What makes a flood happen?

Find out how we assess the risk posed and read details on the different sources of floods.

Flooding is a natural event. It occurs when there is heavy rainfall that fills rivers and streams above their normal capacity, or if there are very high river or coastal tides that cause levels to rise or surge. The excess water that gathers cannot be restrained by normal boundaries (such as a river embankment) and follows the path of least resistance.

This means areas that are low lying and close to the source of a flood will be the most vulnerable. Floods can also occur when rainwater collects on the ground and cannot find a source to drain into. A typical example is surface water run-off (for example, if you are located at the bottom of a hill or slope of ground). Localised flooding mainly happens when the ground cannot absorb any more water in a particular area, or if sewers and underground drains become blocked or cannot cope with the excess water trying to drain into them.

Other sources of flooding

Floods can happen anywhere at anytime, caused by rising ground water levels, burst water drains, hillside run-off from sudden rain and more.
Be prepared

Even if you're not directly at risk of flooding from rivers or the sea, you could still be affected by floods from other sources.

Even if you live miles away from the coastline or a river, there's still a chance flooding could affect you. Take a look at your location. Is it at the bottom of a hill or valley, or in an area below sea level? If the answer to either of these questions is 'yes', it may be worth taking extra precautions.

Properties located at the bottom of hills - even small ones - can be vulnerable to rain water surface run-off, which can be dangerous during excessive rain. You may also be susceptible to groundwater flooding. In addition, blocked sewers can feed back and flood your home.

Flooding from surface water - how are we responding?

Much of the flooding in the summer of 2007 was due to surface water, not to rivers flooding. Government and the Pitt Report have identified flooding from surface water as a priority issue.
What is flooding from surface water?

Surface water flooding happens when the ground, rivers and drains cannot absorb heavy rainfall. Typically this type of flooding is very localised and happens very quickly after the rain has fallen, making it difficult to give any warning.

Flooding from surface water is much more complex to try to predict than flooding from rivers. It often happens in urban areas where there is little open ground to absorb rainfall, and where manmade drainage systems may be overwhelmed.
What have we done so far?

In September 2008, we sent out CDs showing areas susceptible to surface water flooding to Local Resilience Forums (LRFs) and Regional Resilience Teams (RRTs) to help them plan their response to flooding from surface water emergencies.

In July 2009, we updated 40 per cent of the data, and re-released it to professional partners through a secure web site. We also sent the data to Local Planning Authorities for the first time, to point out areas where further investigation is needed.
Collecting historic flood data

We are also collecting historic data on flooding from all our professional partners. While many local authorities, utilities and others have helped already, we need more data. This will let us give you back the most accurate national historical map of flooding in England and Wales ever compiled, and help us all manage flooding.
How will the draft Flood and Water Management Bill help us manage surface water?

The draft Flood and Water Management Bill, published in April 2009, brings some clarity to surface water management. While we would have a strategic overview for all flooding and coastal erosion in England, our executive duties and capital projects would continue to concentrate on risk-based protection for rivers and the sea.

The proposed duty for local authorities is to produce local flood risk management plans - covering flooding from surface water, groundwater and ordinary watercourses.

The bill also proposes removal of the automatic right to connect new developments to the public sewerage system, and legal requirements for sustainable drainage. These measures for surface water and sewerage management would also address important water quality issues, as well as flood risk issues.