General: Slowing the flow

1 to 27 of 27

- CommentAuthortony
- CommentTimeJul 30th 2020
Is it possible to slow the flow of a river, this is a question I asked my children as we built a little dam Across a Scottish brook.

Of course the flow slowed while it filled the dam but after that it remained the same.

You can’t stop flooding by building a dam!

Possibly mitigate a tiny bit but only if the reservoir is emptied before it starts raining

If floods are coming then they are coming,
- CommentAuthordjh
- CommentTimeJul 30th 2020
What you can do is:

- make the flood occur somewhere else. This is the idea behind designated flood plains, where some fields are flooded instead of dwellings further downstream. The excess water can be released later.

- cause the water to be absorbed and 'held' by saturated land for a longer period than it would be if flowing quickly down a narrower stream. This is the idea behind using things like beaver dams. The water eventually evaporates or flows back into the stream or down into an aquifer.

The principles of slowing down water flow and reducing floods by having absorbent surfaces instead of hardstanding, and by using systems such as SUDS are well-proved.
- CommentAuthorWillInAberdeen
- CommentTimeJul 30th 2020 edited
http://www.greenbuildingforum.co.uk/newforum/comments.php?DiscussionID=16476&page=1

Tony's previous thread on this subject, in the members-only area.
- CommentAuthorCliff Pope
- CommentTimeJul 31st 2020
<blockquote><cite>Posted By: tony</cite>I

Of course the flow slowed while it filled the dam but after that it remained the same.

</blockquote>

That reminds me of an incident from childhood sixty years ago. Staying on the farm with my cousins, we built a dam across the river Witham very close to its source. There followed investigations from the water authority and complaints from farmers downstream because the river flow had suddenly stopped.

When I revisited the spot more recently the river had dried up anyway, there was a housing estate where once had been my uncle's fields, and the ditch was partly filled in with rubbish.
The river Witham presumably still flows, but its source must be miles downstream now.
- CommentAuthormarktime
- CommentTimeJul 31st 2020
Aaah! The River Witham. My grandfather operated the swing bridge at Woodhall Spa, replaced in 1968 by the Kirkstead bridge. By gum, good fishin' spot!
- CommentAuthorjms452
- CommentTimeAug 1st 2020
You can't stop a flow indefinitely in the steady state - agreed.

However normal flooding is inherently a transient problem so I'm not sure your steady state conclusion holds tony.

what dgh said +1.

Where I would agree tony is that many of the most effective solutions to flooding don't look like a traditional dam/flood defence.

To overuse a recent analogy reducing flooding is rather like flattening the sombreo but here (unlike with covid), the area underneath the curve is fixed.
- CommentAuthorLF
- CommentTimeAug 1st 2020
Sounds like all operations must be done slowly and in dappled shade for optimum relaxation.

In=Out+accumulation (which can be a + or - term)

So when dam filling accummation is +ve and Out is reduced
When dam full the acc=0 and In=Out again.

Try a bypass ? In=Out 1 + Out bypass
- CommentAuthorJonti
- CommentTimeAug 2nd 2020
It really is not rocket science and I have always been baffled as to why such a basic principle and solution is roundly ignored by national and local planning. You need to control the flow of water so at no point does it exceed the capacity at any one point.

In the UK we seem focussed on holding back excess water at the point where the danger is instead of ensuring that the water levels in that area never gets to the point where flooding occurs. Flood barriers and drainage exacerbate the problem and are no fix.

Of course, it would help if we finally stopped building on floodplains and if we must at least design the building and infrastructure to reflect the situation.
- CommentAuthorCliff Pope
- CommentTimeAug 2nd 2020
One would imagine that the rate at which water can soak in depends on the pressure, ie the depth of water?

That is certainly the case with our garden pond, which has developed a leak somewhere at the bottom. The level goes down rapidly when full, but falls at a decreasing rate as it sinks. That explains why it is so difficult to find the leak - as the level approaches the low mark the inflowing water just seeps slowly and untraceably away. It would be easy if it just went down a gurgling hole.

