- Annual Exceedance Probability (AEP)
- The chance of a flood of a given size occurring in any one year, expressed as a percentage. A 1% AEP event has a 1-in-100 chance each year; it is not a flood that happens once a century.
- Attenuation
- Temporarily storing surface water run-off — in a tank, pond, basin or blue-green roof — and releasing it slowly so a development discharges no faster than the rate the council allows.
- Blue-green roof
- A roof designed to store rainfall above the deck and release it through restricted outlets, providing attenuation on the roof itself. Useful where there is no room for below-ground storage.
- BRE 365
- BRE Digest 365 — the standard method for soakaway testing. A trial pit is filled and the fall in water level timed (normally three times) to measure the ground's infiltration rate.
- Climate change allowance
- The percentage uplift applied to river flows and rainfall to allow for future climate change, set by the Environment Agency by river basin and epoch. The August 2026 NPPF increased the weight on applying it correctly; the figures themselves are set in EA guidance.
- Critical Drainage Area (CDA)
- An area a council or the Environment Agency has identified as having critical drainage problems, where surface water flooding is a particular concern and a drainage strategy or FRA is likely to be required even in Flood Zone 1.
- Culvert
- A pipe or enclosed channel carrying a watercourse underground. Culverting an open watercourse is discouraged by national policy (NPPF Policy F8) and normally needs consent.
- Design flood level
- The modelled water level for the flood event a development is designed to withstand — typically the 1% AEP (1-in-100-year) river event plus climate change, or the 0.5% (1-in-200-year) tidal event. Design levels come from Environment Agency modelling (a Product 4), not from open flood maps.
- Exceedance
- What happens when a drainage system's capacity is exceeded in a storm bigger than it was designed for. Good design routes exceedance flows safely overland, away from buildings.
- Exception Test
- The NPPF test applied where development in a flood zone has passed (or is exempt from) the Sequential Test: it must show wider sustainability benefits that outweigh the flood risk, and that it will be safe for its lifetime without increasing flood risk elsewhere.
- Finished floor level (FFL)
- The level of the finished ground floor of a building, usually set a margin above the design flood level (with freeboard) so the interior stays dry in the design event.
- Flood Consequence Assessment (FCA)
- The Welsh equivalent of a Flood Risk Assessment, prepared under Technical Advice Note 15 (TAN 15) and the Flood Map for Planning (Wales).
- Flood Map for Planning
- The Environment Agency's map showing Flood Zones 1, 2 and 3 for river and sea flooding, plus surface water and other layers. It shows the undefended risk and is the evidence base for whether an FRA and the Sequential Test are required.
- Flood resilience
- Constructing a building so that if floodwater does enter, damage is limited and recovery is quick — for example solid floors, raised electrics and water-resistant finishes. Usually specified above the level flood resistance protects to.
- Flood resistance
- Measures that keep water out of a building up to a set level — most commonly a certified flood-resistant doorset. Resistance is normally specified to around 600 mm above the door threshold, with resilience above that.
- Flood Risk Activity Permit (FRAP)
- The Environment Agency permit required for works in, over, under or near a main river or flood defence (typically within 8 metres, or 16 metres for a tidal main river). Some lower-risk activities are exempt or excluded.
- Flood Risk Assessment (FRA)
- A site-specific report assessing flood risk to and from a proposed development from all sources, and setting out how it will be managed. Required for most development in Flood Zones 2 and 3, sites over one hectare, and — under NPPF Policy F4 — any site at risk from any source of flooding.
- Flood Zone 1
- Land with a less than 1-in-1,000 annual probability of river or sea flooding (less than 0.1% AEP). Low probability — but surface water and groundwater risk can still apply.
- Flood Zone 2
- Land with between a 1-in-100 and 1-in-1,000 annual probability of river flooding (1%–0.1% AEP), or between 1-in-200 and 1-in-1,000 for sea flooding. Medium probability; an FRA is required for development.
- Flood Zone 3a
- Land with a 1-in-100 or greater annual probability of river flooding (1% AEP or more), or 1-in-200 or greater for sea flooding. High probability; an FRA, and often the Sequential and Exception Tests, apply.
- Flood Zone 3b
- The functional floodplain — land where water has to flow or be stored in a flood. Only water-compatible and essential infrastructure development is normally acceptable.
