UKC

Anyone else found themselves shouting at the radio this morning? Lord Smith, chair of the Environment Agency in complete and utter denial that not dredging rivers and drains for 20 years is a contributory factor in flooding that still remains after three weeks.

WTF! How the hell do these people get these jobs?

Somerset levels is not a unique case. Other low lying reclaimed land hasn't flooded and guess what? It is regularly maintained and dredged. Go figure

In reply to woolsack:

I was doing this last night, but at the idiots on TV who thought dredging was the answer. De regulating drainage to the landowners is going to be a disaster, just when the IDBs were finally getting their act together.

In reply to toad:

Essentially allowing the landowners to organise and effect the dredging and maintenance of the water courses is exactly what has happened through the 'Lords of the Levels' of Romney Marsh for the last couple of hundred years and that has worked extremely well

In reply to woolsack:

They had different priorities. plus 2oo years ago people weren't daft enough to live that close to the water and not expect it to flood.

A combination of intensive agriculture and poor catchment management upstream, exceptional flooding and unrealistic expectations of what can be achieved. Dredging isn't going to provide anywhere near the capacity required to move this water.

It's an nice easy obvious "seen to be doing something" with no scientific credibility. Same as the badger cull in many respects, now I consider it

In reply to woolsack:
I'm going to be a little controversial here with the caveat that this is only to start a debate. (I feel desperately sorry for those who have suffered due to the floods; it must be awful).

Anyhoo, whilst there may be contributory factors such as a lack of dredging or other land management issues, perhaps those people whom decide to settle in these areas should stop blaming others and consider that the choices they made for their home had potential risks. I live near the Trent and we get massive floods once every few years and the flood plains are a sight to behold. The consequences of living in these lovely places should be weighed up against the risks and whilst the government and other agencies might have a part to play in what has happened, they cant always take full responsibility.

As I said, I'm interested in the debate on this and to hear other sides to the discussion, not just the understandable rantings and frustration of the poor folks who have been affected.

In reply to TheDrunkenBakers:

In the interest of the debate, not dredging for 20 years will have predictable results. I'll put money on that there will be a lot of machines out this coming year cleaning out 20 years accumulation of silt and reeds

In reply to TheDrunkenBakers:

Just after the 2000 floods I was chatting to a farmer who was flooded out near Collingham and he said it was just something that happened now and again if you lived there.

In reply to woolsack:
I'll put money on that there will be a lot of machines out this coming year cleaning out 20 years accumulation of silt and reeds

I'm sure there will be. What the practical benefits will be in terms of flood risk prevention are also predictable (and have been extensively modelled by the EA over the years). It will also be expensive and the cost will be diverted from rather more useful flood risk management projects, 'cos there won't be any significant new money.

In reply to woolsack: The main rivers on the Levels have earth embankments either side to retain water levels higher than the surrounding land and secondary drainage systems. The secondary systems drain under gravity to permanent pumping stations adjacent to the main rivers where water is pumped up to the higher level to drain out to sea. If the limiting factor is the ability of the pumps to move water from one level to another then dredging river or drain channels will not improve the situation.

So, what the EA say is plausible.

In reply to toad:

Somerset Levels. I wonder where that name came from....

In reply to TheDrunkenBakers:


+1. 5 second of looking at the area on an OS map tells you the natural state is under water. A more detailed look makes it look like drainiage is acive, via raised and embanked channels so it is pumping capacity and not dredging that's the problem. Should Jonny taxpayer suck up the capital costs of pumping kit in anticipation of an exceptionally wet year? No. I don't get exceptional help at tax payers expense to let me live on an idyllic but stupid place for a house.

If you live in a flood plain, suck it up and flood proof your house. Oh and don't go on TV wailing about how unjust it is and how something must be done by someone else.

In reply to woolsack:
I wonder how much is drainage improvements upstream or in upland areas resulting in faster runoff & flooding downstream.

In reply to wurzelinzummerset:

+1

A similar situation exists with the fens in Norfolk / Lincolnshire most of the very productive agricultural land has been 'reclaimed' by a series of man made raised embankments, and pumping stations (back in days of yore they were powered by windmills). It's well known fact that water cannot flow uphill unaided - and will naturally find its own level - if you pardon the pun.

In reply to toad:

Indeed, I live only a few miles away from Collingham. Its a lovely part of the world but if you decide its where you want to live, you better check the reports. There's a big river that flows nearby and from time to time it has a habit of letting humans know that we cant control it.

