What's the best way to prevent flooding?

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Storm Christoph has caused widespread flooding in England and Wales, and more than 2,000 homes have been evacuated.

What are the main methods being used to tackle - and reduce the risk of - floods from risking life and damaging property?

Flood barriers

The Environment Agency uses a range of temporary or "demountable" flood barriers to provide protection in flood-prone areas.

Lightweight metal barriers are relatively inexpensive, can be placed in various positions, and removed when waters recede.

Frame barriers use the weight of the floodwater itself to hold them in place.

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

image copyrightEnvironment Agency
image captionGlass barriers in use in Keswick, Cumbria

Natural flood management

Natural flood management consists of small measures to reduce the flow of water before it reaches larger rivers.

Measures might include using small barriers in ditches and fields, or notches cut into embankments, to divert the water into open land.

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

Natural flood management offers a sustainable approach to managing floods and is intended to complement flood barriers and concrete walls.

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. Such schemes need very careful planning and management.

Sustainable drainage

Sustainable drainage is often used in towns and cities, which are prone to flash flooding after sudden heavy rain.

In urban areas, large areas of ground are covered in concrete and tarmac. Rain is channelled straight into drainage systems, which can 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.

Guidelines suggest the ground should be able to absorb the water - 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.

Dredging rivers

The basic aim of dredging is to remove silt - which consists of fine sand, clay and small particles of rock - from the river's bed, potentially 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.

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.

In some cases dredging can even make flooding worse.

Professor Dan Parsons from the University of Hull says: "it is the water surface slope of the river, rather than the river bed that is most important in moving water through a landscape. In lowland reaches, where rivers meet tidal forces, dredging can actually make matters worse by reducing this surface slope."

Dredging is also costly, sometimes harmful to the environment, and can weaken riverbanks as well as the foundations of bridges and weirs.

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

How much is spent on flood defences?

In the March 2020 budget, £5.2bn was promised over five years to fund flood defences in England.

The money is due to be available from April this year and will help build 2,000 new flood and coastal defence schemes.

The funding was announced after a series of floods caused major damage in the Midlands, Wales and South Yorkshire.

Why are we seeing more floods?

Flooding can come from all sorts of water sources, including groundwater, reservoirs and surface water. But cities and towns that lie on rivers or near the sea are most at risk.

Generally speaking, increased rainfall causes flooding inland, while high tides and storms are behind much coastal flooding - and both can happen together.

Flooding has always been an occasional part of life in such areas, but experts warn that climate change is worsening the situation.

In early 2019, Environment Agency chairwoman Emma Howard Boyd said that some communities might eventually have to move entirely.

"We can't win a war against water by building away climate change with infinitely high flood defences," she said.

Researchers have also warned that coastal areas in Northern Europe are also at higher risk of flooding in the coming years because of rising temperatures.

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