Image copyright AFP/Getty Images Image caption Emergency services members helped residents and their dogs leave flooded houses by boat in south Wales during Storm Dennis

Severe flooding has once again hit parts of the UK, this time during Storm Dennis, damaging homes and leaving many people stranded.

But how can flooding be prevented or alleviated? Here are some of the main methods used in the UK.

1. Flood barriers

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

Lightweight sectional metal barriers are relatively inexpensive, can be placed in various configurations, and can be 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.

Image copyright Environment Agency Image caption Glass barriers in use in Keswick, Cumbria

2. 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 barriers 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.

3. 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.

4. Dredging rivers

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.

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.

Farmers in Somerset claim a lack of river dredging worsened the impact of the flooding that hit the region in January 2014. But planned flood defences in Oxford, for example, have avoided dredging, as it would not provide enough additional capacity to deal with a major flood.

The process is 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 again.

How much is spent on flood defences?

Flooding and coastal erosion can have a huge impact on local communities and economies.

Damage estimated at £1.1bn a year is prevented by the UK's river barriers and defences, according to research by the Association of British Insurers.

The Department for Environment, Food & Rural Affairs (Defra) is responsible for flood policy in the UK. It gives funding to the Environment Agency to deal with main rivers, estuaries, the sea or reservoirs flooding.

Over the last 10 years, total spend on managing flood and coastal erosion risks in England increased by 18.4% to £808.2m in 2018-19.

Spending peaked in 2014-15 at £786.8m. Defra says a lot of that was down to repairing damage after heavy winter storms.

Why are we seeing so many 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 even have to move entirely. She suggested that spending on flood management would need to be raised to £1bn a year.

"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.

So-called "compound flooding" happens when heavy rainfall combines with a storm surge from the sea.