In reply to woolsack:

If you doubled the size of the main rivers you still could not accommodate the amount of. Flood water in the levels..

In reply to Dave Perry:


Removal of 20 years accumulated silt will increase flow. The EA is just trying to justify their cutbacks and lack of maintenance.

By comparison, 3-4 years ago the EA announced that the drains/dykes on Romney Marsh would only be cleaned on a three yearly cycle and there was uproar from the farming community who insisted that the annual cleaning which has worked for 50 years be maintained. No flooding here, same system of pumping from low level to larger channels and out to sea.

One system maintained and working, one neglected and problems

In reply to woolsack:

People in general and builders in particular are guilty of a lot of the problem. Building on flood plains and tarmac roads and even paving over a lawn or a patio are blocking up the normal drainage systems. Simples!

In reply to woolsack:

In Spain houses that were built in the dry summers of a few years back in dried up watercourses are now being washed away in the Gota Fria , which actually lasts much longer than it used to do, but actually it was predictable that come the first real storm the rivers would start flowing again.

In reply to woolsack:

You can call it what you like, but this is effectively about gambling. If you buy a house in an area prone to flooding, it will show up in your searches, but theoretically a one in a hundred year flood is just that: very unlikely. So you take the chance. Sorry, but the value of investments can go down as well as up.

Equally, if you are sec of state for the Environment, you will be in that role for maybe 5 years at most. You've got the eye on a more prestigious cabinet post, so you do what the PM asks - cut your budget. The chances of serious flooding happening on your watch are fairly slim, so you take that chance.

So lets say you dredge, and we get another 1 in a hundred year event in 6 years time. The levels still flood, but You aren't going to be in charge, so what the hell...divert flood defence funding. No one is blaming Justine Greening, are they? But she was the one that started to implement big cuts in the Environment budget, including flood defence.

Flood defence is also about the greatest good. Flood defence doesn't vanish water, it moves it around. for example, Part of the reason that the Trent example (see above) flooded was that Colwick flood defences stopped Nottingham flooding by sacrificing farmland and smaller communities downstream. The EA knows this, but I suspect the owners of the relatively low numbers of properties flooded on the levels haven't grasped that yet

In reply to elsewhere:


See this. Upstream retention of water is one possibility.
http://www.theguardian.com/environment/2014/jan/28/dredging-rivers-flooding...

In reply to Al Evans:

I've been to look at the storm drains and defences in Andalucia. It's a slightly different situation, because they are all about flash flooding in an otherwise arid environment. Having said that, the rains there are in reasonably predictable long cycles - the gov't knew what was likely to happen, but didn't spend the money to build to the standard required - another gamble that someone else would have to pick up the bill

In reply to elsewhere:


And this.
http://www.monbiot.com/2014/01/13/drowning-in-money/

In reply to toad:


It will?!

>Flood defence doesn't vanish water, it moves it around. for example,

See, eg, The Nine Tailors, by Dorothy L Sayers.

jcm

In reply to mockerkin:

That's an interesting article. It's a pity GM shouts so much about so many things; one doesn't know when he's trustworthy and when he's crazy.

jcm

In reply to mockerkin:

yes, this article is saying much the same

http://www.theguardian.com/commentisfree/2014/jan/13/flooding-public-spendi...
Basically adding woodland to significant areas of the central Welsh hills would retain huge volumes of water upstream from the Severn and the Avon

In reply to johncoxmysteriously:

did for me, but then my mrs did the conveyancing, so might have gone the extra mile

The maps are here

http://maps.environment-agency.gov.uk/wiyby/wiybyController?x=357683.0&...

In reply to toad:




Thanks, kind of validates what I said, they're closing the stable door after the horse has bolted



We had 100 year floods last year as well! Seem to have lots of these events yet still they cut budgets and at the same time planners sanction flood plain residential development spurred on by edicts from central Government calling for more housing

In reply to woolsack:

How has The Netherlands been coping with all the rain, does anybody know?

In reply to woolsack: I heard someone blaming global warming as if like some sort of act of God there was nothing you could do. It is curious how little focus there has been on inappropriate housing development. We have pointed out the dangers of flooding and drainage on several developments in our area and they were ignored by planners, desperate to push through their plans. Personally I think global warming is an excuse for avoiding the real issue, overpopulation.

