MORE LAND DRAINAGE TO PREVENT FLOODING - NOT AS CRAZY AS IT SOUNDS!
During the recent floods weather presenters and news reporters were repeatedly heard to say “more heavy rain is forecast and where this rain falls on saturated and waterlogged land there is the risk of flooding” – very true. Subsequent reports have then concentrated on flood defences and how to get rid of the water more quickly, even today attention is focused on dredging.
But for a different perspective just cast back to the opening statement “where this rain falls on saturated and waterlogged land there is the risk of flooding” – true.
But what happens when rain falls on land that is not saturated and waterlogged?
Answer - nothing!
Unsaturated land acts as a sponge and soaks water up. If all the land in Britain could act as a giant sponge it would soak up enormous quantities of rainfall. This would significantly reduce the uncontrolled surface run off of water which is what causes waterways and rivers to flood.
So, how do you make land into a sponge? You drain it!
What! Surely that makes the problem worse! Wrong!
A little bit of soil science tells you that if you lower the water table of land then you dramatically increase its capacity to store water. Also this stored water bleeds off slowly through the land drainage which greatly extends the run-off period and reduces flooding significantly. Land drainage provides large scale attenuation of water. What’s more this could be far more cost effective than building flood defences, barrages and all the other downstream measures that are being talked about.
To concentrate attention upstream
will produce a much more effective long term sustainable
Drainage of land is very straightforward
and can readily be done by a long established land
Land is routinely drained with perforated pipes to provide optimum growing conditions for plants including agricultural crops, fruit and vegetables, grassland for livestock, and even the turfgrass for your local sports club, golf course, or the park where you take the kids to play and walk the dog on a Sunday morning.
The benefits of land drainage, apart from being able to play football on your local pitch or play golf on your course 365 days a year even after heavy rain, are the economic benefits of food production on UK farms which can show up to 30% increase in yield from drained land. As importantly drained land can allow more timely cultivations and sowing and harvesting of crops. Everybody will have sometimes seen on TV undrained crop fields where potatoes or corn has to be left in the ground to rot because the fields are so waterlogged machinery cannot get onto the ground to harvest the crop. This is why farmers drain land, and why land drainage has an economic benefit to consumers in reducing costs of food production.
Soil is drained to get rid of excess water and let air in because plants need air as well as water around their roots to survive. Also to provide a more stable soil structure.
If you pick up a handful of soil half of it is solid particles such as sand or clay and half of it is the spaces between these particles. If all these spaces were full of water the soil would be saturated and you would be holding a handful of mud! If all the spaces were full of air the soil would be very dry and would run through your fingers.
Neither condition is good for plants which require soil to be neither wet nor dry but moist.
Clearly when soil is saturated
like mud it can’t soak up any more water which is why when there
is heavy rainfall on saturated soil you get flooding. Equally when there
is no water in the spaces and they are full of air the soil is capable
of soaking up enormous quantities of water.
This is why more land drainage can help prevent flooding – possibly counter intuitive but it works!
Naturally the water drained from land has to be held but this can be done in a controlled way in watercourses, ponds, lakes, reservoirs and wetlands before it even reaches rivers. These can be used to slow water down to prevent it rushing overland into towns and flooding built up areas.
Ditches which take the water from drained land can have mini weirs to hold the water in the ditch during periods of high rainfall. Further downstream these can discharge into ponds which on a larger scale can be lakes. To be really sustainable these can be reservoirs to hold water for use as irrigation during the drier summer months – the water can even use the drainage system for subsurface irrigation - the ultimate recycling of water!
By controlling the rate of discharge of water from these upstream measures you don’t need to deliberately let land be flooded downstream as sacrifice areas to protect built up communities.
There is nothing new in this.
Many of the measures taken by our forefathers were designed to hold
water back before discharging at a slower rate. Dew ponds were a common
feature, along with village ponds which filled up in winter and dried
out in summer, and a network of ditches and watercourses which have
since been filled in.
However, modern techniques can be thrown at this to get the best of both worlds. Controlled drainage is fairly easy to achieve by creating a series of buffers to slow the passage of water. The starting point is control of the water table in the soil which can done mechanically and is already practiced in Holland. It is being well researched in the USA, not for reducing flood risk, but as a means of reducing pollution of water courses by nitrates which it has been shown to do by up to 50% so is a win win situation.
Other simple measures like grass buffer strips and grassed gulleys in fields can slow the run off of water into water courses. As importantly, these buffer strips can also filter out much of the soil particles and sediment that would otherwise be washed into the watercourse and be washed downstream – less soil erosion and hence less need for dredging.
A larger scale extension of using vegetation to slow and filter water are grassed waterways which can be used to control water run-off and soil erosion. These are already being used very effectively in the UK and their use should be extended.
The ultimate use of vegetation
to slow water with added environmental benefits in reducing pollution
is using areas of land as a watershed to filter and purify drainage
and run off water naturally by allowing it to percolate through surrounding
grassland into wetland areas, ditches and streams before being stored
in a lake or reservoir.
The key to all this is to drain the land in the first place to allow the soil to act as a big sponge to soak the water up so that you can control where the water goes thereafter.
‘Drain before the Rain’
Otherwise, as the weather forecasters predict, you will just get surface run-off which you can’t control – result – floods!
It’s very simple really!
Pictures and illustrations below, and videos of land drainage being installed are available.
Land drainage pipes being installed
A lake under construction which catches and holds water reducing flooding downstream
A lined reservoir for water conservation and storage for irrigation
Land drainage being installed in an arable field
Land drainage pipes being installed on a golf course
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