Glossary of common flood risk terminology

Flood Risk Assessment – A study to assess the risk to an area or site from flooding, now and in the future, and to assess the impact that any changes or development on the site or area will have on flood risk to the site and elsewhere.

PPS 25
Planning Policy Statement 25 “Development & Flood Risk”

Annual Exceedence Probability
The estimated probability of a given flood magnitude occurring, or being exceeded in any year. Expressed as, for example, 1 in 100 chance or 1 per cent.

Climate Change
Long-term variations in global temperatures and weather patterns, both natural and as a result of human activity

Design Event
A notional flood event of a given annual flood probability, against which the suitability of a proposed development is assessed and mitigation measures, if any, are designed.

Design Flood Defence Level
The maximum estimated water level during the design event, plus a freeboard (safety) allowance to which flood defence measures are designed.

Flood Defence
Flood defence infrastructure, such as flood walls and embankments, intended to protect an area against flooding to a specified standard of protection.

Floodplain
Area of land that borders a watercourse, an estuary or the sea, over which water flows in time of flood, or would flow but for the presence of flood defences where they exist.

Flood Zone
A geographic area within which the flood risk is in a particular range, as defined within PPS25.

Fluvial
Flooding caused by rivers.

Freeboard
The difference between the flood defence level and the design flood defence level.

Greenfield
Land that has not been previously developed.

Main River
A watercourse designated on a statutory map of Main River, maintained by Defra, on which the Environment Agency has permissive powers to construct and maintain flood defences.

Ordinary Watercourse
All rivers, streams, ditches, drains, cuts, dykes, sluices, sewers (other than public sewer) and passages through which water flows do not form part of a Main River. Local authorities and, where relevant, Internal Drainage Boards have similar permissive powers on ordinary watercourses, as the Environment Agency has on Main Rivers.

Pluvial
Flooding caused by rain

Resilience
Constructing the building in such a way that although flood water may enter the building, its impact is minimised, structural integrity is maintained and repair, drying and cleaning are facilitated.

Resistance
Constructing the building in such a way as to prevent flood water entering the building or damaging its fabric. This has the same meaning as flood proof.

Return Period
The long-term average period between events of a given magnitude which have the same annual exceedence probability of occurring.

Residual Risk
The risk which remains after all risk avoidance, reduction and mitigation measures have been implemented.

Run-off
The flow of water from an area caused by rainfall

Standard of Protection
The design event or standard to which a building, asset or area is protected against flooding, generally expressed as an annual exceedence probability.

Sustainable Drainage Systems (SUDS)
A sequence of management practices and control structures, often referred to as SUDS, designed to drain water in a more sustainable manner than some conventional techniques. Typically these are used to attenuate or infiltrate run-off from development sites.

Vulnerability Classes
PPS 25 provides a vulnerability classification to assess which uses of land maybe appropriate in each flood risk zone.