Building Homes on Flood Plains

[Abercych]

What worries me about the BBC’s 24/7 coverage of the floods is that they’re not coming up with any insight into what’s going on other than to say rainfall has been unprecedented.
I’ve even heard people from the farming world saying what’s needed is a return to “good, old-fashioned” land drainage. The bloke I was listening to reckoned that fields are flooded because ditches aren’t cleared and rivers aren’t dredged.
But isn’t the problem that for years we’ve been creating a countryside that sheds water rather than holding it? Moving water downhill as quickly as possible has helped make Gloucester and Oxford how they are this week.
What’s needed is a countryside that can behave like a sponge and hold water. That would reduce floods during wet periods and keep rivers flowing when there’s a drought.

[Chiltern Chris]

I’m finding most of the debate about this on the TV and radio a bit simplistic, as although the flood plain is a real place, it’s actual size is variable.

There is a lot of confusion about ‘the flood plain’ as some parts will flood regularly e.g. 3 times a year, others once every few years and others once a century or less. This is called the return period. i.e. a 1:100 year flood or 1% flood is likely to occur once a century (however, this is based on risk which means you may get three 1:00 year floods in 3 consecutive years and then not have one for 200 years). That’s why you constantly hear people saying ‘flooding is definitely getting worse, it’s not flooded this badly before and I’ve been here 50 years’. A single person’s experience is not a good indicator of flood risk increasing or decreasing as they are not likely to have experienced all the different flood / drought level expected to occur in the area.

The debate about building on the flood plain is all about risk. Before we start saying that there should be no development on the flood plain we should say what we mean by the flood plain i.e. do you mean the land covered by a water in a 1:1 year event (annual flood) which is very little land, 1:5 (one flood every 5 years) which is a lot more land, 1:20, 1:50, 1:100? You get the idea, what risk do you want to take? We defend certain structures e.g. the Channel Tunnel Rail Link to a 1:1000 year event.

This isn’t necessarily my point of view but it may be sensible to have all infrastructure out of the 1:200 year floodplain, allow housing on the land which floods between a 1:100 and 1:50 year event (making them flood resilient e.g. no accommodation on the ground floor) and no development except sports fields etc in the less than 1:50 year flood plain. This is further complicated by the topography, in some places a 1:5 flood and 1:100 flood will cover virtually the same land and so lots of land would be sterilised from development, while in others e.g. steep valleys the opposite applies i.e. only land right next to the river would be sterilised.

As for the ‘clear out the ditches brigade’ – Vegetation removal only affects the lower order floods i.e. normally only 1:5 year event or less, where you definitely shouldn’t be building anyway, but it has no effect on large order events like the ones we’ve just seen. Seeing the aerial views of the flooding, does any thinking person really believe that removing watercrowfoot, common reed or some river gravel would make the slightest bit of difference? Butchering the river’s biodiversity to reduce the flood risk very marginally cannot be justified except where human life is threatened.

What causes the problem is not the ecology, it’s the field drains, lack of buffer zones, straightened channels which are shorter and higher gradient than natural channels, the silt from the fields blocking the channels, the constrictions such as bridges and the concrete banks in urban areas i.e. we’ve short circuited this part of the water cycle and then tried to constrain it in small steep and frictionless channels – we’ve made most of the problem and then build in the high risk areas – seems a bit of a no brainer to me. In the end it's a choice of if we want to carry on building in high risk areas then we need to accept the economic and human cost as there's no way we will ever keep that much water in a single or multiple channel system unless the channels are so big they destroy the buidings you're trying to protect - not to mention the catastrophic environmental destruction it would cause. Lets only build stuff in the floodplain we're prepared to get flooded based on the risk we're prepared to take, stop trying to put in larger defences which make the problem worse and use whats left of the functional flood plain.

If anyone is interested in the government’s present guidance to planning authorities on building in the flood plain, it’s contained in Planning Policy Statement 25 (PPS25) which came out in December last year. Planning Policy Statement 25: Development and Flood Risk - Communities and Local Government

Cheers, Chris

[naturelover]

Originally Posted by Kymba

One answer is STOP letting people buy their Council Houses, They were built to house people who cant afford to buy a house.. Also stop letting places get so run down that they have to be Knocked down.. when many large buildings, eg... Hospitals, care homes, Offices close down for what ever reason why not turn them into flats for those who are on waiting lists for a home, They already have Plumbing, electric,heating, and Toilets etc... it has to be cheaper to adapt them rather then knocking them down and re building Luxury apartments for those well off... Then maybe we wouldn;t need to build on Flood plains and green spaces..

