Flood Risk Glossary
Whether you’re applying for planning permission, commissioning a Flood Risk Assessment, or simply trying to understand what your council is asking for, flood risk comes with a language all of its own.
This glossary covers every key term you’re likely to encounter. Each definition is written in plain English, with a real-world example to show how it applies in practice.
Use the A–Z index below to jump straight to the term you need, or browse the full list to build your understanding of how flood risk policy works in the UK.
FAQ
Frequently Asked Questions About Flood Risk
Can’t find what you’re looking for in the glossary? Here are the questions our clients ask most often.
A Flood Risk Assessment (FRA) is a technical report that identifies all sources of flooding affecting a site and sets out how the development will be made safe without increasing flood risk elsewhere. You will need one if your site is in Flood Zone 2 or 3, if it is over 1 hectare in Flood Zone 1, or if your local council’s Strategic Flood Risk Assessment (SFRA) flags additional surface water risk. Your planning application may be refused without one.
Flood Zone 1 is low probability — less than a 0.1% annual chance of flooding. Flood Zone 2 is medium probability — between 0.1% and 1% annual chance. Flood Zone 3 is high probability — greater than 1% annual chance. The higher the zone, the stricter the planning requirements, and the more detailed the Flood Risk Assessment needs to be. Zone 3 developments also require a Sequential Test and potentially an Exception Test before planning permission can be granted.
The Sequential Test is a planning requirement that ensures development is directed to the lowest flood risk areas available. Before a council will grant permission in Flood Zone 2 or 3, they will check whether any suitable alternative sites exist in a lower-risk zone. You will need to demonstrate that no such alternative exists. Rida Reports prepares Sequential Test evidence as part of a full Flood Risk Assessment.
The Exception Test applies in certain situations where development in a high-risk flood zone may still be justified. It requires you to demonstrate two things: first, that the development delivers wider sustainability benefits to the community that outweigh the flood risk; and second, that the site will be safe for its lifetime without increasing flood risk elsewhere. It is only considered once the Sequential Test has been passed.
SuDS are drainage features designed to manage surface water runoff in a way that mimics natural drainage — slowing water down, storing it, and allowing it to infiltrate into the ground rather than overwhelming sewers. Most new developments in England are now required to include SuDS under the NPPF and the national SuDS standards. Common examples include permeable paving, swales, rain gardens and attenuation tanks. Rida Reports designs SuDS strategies as part of every Flood Risk Assessment.
Most site-specific Flood Risk Assessments are completed within 5 to 10 working days, depending on the complexity of the site, the flood zone, and whether hydraulic modelling or drainage calculations are required. Rida Reports works to fast turnarounds to keep your planning application on track.
The cost of a Flood Risk Assessment depends on the size of the site, the flood zone, the sources of flooding present, and whether SuDS design or Sequential Test evidence is also required. Rida Reports provides transparent, fixed-fee quotes. Visit our FRA cost guide for more information.
Fluvial flooding occurs when a river or watercourse overtops its banks and inundates adjacent land. Pluvial flooding, also known as surface water flooding, occurs when intense rainfall exceeds the capacity of drainage systems and water accumulates on the surface. Many sites are at risk from both sources, which is why a thorough Flood Risk Assessment considers all potential flood pathways.
The Environment Agency (EA) is a statutory consultee on planning applications where flood risk is a material consideration. They provide standing advice for lower-risk applications and bespoke responses for higher-risk or more complex sites. The EA also maintains the Flood Map for Planning, which defines flood zones across England. A well-prepared Flood Risk Assessment that follows EA and NPPF guidance gives your application the best chance of success.
Yes, in many cases development in a flood zone is possible — but it requires a robust Flood Risk Assessment that demonstrates the site can be made safe, that flood risk will not increase elsewhere, and that the development passes the Sequential Test and, where required, the Exception Test. Rida Reports specialises in securing planning approval for sites in Flood Zones 2 and 3.