The meaning of flooding
What do we mean by flooding? Mention the word 'flood' and most people think of heavy rainfall causing
rivers to burst their banks. But there are many reasons for and definitions of flooding:
- Surface water – this occurs when heavy rainfall cannot be absorbed into the ground or enter the drainage
- Ordinary watercourses – this occurs when smaller watercourses, such as streams, ditches, drains, cuts,
dykes and sluices cannot hold the volume of water flowing through them and overflow their banks onto
- Groundwater – this occurs when water levels below ground rise above the surface. This is most
likely to occur in areas underlain by permeable rocks, and is likely to occur after seasonal periods of
- However, the most severe flooding is often caused when different types of flooding combine.
The River Teme and Linney recreaction ground: an example of combined fluvial and
Other sources of flooding include:
- Rivers – this occurs when a watercourse cannot cope with the volume of water draining into it and overflows
its banks onto surrounding land.
- Drainage/Sewer - this occurs when combined or surface water sewers are overwhelmed by a heavy rainfall
event which exceeds the capacity of the sewer/drainage system, the system becomes blocked by debris or
sediment, and/or the system surcharges due to high water levels in receiving watercourses. Flooding from the
foul sewer can also occur through blockage, illegal connections or under capacity
- Reservoirs - this occurs when reservoirs which hold large volumes of water above ground water overtop, i.e.
cannot contain the amount of water flowing into them, or when part of the reservoir fails resulting in a fast
release of water.
Primary effects of flooding
The immeidiately obvious risks include loss of life, damage to buildings and other structures, including
bridges and sewerage. Floods also frequently damage power transmission and sometimes power generation, which then
has knock-on effects caused by the loss of power. This may include loss of water supply, loss of drinking
water, or severe water contamination. It may also cause the loss of sewage disposal facilities.
Lack of clean water combined with human sewage in the flood waters and the increased prevalence of rats raises
the risk of waterborne and other diseases. These can include typhoid, giardia, cryptosporidium, cholera
and Weil's disease to name a few.
Damage to roads and transport infrastructure may make it difficult to mobilize aid to those affected or to
provide emergency medical treatment.
Flood waters typically inundate farm land, making the land unworkable and preventing crops from being planted or
harvested, which can lead to shortages of food both for humans and for farm animals. Entire harvests for a country
can be lost in extreme flood circumstances. Some tree species may not survive prolonged flooding of their root
Secondary and long-term effects
Economic hardship due to a temporary decline in tourism, rebuilding costs, or food shortages leading to price
increases is a common after-effect of severe flooding. The impact on those affected may cause psychological damage,
in particular where deaths, serious injuries and loss of property occur. Some businesses never recover, whilst
insurance goes up for both domestic and commercial properties.