I made My partner laugh yesterday when we were watching the news of all the flooding as i looked up and said quite seriously,, why has no one mentioned how many animals and wildlife has been killed in these floods!

hi funny you should mention the wild life in the flood reports,i have been thinking the same thing,there must be hundreds of animals drown in all this water,i hate to even think how many,if for instance the fields are flooded what about the cattle they are fenced in and cant escape,thats the trouble when you are an animal lover you worry about the animals,the wild life i think would have been ok all but babies,bacause they would have moved with it they are not stupid they have special instinks that we dont have unfortuneatly,its very sad about the baby birds that have died in their nests the birds that nest on the ground they didnt have a chance to servive,i am afraid this is one thing man canot controle nature.

[Tormentil]

Originally Posted by Chiltern Chris

....... stop trying to put in larger defences which make the problem worse......

Stop building flood defences, period.

[Chiltern Chris]

Hi Tormentil,

We can’t just stop all flood defence work or many of our urban areas would flood. If you think the floods are bad now, you should try and imagine what it would have been like without flood defences. Rivers are dynamic systems and we’ve altered the system so much that the rivers are not in a stable state, so just like most habitats in the country, the rivers must be managed. Nearly every river you see will be managed in some way with altered flow regimes, structures and artificial gradients. What we’re really bad at is working with nature rather than against it i.e. we don’t use the rural flood plains in a sustainable way to save the towns and reduce river maintenance to increase small floods to keep wetlands wet. We can’t stop undertaking flood defence, but we can do it a lot better and increase biodiversity as we go rather than reduce it as we did very successfully in the latter part of the last century.

Chris

[Tormentil]

But don' flood defences just cause problems further up stream? Did n't the yanks find this out when the Mississippi flooded several years ago.

[Chiltern Chris]

They can cause problems upstream, but only if they cause what’s called a ‘head loss’ i.e. it makes it harder for the water to get through and you get a back up of water in high flows and deposition of silt in low flows (which then makes the flooding worse next time there’s a flood). The problems with most flood defences are down stream with higher water levels (as the water gets down stream faster), more erosion as there’s more energy kept in the river rather than it dissipating over the flood plain and faster water speeds as flood defence channels and structures have less friction (concrete and sheet piles are smoother than vegetation) and are shorter and steeper than natural channels.

There are other problems with many flood defences as the channel size is artificial. All rivers have a natural width and depth depending on the flow and gradient and the river is always trying to get back to that natural width. In general flood defence works make rivers too wide and the river’s energy is dissipated so much that silt drops out, which them smothers all the wildlife. That why many channels in the UK have silty beds with little life. They were dug too wide, so silt drops out, then people complain that the river isn’t being cleaned out because it’s all silty, so they are dug out again and so you start again. The answer is to stop clearing the rivers (except in urban areas) so they can recover and accept that they will then flood agricultural land.

Chris

[Kayleigh]

The rivers used to be a lot larger than they are today due to extraction to supply houses. They have become little less than a trickle in places.

Also the councils used to clean the drains from debris on a regular basis. Which they do not do now.

[Goosegogs]

I know it is slightly off-topic, but I was thinking of volcanoes for some reason. People live near some volcanoes - the soils are fertile. How often does a volcano erupt?

There wouldn't be enough land if people didn't live on floodplains.

On a fictional note - I've started to read John Wyndham's book The Kraken Wakes. I heard the serialisation on BBC7 a while back. Gripping!

[Loopy2]

Originally Posted by Goosegogs

I know it is slightly off-topic, but I was thinking of volcanoes for some reason. People live near some volcanoes - the soils are fertile. How often does a volcano erupt?

There wouldn't be enough land if people didn't live on floodplains.

Good point Goosegogs. Just look at the vineyards that ire evidence on the slopes of Vesuvius where the surrounding area is densly populated.