In reply to toad:

Not often I disagee with you, but the 'dredging doesn't make any difference' argument put forward by the EA is something akin to 'if I hit a brick wall at 100mph in my car I'll die, so no point in having seat belts or air bags'. It's not a universal cure all, but watetways have had massive neglect over the years. Purely anecdotal, but since the last major flood in 2009 removed 12,000 tonnes of gravel from the Derwent at Cockermouth, (that the EA had allowed to build up in the relief channels), river levels seem far more stable, despite significant rainfall in 2012.

In the case of the Somerset levels, it's like emptying a swimming pool with a hose pipe and the EA saying a 100mm drain line won't make any difference. Dredging won't prevent flooding, but it reduces the frequency, severity and duration.

In reply to Ridge:

I don't think dredging is never a solution, but these are very different catchments. The Derwent is much more "flashy" as it's relatively small and takes a lot of water from the mountains - in this circumstance it makes sense to get it out to sea as quickly as possible.

The levels are a much bigger drainage system and have a very different flood response (I really need to be able to draw to explain!). In this instance it would only have a marginal effect on flooding as there are so many other factors at work. Plus in this instance, moving water quickly only moves the problem downstream. Ultimately because the water has to be shifted uphill, there's nowhere for the flood water to go

In reply to toad:

Individual waterways that are 60% silted up have 60% less capacity to hold water. 60% less storage irrespective of actually getting the water away.

Again, Romney Marsh, in the autumn and winter the dyke levels would be run low to provide an initial buffer but then the pumping to the main drains was effective as the smaller channels weren't choked with weed and silt and there was effective flow.

The Marsh is a good example of how it is done properly

In reply to woolsack:

I know this area well, one problem with the flooding,and any attempt to hasten its dispersal, is the down stream effect on Bridgewater, which would affect far more properties

In reply to Ridge:

Got this via UKC a few months ago.
http://www.forestry.gov.uk/pdf/Derwent_flooding_final_report_2010.pdf/$FILE...

In reply to woolsack:

A quick look at the EA flood risk maps of the area (or the recent aerial footage) and a bit of mental maths on the volume of water currently lying on the land vs the volume of the drainage channels shows that their initial level would have basically no effect on the current situation.

Without having details on what is pumped vs what is gravity drainage it is not possible to make an informed estimate on dredging, although I would note that for most rivers the increase in capacity when in flood is very large (much wider channel) than the capacity lost to silting which affects only the smaller permanent channel.

A confused article from Christopher Brooker

http://www.telegraph.co.uk/earth/environment/10595534/Somerset-Levels-There...

In reply to woolsack:

In the middle ages the Bishop of Bath & Wells used to visit his parishes by boat, since then a vast civil engineering program has drained the levels. If the civil engineering works are not properly maintained there will be flooding.

The EA has been caught out fair and square.

In reply to blurty:

I have to say our stream has been subject to flood schemes and dredging and cleaning, guess what it made not one jot of a difference, our neighbour was flooded and we were within a cm, it has more to do with heavy rainfall, rainfall is the culprit

In reply to woolsack:

The problem is the environment agency hasn't got enough money or people...

http://www.insidetheenvironmentagency.co.uk/index.php?controller=post&a...

Oh wait.

Cheers

Toby

In reply to dale1968:

, rainfall is the culprit

I take your point, but note that the flood map shows that the flooded areas are in-land now. If the water could 'get away' along properly maintained water courses & drains, then the problem would come to an end.

I can see that rainfall + high tides can lead to problems in the tidal part of the Levels, but further in-land I don't think there should still be problems 2,3 weeks after the event.

It's bad maintenance that's all

In reply to armus:



Ooh. Ta, looks interesting!

In reply to toad:

sorry toad, I appreciate your amphibian expertise but:
a 1:100yr flood isn't unlikely it's inevitable, just a question of when.

I know what you mean, unlikely in any one year, but that's what leads to complacency, and it's the inevitability bit that folks really need to understand.
thanks, I have just awarded myself 10 pedantry points.

In reply to Rigid Raider:

I don't think the Netherlands has had as much rain as the UK has in the last few months. More generally they're coping through forward planning, integrated thinking/management and spending significant amounts money on flood prevention.

In reply to daWalt:
I have just awarded myself 10 pedantry points.

