The Most Common Types of Flooding

In the UK, there are five main types of flooding that can all cause serious damage to property and threaten the lives of people.  These five flood types have various causes and consequences, and they can strike anywhere and at any time.  Understanding what makes each type of flood different will help you know what to do to protect yourself, as well as who to contact for assistance.  

River flooding

Flooding from rivers is usually caused by too much rain entering streams and other minor watercourses, then running into rivers. The way that rivers respond to this excess water varies, depending on their capacity and available access to the floodplain.

Any watercourse that is big enough to cause significant flooding is designated by the Environment Agency as a ‘main river’.  Importantly, this does not necessarily just mean larger rivers, as smaller watercourses can also cause serious flooding.

When floodwaters have access to the floodplain it allows storage of surplus water and slows down the flow in a river. However, when building development has taken place on the floodplain, this increases the risk of flooding. Sometimes, developers who are allowed to build on floodplain land are asked to contribute to the cost flood protection measures as part of the planning process.

Flooding from rivers is usually known as the river ‘going out of its bank’.  When forecasts show there is a possibility of flooding from rivers, the Environment Agency issues flood warnings to householders and businesses that may be affected. However, not all areas subject to possible flooding are registered to receive flood warnings from the Environment Agency. The annual public awareness campaign run by the Environment Agency encourages householders at risk to join the Flood Warnings Direct service.

Households and businesses can now register to receive flood warnings through a variety of methods, including to their mobile phone or computer, or to a landline in their property.

People who own land adjacent to rivers (known as ‘riparian owners’) are responsible for making sure that water passes smoothly through their land. They must not allow blockages in the watercourse and should remove any obstructions that appear. These obstructions can be natural blockages such as fallen trees or overgrowth on the banks, or it could be rubbish that has been dumped in the watercourse or flowed downstream from another area.

The Environment Agency has permissive powers to maintain and improve the flow of water on main rivers, and it is also responsible for building and maintaining flood alleviation schemes.

Localised or flash flooding

Flash floods happen when an exceptional amount of rain falls in a very short time. It is usually a result of storm-level torrential rain and often causes problems in urban areas. Heavy rainstorms can mean that drainage systems and sewers fill to capacity very quickly, and then ‘over-top’.

Flash flooding can be a major problem if the weather has been very dry for some time before a storm breaks, as the land can become baked hard and can’t absorb the rain when it comes. Rainwater can then run off the land and lead to serious flooding.

Flash flooding can be very localised, but it can do as much damage as flooding from rivers or other watercourses. Localised flooding can also happen after a period of prolonged rainfall, when the ground becomes saturated and unable to absorb any more water, causing it to flow off the land.

Watercourses are often culverted (meaning that previously open channels are diverted through pipes) during the building of new developments.  This helps to channel water away from – or even underneath – buildings. These culverts can work well for many years, but they can also cause flash flooding problems following rainstorms or prolonged rain.  This is because culverting dramatically reduces the capacity of a watercourse.

Flash flooding can also be made worse when the capacity of small watercourses or culverts is reduced by blockages of rubbish, perhaps caused by fly-tipping. Rubbish can also travel down watercourses until it reaches culverted sections and then blocks them. To help avoid this, people who own or manage land next to watercourses are encouraged to keep those watercourses clear of debris that might cause problems further downstream.

Flash flooding can happen suddenly. It is important to know what signs to look for in your location so you can ACT FAST.   Those signs are:

  •  heavy rain or severe weather reports
  •  rising water levels with churning dark water
  •  a build up of debris in rivers or streams

Although flash floods are rare, climate change and an increase in thunderstorms means they are more and more likely to happen. Flash floods are extremely destructive, so being prepared could save property, belongings and even lives.

Flooding from sewers

Localised flash flooding from blocked or overloaded drainage systems can happen at times of heavy rainfall. This type of flooding is unpredictable, and it often occurs in unexpected locations depending on the location and intensity of rainfall.

Such drainage systems include open drainage ditches and culverts, or buried drains and sewers. Where flooding occurs from foul sewers, the floodwater will be contaminated with sewage. In some cases, contaminated floodwater can flow back through foul sewers causing flooding inside buildings.

The water companies are responsible for the foul sewerage system, and the surface water sewerage system if it has been adopted. They do not have responsibility for any other watercourses or land drainage systems.

If floodwater has been contaminated, a filthy layer of silt is usually left behind when the water recedes. Contamination can add to the cost of cleaning and disinfecting buildings that have been flooded and it also poses risks to health. Protective clothing should be worn while cleaning after the flood, and other hygiene precautions should also be taken.

General flooding can create blockages in drains and sewers and this can lead to the backflow of sewage into properties through low-level drain gullies, toilets and washing machine/dishwasher outlets. Backflow through drains is particularly likely where floodwater has been prevented from entering the property by using temporary barriers.  It is also more likely where the flooding depth outside is higher than the internal level of the drain entry points.  This flooding can often be effectively controlled by installing ‘non-return valves’ within the private sewer of a property upstream of the public sewerage system.

Anti-flooding devices require careful installation and must be regularly maintained. Maintenance depends on the type of device fitted, but is normally required every six months.

Some properties have private sewage systems and flooding of these systems can also create a back-up of sewage into the property. Damaged septic tanks, cesspools or small package sewage systems should be inspected by a qualified engineer before the drainage system is used after a flood.

Surface and highway flooding

Roads and paths are usually drained into highway drains that run alongside them. In rural areas, roads may drain into ditches along their edges. In heavy rain and storm conditions – or following periods of prolonged rain – these drains can also sometimes find it difficult to cope with run-off from roads and paths.  This can lead to flooding.

In rural areas, keeping drainage ditches clear of obstructions is the responsibility of the landowners whose land they flow through (known as riparian owners). Excess water usually drains away gradually into the adjoining land.

Landowners are encouraged to maintain land ditches and avoid reducing capacity by culverting. This ensures good flow and reduces the risk of flooding. Improvement work on land drains is carried out by the highway authorities, local authorities and landowners.

Highway drains are the responsibility of the highway authorities, who maintain them and attempt to keep them clear so that they can cope with run-off. Excess water build-up in urban areas is sometimes diverted away from the highway into ‘soak-aways’ near to the drains, and then gradually into the ground.

New developments can increase run-off because of a rise in the numbers of roads, driveways and hard landscaped gardens.  Some are now built with balancing ponds to compensate for the loss of land for rainwater to soak in to. In certain areas, developers are also now encouraged to avoid too much hard landscaping in new housing estates and to include ‘green open space’. These spaces can absorb some of the run-off from roads after periods of heavy rain.

Reservoir flooding

Reservoirs hold large volumes of water above ground level, contained by dams. Although the safety record for reservoirs is excellent, there is a very small possibility that a dam could fail, causing a large volume of water to be released very quickly. Reservoirs are regularly inspected and essential safety work is carried out. Local authorities are responsible for coordinating emergency plans for reservoir flooding as well as ensuring communities are well prepared. They work with other organisations to develop these plans for your area.