Flooding can be defined as any area of land covered by water, which is normally dry. Sometimes water levels can rise slowly and unnoticed, other times flooding can be rapid, sudden and unexpected. Floods are one of the main weather hazards that can be made worse by where we chose to live and how we manage our environment.

The Great River Stour overflowing into Broad Oak Nature Reserve in November 2000

Rain is the most important factor in flooding. How much rain, how heavy and for how long it falls will all have an impact on the level of flooding. What happens to the rain once it hits the land can also decide which areas will flood. Developed land surfaces or recent weather conditions will both play a part in how serious the floods are. The most common influencing factors are:
Recent weather – The more rain in recent days or weeks, the more likely a flood is to occur. Even if rain falls land affected by drought, the soil may be too hard and dry to absorb any water.
Urbanization – water flows much faster over pavements and concrete than grass and soil.
Soil Type – Rainfall moves through sandy soil much faster and more easily than through thick clay.
Topography – Most of the world’s flash floods occur in mountains where small streams can become raging rivers in a matter of seconds, as they fall almost vertically. In lowland areas the slopes are gentler with floods taking longer to develop.

Experts have named two types of floods:
1. River floods
2. Flash floods
A river flood forms slowly, normally because of melting winter snow or a very wet season. Smaller streams will eventually join larger rivers and the waters power can be great. Flooding can cause disruption for weeks on end, destroying buildings, washing away vehicles and roads.

Flooding at Littlebourne on the Little Stour River, November 2000.

Flash flooding is far more dangerous and can happen very quickly. It is usually caused by large amounts of heavy rainfall from thunderclouds moving slowly over a land area that is likely to flood. Very heavy rainfall over an hour or two can create a ‘wall of water’ that can raise river and lake levels by just one meter in a minute. Flash floods are much more likely in area where it has been raining for a few days and the soil is saturated. The natural environment is also an important factor, and some of the most devastating flash floods have occurred in narrow, steep-sided canyons. For example the Lynmouth Flood Disaster 1953 on the edge of Exmoor, Devon.

In the UK the Environment Agency (previously the National Rivers Authority), is in charge of monitoring river and lake levels and making flood warnings when necessary. Sometimes floods are very difficult to predict. With the help of computer models, the monitoring of rainfall and water levels can help predict when and where flooding may occur. However, a large river system is harder to understand as there are many smaller streams involved, which are not monitored.

 

A sign near Broad Oak Nature Reserve following flood defence work on the Great River Stour
www.environment-agency.gov.uk

When flooding is likely, the Environment Agency (EA) releases different levels of warning:

Flood Watch – Flooding possible.
Flood Warning – Flooding expected to affect buildings and roads.
Severe Flood Warning – Severe flooding expected very soon. Take immediate action to protect life and property.
All Clear – issued when flood warnings are no longer in force.

These warnings are issued by local television and radio stations as well as by phone and on the Environment Agency (EA) website. Siren systems or loud hailers may also be used on a local scale if a more immediate and severe flood is likely.

On the night of October 10th 2000 the worse floods in South-east England for 40 years began. The rivers of Kent and Sussex already had very high water levels because of significant rainfall. In just 72 hours, North-west Kent and East Sussex had 18cm (7 inches) of rainfall. 13cm of this had fallen in just 12 hours. 16 severe flood warnings were issued by the EA for rivers in Kent, Sussex, Hampshire and the Isle of Wight. In some places the water flowed over the river banks and towns and villages were flooded. More than 3000 homes had to be evacuated in Kent and the damage bill covering southern England reached millions of pounds.

For more information on rivers and the water cycle have a look at our Investigating Rivers pages