment and moving people out of the office and back into the field onto the shovel. SRA is showing it works, but EA still resistant to using simple methods tested over the years. One assumes nobody in EA lives in any area where rewilding is being encouraged or where dredging has been stopped.

    Reply
  8. Comment by John Spokes posted on

    I'm very dismayed that when you look into any river you see vast amounts af sand and large banks in the middle of rivers arch ways that allow the free flow of water under the bridges blocked up with sand and gravel.
    I wonder if this is the result from when The European Union stopped The British Goverment Dregging rivers This was Directive in 2005
    What The Goverment Does not Want You To Know
    It's Obious the huge amounts of sand banks we see in most rivers need to cleared to make space for the huge amounts of water that causes all the misery for so many people.

    Reply
  9. Comment by Rita Barnard posted on

    The river Trent where I live has been dredged in the past but not since the late 80s. Since then large sand banks have developed near Keadby and other areas. Surely if these are dredged then it gives the river more capacity thus requiring less areas for excess flooding water to be held. There have been more incidents of flooding along the Trent since dredging stopped and to say that dredging damages wild life is laudable but peoples homes and businesses and the cost of clean up and the loss of usuable farm land surely outweighs the problems for the wildlife which is also affected by floods.
    It seems that most of the reasons against dredging can be seen as financial.
    Having said all this I appreciate that global warming will reult in levels rising and more rainfall but knowing that does not mean we spend millions on defences when increasing the capacity of the rivers will reduce the risks.

    Reply

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