Consider my cap doffed

In reply to Brev:

The design criteria used in the Netherlands is 1:10,000 year flooding I believe

In reply to Brev:


Indeed, and not taking this stance could be a countrywide disaster rather than a regional one as we have in the UK judging by the altitude of much of the Netherlands.

In reply to blurty:

I'd assume that's coastal flooding; bit of a different nut to crack.
the main diference is the tidal gates that are everywhere; presume they close these for fluvial events (same at the Thames Barrier) but I don't think that'll acheave the same standard of protection.

In reply to woolsack:

But the total volume of rivers is usually irrelevant, in that it is much smaller than the total volume of water that falls on the watershed in a 'rain event' (whether that is a single heavy burst of rain or a few weeks over a winter). If you took 'dredging' to extremes, you would need colossal storm drains that would dwarf the rivers.

Consider the River Severn. It has a watershed of 11,420 km^2. It also has an average flow of 61.17 m^3/s, and a maximum recorded flow of 533.48 m^3/s. Now if heavy rain (40mm) falls over only 20% of the catchment area, the total volume of rain is 91.36 million m^3, which is 414 hours or about 17 days at the average rate of flow (or just under 2 days at the highest recorded flow, which was presumably associated with flooding). The River Severn has a lag time of about 3-4 days according to Google (quite slow I think).
If you (hypothetically) dredged, straightened and canalised every mile of river and every major tributary so that water which previously took days to reach the mouth now took (say) 12 hours, you would need a channel that could take 2114.8 m^3/s, roughly the average flow of the Rhine, and ~17 times larger than the current Severn's average flow.

Water comes from upstream. If it comes quickly, you will get flooding; if it comes slowly you are fine.

PS if you assume a high flow rate of (say) 300 m^3/s, and that the time this water takes to flow out is roughly equal to the lag time (4 days), you get a total volume of ~100,000,000 m^3, fairly (surprisingly!) close to my estimated 'rainfall' event, which suggests my estimates aren't too insane.

In reply to andrewmcleod:

Ive no idea if your stats are correct or incorrect however, I like your working.

Assuming your calcs are correct and the inputs are good then I think we have a simple fact of too much rain over too short a period of time X people living in a flood plain = problem which simple dredging might not alleviate.

In reply to andrewmcleod:

I too like your workings and with the Somerset Levels, a four week flow out time hints at 20 years worth of maintenance not having been done

In reply to TheDrunkenBakers:

yea, that sums it up.

any artificial drainage system (channels, pipes, pumps etc... ) will have a limit to its capacity, which at some point will be overwhelmed.
without knowing the extremity of the event we shouldn't say the system has failed.

As regards dredging; digging out drainage channels can have a major effect on the “normal” (non flood) situation. But if the pumps are the limiting component of the system (operating at max capacity and still flooding upstream) dredging won’t have made the slightest difference.

The biggest EA mistake was to disagree with an angry farmer.
schoolboy error.................

In reply to johncoxmysteriously:

true, but he is right on this one; it also relates to removal/drainage of upland bogs/mosses/etc which act as giant sponges controlling the outflow of water.
and in case no-one else has said it - a 'hundred year event' (flood, tide, wave, World cup victory, etc) doesn't mean it happens once every hundred years, it means that the probability is 1 in 100 (or 1%) EVERY year.

and as for the levels...places don't have names like the Isle of Wedmore just to trick visitors, you know!

In reply to woolsack:

did you consider the cost and environmental considerations when you posted?
thought not..

In reply to Removed User:




You would have been safe Lardbrain, even Chris has not been flooded yet although mysteriously the cycling to work seems to have ceased in favor of the evoque!

In reply to Dr.S at work: he's not sailing to work then?

In reply to Removed User:

Not enough depth for a kayak apparently so I guess the super-duper-dinghy would struggle - mind if he sailed down the 'stream' out the back of his garden he could probably come up the congrebury yeo and moor up on Iwood lane?

In reply to Dr.S at work:

i assume coming up the congresbury yeo isn't a euphemism?
He could always give Ed a lift on a wake board!

In reply to Removed User:


As someone who spends his life hearing "This project will be completed in 5 years so we don't need to consider a 20 year event" I suspect I can feel the pain behind that comment.

In reply to toad:

Yes this happened to hundred year old bridge just down the road from me, about five or six years ago, several houses were washed away as well.
http://www.panoramio.com/photo/5340277

Regarding dredging or not dredging, as others have pointed out the total volume of water falling within the catchment massively dwarfs the total capacity of the rivers, therefore dredging the rivers won't stop the flooding occurring in itself.

