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)

HOW do you STOP FLOODING? 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 Dredging 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.