Updated 01 Feb 2014

Preventing floods

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Preventing Floods

Images and text that appeared in the media around end of January 2014. The month had been the wettest on record, and the Somerset Levels flooded for 5 weeks.

Corfe Mullen (East Dorset) just lies in the black (>225%) area.

                                Rainfall in Dorset (mms per month)
    2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014  Av 
Jan   44  108   98  124  128   55   26  161  133   99   72  106   47  120  292 105 
Feb   94   83  111   41   50   18   59  105   49   92   89   84   21   48       66 
Mar   46  146   56   43   51   47   77   75  102   26   80   18   33  100       62 
Apr  152   88   54   52   80   69   24    4   76   51   34    4  139   58       65 
May   64   22  107   44   37   31  101  111   91   25   13   32   65   54       51 
Jun   21   21   35   47   44   33   24  109   51   68   38   98  160   25       55 
Jul   54   64   49   50   45   60   40  132   82   80   28   33  132   32       53 
Aug   41   23   36   14   86   59   52   86  119   52  109  123   67   28       63 
Sep  111   59   49    5   50   26   45   32   75   23   24   58   92   42       66 
Oct  173  168  167   82  177  129  145   36  100   92   37   58  169  141      107 
Nov  167   44  220  125   25   81  153  115   82  196  110   49  151   61      111 
Dec  164   26  139   74   61   89   84   96   22  150   61   92  176  213      104 
Tot 1131  852 1121  701  834  697  830 1062  982  954  695  755 1252  922      902 
(Data derived from that supplied by Environmental Agency for Rivers House,
 Blandford, 1991-2007, for 2008-2013 on from Author's measurements at Broadstone)


With parts of the UK continuing to suffer the aftermath of the most severe winter floods in years, attention has focused on how flooding can be prevented or alleviated.

Here are some of the main methods and principles in use.

The Somerset Levels have been flooded for several weeks


Farmers in Somerset claim a lack of river dredging has worsened the impact of the flooding in the area in recent weeks.

But the issue of whether rivers should be dredged is not clear cut.

The Environment Agency says that while dredging can improve general land drainage, it cannot prevent rivers from flooding, due to the huge volumes of water involved during major floods.

The basic aim of dredging is to remove silt - a sedimentary material made of fine sand, clay and small-sized particles of rock - from the river's bed, therefore increasing its capacity to carry water downstream.

The process usually involves an excavator, or vacuum pump, mounted on a barge or on the riverbank, to remove silt from the river.

The process is costly, sometimes harmful to the environment, and can weaken riverbanks as well as the foundations of bridges and weirs, the Environment Agency says.

After a major flood, large volumes of silt may accumulate in slow-flowing areas and the river may need to be dredged again.

Flood barriers

In recent years the Environment Agency has used a range of temporary or "demountable" flood barriers to provide additional protection to flood-prone areas.

Lightweight sectional metal barriers are relatively inexpensive and can be placed in various configurations and removed completely when waters recede.

Frame barriers consist of rigid frames holding an impermeable membrane and use the weight of the floodwater itself to hold the barrier in place.

Temporary barriers can also be added to existing permanent flood defences, such as raised embankments, increasing the level of protection.

Metal frame barriers in use at Bewdley, Worcestershire.

Natural flood management

Natural flood management offers a sustainable approach to managing floods and is intended to complement traditional "hard engineering" techniques, such as flood barrier and concrete walls.

These schemes rely on a combination of small-scale interventions with the aim of reducing the speed of the flow of converging water before it reaches larger rivers.

Natural flood defence features include small barriers in ditches and fields, or notches cut into embankments, all of which divert the water into open land.

Letting pools form outside the main channel of the river means the water is temporarily removed from the main flow reducing the power of the floodwaters.

Trees can also help defend against floods. Planting more trees catches rainfall and helps take water from the soil - although the Environment Agency says large areas must be reforested to make a real difference.

Felled trees can also be laid across streams in wooded areas and help push unusually high waters into surrounding woodlands, although such schemes need very careful planning and management.

Sustainable drainage

Sustainable drainage is a concept often applied to towns and cities which are especially prone to flash flooding after sudden heavy rain.

In urban areas, large areas of concrete and tarmac, as well as the roofs of buildings, are impermeable to water. Rain is channelled straight into drainage systems which can quickly become overwhelmed.

In the UK, the Flood Act of 2010 obliges builders to landscape developments so that water from roofs and driveways seeps into open ground rather than rushing into the water system.

Sustainable drainage guidelines suggest that impermeable surfaces should be replaced with permeable material, allowing rainwater to drain into the ground - a process known as infiltration.

Large "detention basins" can also be built to collect rainwater and hold it, managing the volume of water entering urban rivers, while ponds offer further water-holding capacity.

From: BBC News

John Austen, a Norfolk farmer and chairman of a drainage board, said he supported the lazy river scheme.

"Here we have a flood plain protecting the village of Litcham. It's full of wildlife… absolutely fantastic. Drainage is not about diggers and silt - what we have to think about is the whole river starting with the catchment right the way down to the sea."

But Mr Austen opposed the idea that grants should be made conditional on capturing water.

"It shouldn't be compulsory but to really incentivise the farmer to do these projects he will want a little bit of icing on the cake - I suggest £200 a hectare to allow your grass to be flooded."

In theory farmers can already get extra EU grants to hold water on their land, and some of the participants in the Nar trial are receiving extra subsidies for wildlife - but experts say subsidies for water storage are much harder to obtain than grants for benefiting wildlife.
An NFU spokesman said:

"Although we agree that 'slowing the flow' should have an important role to play in reducing flood risk from hilly upland catchments, techniques such as tree planting need to be located carefully.

"They are not a panacea, and should not be expected to significantly reduce flooding everywhere and on their own.

"For this reason we would not support a requirement for all farmers to have to capture water on their land in order to access grants from the EU.

"Our approach to river flooding must be balanced, looking at river systems as a whole; attenuating flows upstream where needed and maintaining capacity downstream, including de-silting and vegetation management.

"The NFU would welcome guidance and environmental stewardship options that facilitated farmers to use natural processes to help control flows in, over and around farms and where appropriate store water."

Compiled, hand coded and copyright © 2014, John Palmer, All Rights Reserved.