What is Flood Risk?
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.
What is 'local' flood risk?
Bedford Borough is
responsible for managing local flood risk, this is made up of
flooding from the following sources:
a) Fluvial (ordinary watercourses
Occurs when 'ordinary
watercourses' (e.g. streams and ditches) are unable to contain
large volumes of water falling or flowing into them, such as during
or after heavy rain.
b) Pluvial (surface water flooding due to
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.
c) Groundwater (saturated conditions reaching the
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
What other sources of flood risk are there?
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 Other Flood Authorities. The other sources
of flooding can be categorised as:
a) Fluvial (main river flooding)
Occurs when a main river (e.g.
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.
b) Coastal flooding
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.
c) Resevoir flooding
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
d) Sewer flooding
Occurs when sewers are
overwhelmed by heavy rainfall or when pipes become blocked In urban
areas, surface water flooding and sewer flooding often combine,
e) Burst water mains
cause localised disruption to transport links and damage to
buildings, particularly properties with a basement. This type
of flooding is not related to rainfall.