12 March 2014

LDCA advises that more land drainage will prevent flooding

The Land Drainage Contractors' Association (LDCA) is advising that more land drainage is needed to prevent flooding. While at first this might seem counter-intuitive, this is not as crazy as it sounds, as Bruce Brockway, Secretary of the LDCA, explains in this article.

LDCA advises that more land drainage will prevent flooding

Flooding in Somerset

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 focussed on flood defences and how to get rid of the water more quickly, even today attention is focussed 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. 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.

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 centuries 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 practised in the Netherlands. 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 watercourses. 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!

MJ Abbott Limited is one of the longest-serving members of the LDCA.

LDCA website:  http://www.ldca.org/

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