Isn't there a big river somewhere in southern Africa that just gets lost in a desert - soaking in or evaporating?
That must happen to an extent with any floodplain, just not normally enough to absorb the entire flow indefinitely.
- CommentAuthorDominic Cooney
- CommentTimeAug 2nd 2020
Massive upland catchment areas of closely-nibbled sheep pasture.

= The rain runs straight off.

Allow the trees to grow (they don’t even need planting, just the sheep removed)

= the rain soaks in.

Once you see it, you can’t un-see it.
Building in floodplains, although a bit stupid, is a diversion from the real problem.
I have read that farm subsidy requirements state land has to be in “a farmable state” (i.e. cleared of trees) to get payments.
- CommentAuthorEd Davies
- CommentTimeAug 2nd 2020 edited
Posted By: Cliff Pope“Isn't there a big river somewhere in southern Africa that just gets lost in a desert - soaking in or evaporating?”

You're probably thinking of the Okavango Delta in Botswana: https://en.wikipedia.org/wiki/Okavango_Delta
- CommentAuthorfostertom
- CommentTimeAug 2nd 2020
River Jordan, mostly below sea level, evaporates in the Dead Sea.
- CommentAuthortony
- CommentTimeAug 2nd 2020
Try that with the big river in China, the vast majority of river flow into the sea, just a few into lakes and one into a desert
- CommentAuthorJonti
- CommentTimeAug 3rd 2020
Posted By: Dominic CooneyMassive upland catchment areas of closely-nibbled sheep pasture.

= The rain runs straight off.

Allow the trees to grow (they don’t even need planting, just the sheep removed)

= the rain soaks in.

Once you see it, you can’t un-see it.
Building in floodplains, although a bit stupid, is a diversion from the real problem.
I have read that farm subsidy requirements state land has to be in “a farmable state” (i.e. cleared of trees) to get payments.

Certainly partly true. Trees are a great way to slow the flow of water and there really needs to be a reintroduction of hedgerows as the default way to enclose fields, roads and of course gardens. It also needs the drainage systems to regulate the flow and so prevent flash flooding.

Sheep are not really the problem and indeed grazing by livestock is a very important process for habitat maintenance but over grazing certainly does lead to the 'closely nibbled' pastures you refer to.

I do however completely disagree with building on floodplains being a distraction. Were it not for housing flooding then this would not be an issue. Nobody really cares about farmer's fields flooding except the farmer. It is the fact that we build ill designed housing and infrastructure in areas where they should not be which creates this whole conversation. The solution to that is simple. Don't build on flood plains and if you have to then design the building/infrastructure appropriately.
- CommentAuthorWillInAberdeen
- CommentTimeAug 3rd 2020
One of my family is a flood defence engineer, her industry would love to manage floods by changing land usage and by reforesting and re-bog-ing the upland, instead of building expensive concrete floodwalls round towns downstream which will never be quite tall enough.

The problems they have are:
1) it takes decades to reforest large enough areas, but flood-affected communities understandably want faster responses than that;

2) flooding authorities have no legal power to pay or enforce upland communities to stop traditional farming on their land, but they do have legal powers to build floodwalls through cities.

A lot has been written about Belford where the village couldn't get floodwalls, so instead they blocked ditches and built simple dams to store water on farmland and release it slowly into the river

http://www.thejournal.co.uk/north-east-analysis/analysis-news/northumberland-pioneering-flood-defence-scheme-9347131

Building new houses on floodplains is setting up problems, and stopping that is necessary but not sufficient. Most of the houses we will live in this century have already been built, and many are on land that is now becoming more vulnerable to flooding because of climate and land-use change. Even houses on steep slopes can be flooded when ditches etc overflow in heavy rain.
- CommentAuthorWillInAberdeen
- CommentTimeAug 3rd 2020
Belford flood dam.
Image from https://www.theflowpartnership.org/belford
- CommentAuthorPeter_in_Hungary
- CommentTimeAug 3rd 2020
Posted By: WillInAberdeenBuilding new houses on floodplains is setting up problems,

