Flash Flood Risk

What is Flash Flooding?

Flash floods occur extremely quickly and with massive force. They can move rocks, tear out trees, sweep away vehicles and destroy buildings and bridges.

Flash flooding is often associated with surface water rising when drains are unable to cope with heavy rainfall. Flash flooding may also occur from watercourses which are very responsive to heavy rainfall, meaning river levels rise quickly.

These watercourses are within drainage areas called ‘Rapid Response Catchments’. The extremely quick response to heavy rainfall is often due to steep valley slopes and urban areas in these catchments, where water runs off the land straight into the river.

Across England, the Environment Agency have identified 384 ‘Rapid Response Catchments’, with over 75,000 properties being identified as being vulnerable to this risk. There are 16 Environment Agency classified Rapid Response Catchments in Cornwall, that are shown on the map below as blue markers. Click on a marker to view communities at risk from flash flooding.

How can you stay safe in the event of a flash flood?

Flash floods can even occur on sunny days when a storm has struck higher ground upstream. Therefore, it is important to know the signs of flash flooding even on dry days.

The localised nature and speed of the river response associated with this type of flooding means flooding can occur before warnings are issued. It is therefore crucial that you know what to do and how to stay safe. Examples of the devastating impacts of flash flooding in Cornwall were seen in Boscastle in 2004 and Coverack in 2017.

Those members of the community particularly at risk are those:

• In basements or single-storey properties
• In a mobile home, tent, caravan, boat or wooden structure
• Unfamiliar with the area e.g., tourists

Read the poster below to find out more information on how to stay safe in the event of a flash flood.