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
solution.
Drainage of land is very straightforward and can readily be done by a long established land
drainage industry.
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 w
hy
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.
For plants the ideal is for the spaces to be half water and half air. So even a well
drained soil that provides good growing conditions for plants still has lots of capacity
to soak up water in periods of heavy rainfall. 25% or more of its volume! This excess
water is then drained away more slowly to restore the right balance for the plants. It
can be released to water courses at a controlled rate over a longer period of time
which considerably reduces flood risk.
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.
Even the century’s old ‘ridge and furrow’ land was a form of flood prevention.
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.
These natural attenuation areas slow the passage of water which not only reduces
the flood risk but also allows time for the natural biological action of vegetation to
remove pollutants from the water. These areas used as watersheds to slow and
purify water in this way can even result in water being cleaner when it leaves the
area than when it arrived
– how about that for a good news story!
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!
Land Drainage Contractors Association
Tel: 0845 5191 243 Email: secretary@ldca.org Website: www.ldca.org
Pictures below – and videos of land drainage being installed are available
LAND DRAINAGE TO REDUCE FLOODING