I went to Poland a few years back an wondered why all the houses in the area seemed to have unfinished swimming pools in the garden - but no - it was the crater left after creating a 1.2M high mound on which to build the house. The area was a flood plain and their solution was to first build the mound and then build the house on it up out of the flood waters. Of course it needs a fair size plot to make this approach feasible - far beyond the typical new build plot size in the UK.
- CommentAuthorJonti
- CommentTimeAug 3rd 2020
Posted By: Peter_in_Hungary
Posted By: WillInAberdeenBuilding new houses on floodplains is setting up problems,

I went to Poland a few years back an wondered why all the houses in the area seemed to have unfinished swimming pools in the garden - but no - it was the crater left after creating a 1.2M high mound on which to build the house. The area was a flood plain and their solution was to first build the mound and then build the house on it up out of the flood waters. Of course it needs a fair size plot to make this approach feasible - far beyond the typical new build plot size in the UK.

Or you build a basement level for things that can be moved or are water safe then put the house on top. It is not rocket science but the building industry here is not very innovative alas.
- CommentAuthorWillInAberdeen
- CommentTimeAug 3rd 2020
Um.... the point of a flood plain is that water can flow freely onto and across the land, and stand there for a few days, before returning gradually to the river.

If you build private islands on the land, your house might stay dry, but there's a good chance they will impede the natural flow of water across the land. If the flood plain doesn't function as before, then you will have displaced the water to rush on downstream and flood somebody else....?
- CommentAuthorowlman
- CommentTimeAug 3rd 2020 edited
Posted By: Jonti....................................
Or you build a basement level for things that can be moved or are water safe then put the house on top. It is not rocket science but the building industry here is not very innovative alas.

Yes, I've watched this technique in Germany, it's commonly used there, The usually, specialist Kellerbau company, first dig the house footprint plus a metre or so all round, often only about 1.5metres or so deep, then construct a solid externally waterproofed and insulated blockwork cellar. The builder then continues to extend the cellar to full height out of the ground and build the single floor above. Finally heap and landscape the spoil all around the cellar to leave a single storey.
- CommentAuthorfostertom
- CommentTimeAug 3rd 2020
A great idea, esp if it can be efficiently systematised like that sounds. Every house shd have one, whether flood plain or not.
- CommentAuthorJonti
- CommentTimeAug 3rd 2020
Posted By: owlman
Posted By: Jonti....................................
Or you build a basement level for things that can be moved or are water safe then put the house on top. It is not rocket science but the building industry here is not very innovative alas.

Yes, I've watched this technique in Germany, it's commonly used there, The usually, specialist Kellerbau company, first dig the house footprint plus a metre or so all round, often only about 1.5metres or so deep, then construct a solid externally waterproofed and insulated blockwork cellar. The builder then continues to extend the cellar to full height out of the ground and build the single floor above. Finally heap and landscape the spoil all around the cellar to leave a single storey.

In Switzerland too.
- CommentAuthorEd Davies
- CommentTimeAug 3rd 2020
Posted By: WillInAberdeenUm.... the point of a flood plain is that water can flow freely onto and across the land, and stand there for a few days, before returning gradually to the river.
Exactly. Much better IMHO to build on stilts, Segal style, to allow the water to flow past safely. I'd think that in many cases they wouldn't need to be very high, maybe 300mm clearance or so, so long as there's nothing for the water to pile up against.
- CommentAuthorMike1
- CommentTimeAug 3rd 2020 edited
Posted By: JontiSheep are not really the problem and indeed grazing by livestock is a very important process for habitat maintenance but over grazing certainly does lead to the 'closely nibbled' pastures you refer to.

