Flooding originates from a variety of sources, however it should be pointed out that, in reality, these types of flooding will often occur in combination.
Flood risk takes into account the harm that a flood actually causes. It is a combination of the probability (likelihood or chance) of an event happening and the consequences (impact) if it were to occur.
Local flood risk
We are responsible for managing local flood risk, this is made up of flooding from the following sources:
Fluvial (ordinary watercourses flooding)
Occurs when 'ordinary watercourses' eg streams and ditches are unable to contain large volumes of water falling or flowing into them, such as during or after heavy rain.
Pluvial (surface water flooding due to intense rainfall)
Occurs when rainfall cannot absorb into the ground (due to saturated soil or hard surfaces such as tarmac) or when the amount falling exceeds the capacity of the drainage network to take it away, causing it to flow across the ground.
Groundwater (saturated conditions reaching the ground surface)
Occurs when rainfall makes the groundwater table rise above its normal level. This type of flooding can last for weeks or months and is most likely to occur in areas above an aquifer.
Other sources of flood risk
These sources of flooding are managed by other organisations, known as risk management authorities, who are better equipped to respond to these types of flooding than the Council. To find out more on how they do this, see our other flood authorities page.
The other sources of flooding can be categorised as:
Fluvial (main river flooding)
Occurs when a main river eg the River Great Ouse cannot accommodate the volume of water draining into it from the surrounding land. It is generally infrequent and can be predicted to some extent.
Results from high tides, waves driven by strong winds and surges of seawater caused by storms. The most severe coastal flooding often occurs when surges and high waves coincide with high tides.
Occurs after the failure of the reservoir’s walls or earth embankments. This may be caused by erosion due to seepage, overtopping of the dam or by accidental damage to the structure. Reservoir failure is extremely rare in the UK.
Occurs when sewers are overwhelmed by heavy rainfall or when pipes become blocked In urban areas, surface water flooding and sewer flooding often combine, polluting floodwater.
Burst water mains
This can cause localised disruption to transport links and damage to buildings, particularly properties with a basement. This type of flooding is not related to rainfall.