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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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  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 -

    Ian Moodie
    Technical Manager
    ADA (Association of Drainage Authorities)

  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.

  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.

  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

  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.

    • 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!

    • Replies to Rob Coleman>

      Comment by Vin posted on

      Exactly this point. I recall as a youngster in the 80's and to a point in the 90's, as I was always out fishing canals and rivers, that there was regular dredging and these floods were far less common.
      It's simple, if you dredge you increase capacity and volume of the waterway. With modern life and all that gets thrown in and discarded in waterways, along with the build up of silt and sludge, the capacity gets dramatically reduced, leading to waterways bursting their banks and these floods. The investment in dredging seems to have been largely removed. Building on flood plains has also not helped and that is widespread. So its been government and council mismanagement that causes this issue, not the rain and weather itself, and its not climate change either!

  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?

  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.

  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.

  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.

    • Replies to Rita Barnard>

      Comment by Mick Thompson posted on

      Totally agree , I live next to the trent where the derwent meets it and parts of these rivers where I have fished are so full of sand , silt and rocks they practically form islands that cause them to back up and flood even with a small downfall

  10. Comment by Phil Watkins posted on

    What about the boats that use the rivers. I navigate the river Ancholme in a 22ft Teal. The sand and silt is felt under my bow and kicked up by my prop. Heavy rains over winter has washed fine dirt into the river. I can not even get to Harlem Hill lock as that final half mile is too silted. River closed almost but still take my taxes.

  11. Comment by Felicity Botham posted on

    How can I find out the dates of the highest tides in the River Dee in Chester? I want to put some flood defences in place and the builder cannot do it until the end of October. Will my cellar be wet then?
    Thank you

  12. Comment by stephen isom posted on

    rivers in the Warwickshire area the up stream leam and itchen are full of trees blocking and stopping the flow ,now they are silted up full of reads from bank to bank the water is there somewhere ,why are we leaving our rivers to get into this mess,,flooding is worse not better as this has been used as an excuse to do nothing so cost nothing,the fishing has long gone and the wild fowl ,now the rivers look a mess,nothing is done,very rarely an ea personal will take a sample thats it ,where is the money going ,a question being asked more and more ,its the can,t be bothered attitude ,keep the money and do as little,as possible ,who looks after our rivers and why are they getting into such a mess,sewage is turning these once smashing rivers into green soup ,the fish have gone and the birds,some one needs to get out of their chair an walk these rivers and do a proper job and sought this out the N R A are doing zilch anglers licence money going on trout streams when most course fish,ext ext ext ,when will this matter be dealt with properly..

  13. Comment by Paul Durber posted on

    If we dredged and subsequently deepened our waterways in the floodplain areas at first we do 2 things:- the bucket to catch the water is significantly increased and the speed of water flow is also decreased If the environment agency don't have funds to carry this out maybe insurance company's could also contribute? Just a thought

  14. Comment by Ian Miller posted on

    It is our Green government intention to force us to believe it is all down to 'Climate Change'.
    Although last year when we voted 650 MP's into Westminster, only ONE green MP got in. voting for a Conservative PM how come - we got a GREEN one !
    Although we managed to free ourselves from the EU, we will still have to reclaim our democracy, sack unwilling MP's and enable us to dredge our rivers again. ( "DRAIN THE SWAMP" comes to mind)

  15. Comment by Ian Miller posted on

    I left a comment somewhat critical of our government having little regard for local democracy, which has been unsurprisingly disallowed.

  16. Comment by J B posted on

    I have lived in Chertsey by the Thames for over 40 years. We used to have dredging regularly but since this stopped when I understand the EU were against dredging, the river frontage by us is now at least 2 feet deeper of thick silt. Penton Hook Island has fallen trees and is silting up causing sand banks, making the channel narrower to Penton Marina. EA been made aware of this but nothing has ever been done.

    As locals who have lived here for all these years know what the impact of stopping dredging has caused. It's not only silt building up it's all the debris of green waste, tree stumps, branches and I've even witnessed trees floating past! On top of this there's all the plastic waste we see. Much gets caught up at the weirs but much of it sinks. Many a time I fish out rubbish and plastic with a net or rake out sunken branches caught up in the silt and dispose of it using my own bins.

    Up until the 90's, I used to watch the dredgers by us and all that was dredged out. Huge amounts of silt and rubbish came out. Now this is just left to build up, silt up, decreasing the Thames flood water capacity.

    Many with boats have had to take their jetty's further out so their boats can still be moored and not damage their propellers as it's become so much shallower over the years. This then causes their jetty licences to increase hugely, more money to the EA.

    Not only The Thames is not cleared and maintained but neither are surrounding ancient ditches and rivers. No one seems to care.

    The Burway Ditch/ Watercourse that once ran between the Thames and Abbey River, shown on all maps was passed and permitted by Runnymede Council and EA for the land owners of our local golf course at Retrospective Planning to be infilled, levelled and blocked. This ditch/ watercourse is still held as an Act of Parliament that was meant to kept 'scoured and cleared at all times'. This Act was ignored and allowed to be destroyed, infilled and blocked, all for the sake of a golf course, that has now since closed down! This was once a long wide ditch/ watercourse that helped drainage , flood water in our area and locals were shocked at it's destruction and being passed by Runnymede Planning and the EA and not made to be reinstated, even though a hydrology report was NEVER done. How could this have been passed with NO hydrology report. Not only did The Burway Ditch get infilled and destroyed but also many ancient drainage ditches on the golf course, dug by the Benedictine Monks from Chertsey Abbey. Total destruction on a flood plain between 2 rivers and permitted by the authorities without a hydrolgy report.

    The other river that many locals have campaigned to get cleared of debris is the Abbey River. This has been completely forgotten about and left to get clogged up with debris. We tried kayaking down it last year but gave up after unable to get past fallen branches/debris and too shallow from silt build up.

    Yes we do have climate change but this can't always be used as an excuse for flooding when it's also from the lack of maintenance, clearing of rivers, ditches and streams of silt build up and debris.

    We have experienced flooding and the heartache of the mess, smell and destruction it leaves behind. We've been flooded more since dredging stopped than we ever did the years before. Flooding not only destroys our lives but also the wildlife that I have seen decrease over the years.

    Worryingly as I type this, it looks like ourselves and our neighbours will be flooded once more as the river is rising drastically and coming up our gardens towards the houses. Another sleepless night of watching the river levels.

  17. Comment by Gary Alan posted on

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


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