Professor Stuart Lane, Leeds University, seems to think that sheep can be a significant factor:

The second major explanation of the increase in flood magnitude and frequency at York relates to the increases in stocking densities that have been observed since the 1970s... APEM (1998) note that high stocking levels: (a) may lead to biomass loss, which reduces evapotranspiration rates, so maintaining high levels of soil wetness, and also reduces root depth which reduces infiltration into the soil; and (b) leads to increases in surface soil compaction, which also reduces infiltration. Sheep are of particular concern: Betteridge et al., (1999) demonstrated that different types of cattle had different effects upon the soil surface: cattle caused upward and downward soil movement leading to high levels of soil disturbance; sheep caused more surface compaction. These observations are supported by a wealth of studies from a range of different environments. http://www.therrc.co.uk/pdf/References/Lane_2003.pdf

George Monbiot's not too keen either:
Sheep have done more damage to Britain’s environment than all the building that has ever taken place
https://www.spectator.co.uk/article/meet-the-greatest-threat-to-our-countryside-sheep
- CommentAuthordjh
- CommentTimeAug 3rd 2020
'Livestock' is not a uniform group. Different animals graze in different ways - in the way they eat, the things they eat, the way they move about and what they leave behind. As indeed Prof Lane says in the quote above. There are lots of examples of particular species being chosen for particular conservation 'tasks'. Cattle, goats, pigs and buffalo seem to crop up on the useful lists, whilst sheep and deer can be problematic.
- CommentAuthorJonti
- CommentTimeAug 3rd 2020
Posted By: Mike1
Posted By: JontiSheep are not really the problem and indeed grazing by livestock is a very important process for habitat maintenance but over grazing certainly does lead to the 'closely nibbled' pastures you refer to.

Professor Stuart Lane, Leeds University, seems to think that sheep can be a significant factor:

The second major explanation of the increase in flood magnitude and frequency at York relates to the increases in stocking densities that have been observed since the 1970s... APEM (1998) note that high stocking levels: (a) may lead to biomass loss, which reduces evapotranspiration rates, so maintaining high levels of soil wetness, and also reduces root depth which reduces infiltration into the soil; and (b) leads to increases in surface soil compaction, which also reduces infiltration. Sheep are of particular concern: Betteridge et al., (1999) demonstrated that different types of cattle had different effects upon the soil surface: cattle caused upward and downward soil movement leading to high levels of soil disturbance; sheep caused more surface compaction. These observations are supported by a wealth of studies from a range of different environments.http://www.therrc.co.uk/pdf/References/Lane_2003.pdf

George Monbiot's not too keen either:
Sheep have done more damage to Britain’s environment than all the building that has ever taken place
https://www.spectator.co.uk/article/meet-the-greatest-threat-to-our-countryside-sheep

Mike,

I am assuming you have posted this to back up my point as that is what it does. Thanks for adding it.
- CommentAuthorWillInAberdeen
- CommentTimeAug 4th 2020 edited
Posted By: WillInAberdeenUm.... the point of a flood plain is that water can flow freely onto and across the land, and stand there for a few days, before returning gradually to the river.
If you build private islands on the land, your house might stay dry, but there's a good chance they will impede the natural flow of water across the land. If the flood plain doesn't function as before, then you will have displaced the water to rush on downstream and flood somebody else....?
Posted By: Ed DaviesExactly. Much better IMHO to build on stilts, Segal style, to allow the water to flow past safely. I'd think that in many cases they wouldn't need to be very high, maybe 300mm clearance or so, so long as there's nothing for the water to pile up against.

Sounds fun, but rather brave...

The 2016 flood level round here was 0.6m higher than the previous record, so the insurers would probably want the stilts to be rather more than 0.3m tall. Debris was left above my head height in trees on the flood plain, and many were bent by the force of the flow. I think you're better off on 2-3m tall stilts and they'd better be strong..!

Obvs the access roads, EV chargepoint, bin store and polytunnel would also need to be up on 2m stilts.

The other function of a flood plain is to slow the flow so that some of the sediment load can be deposited, so it doesn't end up restricting the main river channels. That regular silt recharge makes floodplain land so fertile. Guess that would be good for your veg plot... but the siltation might collect under the stilt house and reduce the clearance.

Alternatively, we could recognise that floodplain land has a high-value job to do storing floodwater, and value it as such, and then people wouldn't be so tempted to build things on it!

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