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.

Flooded Somerset Levels, Jan 2014 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.

Graphic: How dredging works

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.

Graphic: Two types of flood barrier

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

Flood defences, Bewdley, Worcs. Metal frame barriers in use at Bewdley in 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.

Map showing natural flood prevention measures around Belford in Northumberland

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.

sustainable_drainage

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.

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