Flood Protection Group

The risk of flooding is growing. Flooding could seriously affect the value and amenity of your home or business premises.

There will probably be an increasing number of floods in the future due to changes in weather patterns, the amount of new building on low-lying areas in recent years, and other local factors.

Many properties which have not previously been at risk of flooding now are. Of the 28 million homes in the UK, over 5 million are currently at risk, as well as over 300 000 business premises and many more public and utility services buildings. For most of these properties the risk of being flooded in any one year is still small, but for several hundred thousand properties, especially those which have been flooded in recent years, the risk is more significant.

The increasing risk of flooding can reduce the value of your home or business premises and may make it more difficult and expensive to get insurance cover.

A flood can threaten your safety, cause serious damage to your property and its contents, and will result in many months of dislocation and disruption. This Guide will tell you what you need to know about flooding and flood risk to your property (the land and the buildings on it), and what you can do to deal with this risk.

What is the risk to property from flooding?

There are a number of causes of flooding. A property can be flooded by…

Surface water flooding in times of heavy rain

In prolonged, exceptionally heavy downpours, which are becoming more frequent, the ground may become saturated and the drains and sewers which carry away surface water may not be able to cope, leading to surface water flooding. Although this is more likely in low-lying areas, and to premises at the foot of slopes, it can happen to many other properties which are not specifically designated as being at risk of flooding on the Environment Agency’s flood risk maps

Surface water flooding may be triggered or made worse in urban areas where the ground consists of mostly hard surfaces such as concrete or tarmac so the rainwater flows straight off rather than soaks away into the ground. It is estimated that nearly 4m properties are at risk of surface water flooding in the UK. Surface water flooding can affect one or two individual properties at a time, or may affect many more where this kind of flooding extends throughout the neighbourhood.

Sewer flooding

When sewage escapes from the pipe through a manhole, drain, or by backing up through toilets, baths and sinks this is known as sewer flooding. Sewer flooding can be caused by: a blockage in a sewer pipe;[1] a failure of equipment too much water entering the sewers from storm run-off (from roads and fields) and rivers and watercourses which overflowed; or the sewer being too small to deal with the amount of sewage entering it. The cause of the problem may be some distance away from where the flooding is happening. If the sewage enters a building, it is called ‘internal flooding’. If it floods gardens, or surrounding areas such as roads or public spaces, it is called ‘external flooding’.

Groundwater flooding

Rising groundwater levels resulting from heavier rainfall and reduced abstractions can present problems. Groundwater flooding generally occurs during long and intense rainfall when infiltration into the ground raises the level of the water table until it exceeds ground levels. It is most common in low-lying areas overlain by porous soils and rocks, or in areas with a naturally high water table.

Irrespective of whether water shows at the surface, rising groundwater levels are posing an increased threat to buildings with basements. Such flooding may occur separately or in conjunction with flooding from other sources such as surface water flooding.

River flooding

River flooding occurs when rivers and streams are unable to carry away floodwaters within their usual drainage channels. Adjacent low-lying properties and land are then liable to be flooded. River flooding can cause widespread and extensive damage because of the sheer volume of water, and may be longer-lasting and more difficult to drain away. Fast-flowing floodwaters can also be a threat to peoples’ and animals’ safety and can structurally damage buildings. Breaches in reservoirs pose a particular hazard, with the potential to release large quantities of water if the failure is catastrophic.

Coastal flooding

Coastal flooding is caused by high tides coinciding with a low-pressure storm system which raises sea and tidal water levels, overwhelming coastal defences. This may be made worse by gale force winds blowing the raised body of water onto the coast. Coastal flooding may affect not only property on the coast itself but also property in tidal river basins some distance from the coast, due to floodwater being forced up the tidal reaches of rivers and estuaries by raised sea levels and gales. Such flooding may become more frequent in future years due to rising sea levels.

Reservoir or dam failure

The UK has approximately 5 000 reservoirs. Many of these were created by building a dam across a river or stream. Dam failures in the UK are however uncommon. Nevertheless, there are a significant number of “large” raised reservoirs in the UK which may pose a potential risk. It is recognised that whilst the chances of reservoir failure are remote the consequences are potentially catastrophic and could affect areas several kilometres from the dam itself. A flood can happen to any property, from one or more of these causes.

For most property in the UK, the risk is still small. But some premises are more at risk than others because of their geographic location and particular local situation.

Properties in river basins (areas of land drained by a river and its tributaries), coastal properties on low-lying ground, properties in urban areas with old drains and sewers at the limits of their capacity, properties on absorbent ground with a high water table, and properties in hollows, or at the foot of slopes or gradients down which surface water could flow, may be at a significantly higher risk of flooding.

Content provided by RICS ‘Clear Guide to Flooding’