Storm Franklin: How do flood prevention schemes work?

Storm Franklin has led to flooding and homes being evacuated in some parts of the UK.

In areas at risk of flooding, several different techniques are used to try to reduce the threat to people and property.

The Environment Agency uses a range of temporary 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.

Simple measures to reduce the flow of water before it reaches larger rivers are also used.

They include the use of 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 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 is often used in towns and cities, which are prone to flash flooding after sudden heavy rain.

This happens in urban areas because large areas of ground are covered in concrete and tarmac. Rain is channelled straight into drainage systems, which can become overwhelmed.

Builders are now required 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, while ponds offer further water-holding capacity.

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.

Prof 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.

The government has promised to spend £5.2bn over five years to fund flood defences in England, with £860m being spent on 1,000 schemes this year.

Scotland, Wales and Northern Ireland control their own flood schemes.

The Welsh government promised to spend £36m this year in flood defences, maintenance and management, and Northern Ireland's 2021/22 Budget allocated £19m. The Scottish government said it had spent at least £42m a year on flood protection since 2007, and in 2020 pledged an extra £150m over the next five years, and spends a further £195,000 per year to "build flood resilience within communities".

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.

And while many factors contribute to flooding, climate change makes extreme rainfall more likely.

A warmer atmosphere can hold more moisture and so these storms become more intense.

In 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.