However, the channels of both the Parrett and Tone are leveed, and the surrounding farmland is lower hence the need to pump the water back into the channels. There’s an extensive system of rhynes, major artificial channels, sluice gates and pumping stations to control water levels. Pumping rates are slowed down when the river levels are high (so as not to increase flood risk downstream) and the regular rainfall since Christmas has keep river levels high (the Parrett also has a large tidal range which complicates things).

The EA modelled the effect of dredging the rivers following the winter 2012/2013 floods (I've previously seen a summary of the report but can’t find it now) which showed that dredging the rivers would enable pumping at a greater volume for longer, and reduce the time the levels were flooded, and also reduce the maximum height and extent of the floods. Dredging also IIRC, increases the height of the rivers during high tide/storm surge events, as there's more capacity in the rivers for water to be forced in.

An additional problem of the large volumes of standing water is the low dissolved oxygen levels which result due to rotting vegetation etc. If you then pump the water back into the rivers you potentially kill all the fish, so they need to aerate the water and add massive quantities of hydrogen peroxide. So getting the water back into the rivers as quick as possible is beneficial for many reasons.

I can fully see the point people are making regarding other solutions to this problem, including better upstream catchment management etc. The problem with the levels is they have evolved to the current situation over 100s of years, they can’t be kept dry without pumping, and there are large areas of productive farmland reliant on being kept dry. There is also no large upland area to the catchment (as with many other large rivers) which could benefit from reforestation etc to slow the water down.

I guess long term a better managed system looking at the whole catchment needs to be developed, which may well include giving up some areas of farm land to allow greater flood storage, but pumping large volumes of water is likely to remain part of that.

In reply to woolsack:

Its gonna get worse in Somerset at end of week. Big surge tides again, and more rain a Coming.

In reply to Phil79:

Dredging might help locally but the problem comes downstream were bed levels are back to existing values. The Levels are an extensive area and dredging would be required over a long length of the river and would be very expensive plus there might be a disposal problem of the dredged material. They are also thinking about trying to increase the time taken for flood water to enter the river. In natural landscapes, trees, bogs, low lying areas all soak up water or form flooded areas in non- developed positions. Unfortunately, farming methods over thousands of years have cleared and levelled such areas: difficult to go backwards. On the Mersey, they provided large areas just upsteam of a particular flooded section where water was diverted when the river level rose to a particular value: this needs large areas to be available which can be flooded on a temporary basis - not possible in built up areas. Of course, the primary problem is allowing residential development on river flood plains.

In reply to Removed User:



Finally, the voice of reason!

In reply to nw:


Isn't it more like a 3 in 100 chance? That way the odds of the event not happening in 100 consecutive years 0.97^100 is less than 5%. I chose 5% meaning unlikely, that is a 95% chance it will happen in 100 years.

In reply to Philip:

You really need to speak to a statistician, but let's say you bought 10 lottery tickets every week (at odds of (off the top of my head) 1:14,000,000 that you identify the winning numbers of the jackpot). After 1,400,000 weeks unfortunately, you stand no greater chance of having won the jackpot than you did in year one. And you'd be quite old & a lot poorer. Part of the problem comes from Actuaries - i.e. predicting the likelihood that something will happen - pretty much it's guesswork - it's easy to predict things that show a pattern, but not so much if they're random (or more accurately there's a pattern that we can't determine easily)
Probability is funny like that - 1 in 14million sounds great when you're sat on your private island in the Bahamas, but not when a piece of Skylab lands on your head

lardbrain

In reply to woolsack:

I'm not sure it's as simple as you make out.
http://www.monbiot.com/2014/01/13/drowning-in-money/

In reply to Removed User:

'winning' rather than 'having won' shirley?

>. After 1,400,000 weeks unfortunately, you stand no greater chance of having won the jackpot than you did in year one. And you'd be quite old & a lot poorer. Part of the problem comes from Actuaries - i.e. predicting the likelihood that something will happen - pretty much it's guesswork - it's easy to predict things that show a pattern, but not so much if they're random (or more accurately there's a pattern that we can't determine easily)

In reply to Removed User:

The lottery is random. The weather is chaotic.

In the lottery you don't improve your odds with each loss.