- Freeboard
- A safety margin added above the design flood level when setting floor levels or defence heights, typically 300 mm, to allow for uncertainty in the modelling.
- Functional floodplain
- See Flood Zone 3b — the land that naturally floods and stores water in a flood event, which policy protects from development.
- Greenfield run-off rate
- The rate at which rainfall would run off a site in its natural, undeveloped state. Developments are generally required to limit their surface water discharge to this rate (or a betterment of the existing rate on brownfield sites).
- Groundwater flooding
- Flooding that occurs when the water table rises above ground level or into basements and services, common on chalk and in low-lying areas. Assessed separately from river and surface water.
- Internal Drainage Board (IDB)
- A local public body managing water levels and drainage in low-lying areas prone to flooding, with its own byelaws and consenting powers over watercourses in its district.
- Infiltration
- Disposing of surface water by allowing it to soak into the ground through soakaways, permeable paving or infiltration basins — the top of the SuDS hierarchy, where ground conditions allow.
- Lead Local Flood Authority (LLFA)
- The county or unitary council responsible for managing local flood risk from surface water, groundwater and ordinary watercourses. A statutory consultee on major planning applications, and the body that consents works to ordinary watercourses.
- Main river
- A watercourse designated on the Environment Agency's main river map, for which the EA is the regulating authority. Works affecting a main river need a Flood Risk Activity Permit.
- National Planning Policy Framework (NPPF)
- The Government's planning policy for England. The August 2026 version places flood risk in Chapter 18 as coded policies F1 to F9, with the flood zones and vulnerability classification in Annex F.
- National Standards for SuDS
- The Government's technical standards for sustainable drainage design (2025 edition), which NPPF Policy F8 requires all development to follow.
- Ordinary watercourse
- Any watercourse — river, stream, ditch, drain or culvert — that is not a designated main river. Works to an ordinary watercourse need consent from the Lead Local Flood Authority (or IDB) under section 23 of the Land Drainage Act 1991.
- Permeable paving
- Paving that lets rainfall pass through its surface into a stone sub-base below, providing infiltration or attenuation and treatment — a common SuDS component for drives and car parks.
- Planning Practice Guidance (PPG)
- The Government's detailed guidance supporting the NPPF, including the Flood Risk and Coastal Change guidance used alongside the Framework.
- Product 4
- The Environment Agency's detailed flood risk data product for a site: modelled flood levels, extents, depths, defence details and historic flooding. The proper basis for design flood levels in an FRA.
- QBar
- The median (mean annual) flood flow from a site in its natural state — the base figure from which the 1-, 30- and 100-year greenfield run-off rates are calculated.
- Residual risk
- The flood risk that remains after defences and mitigation are taken into account — for example from a defence being overtopped or breached. An FRA must show residual risk is managed.
- Risk Management Authority (RMA)
- The bodies with flood risk management functions under the Flood and Water Management Act 2010: the Environment Agency, Lead Local Flood Authorities, district councils, water and sewerage companies, highway authorities and Internal Drainage Boards.
- Section 19 investigation
- A Lead Local Flood Authority's duty under section 19 of the Flood and Water Management Act 2010 to investigate flooding incidents in its area and publish the findings, identifying which risk management authorities have relevant functions.
- Section 23 consent
- Consent from the Lead Local Flood Authority (or IDB) under section 23 of the Land Drainage Act 1991 for works that affect the flow of an ordinary watercourse, such as a culvert, bridge or diversion.
- Sequential Test
- The NPPF test that steers development to the lowest available flood risk: sites in Flood Zone 1 first, then Zone 2, and only then Zone 3, where no reasonably available lower-risk site exists. Householder extensions and certain changes of use are exempt, and NPPF Policy F5 provides a route for surface-water-only sites.
- Soakaway
- An underground chamber or crate system that stores surface water and lets it infiltrate into the surrounding ground. Sized from a BRE 365 test and normally set at least 5 metres from buildings.
- Source Protection Zone (SPZ)
- An area around a public water supply borehole where the Environment Agency restricts activities — including some infiltration drainage — to protect groundwater quality.
- Strategic Flood Risk Assessment (SFRA)
- A council-wide assessment of flood risk used to inform the local plan and planning decisions, including the boundaries of the functional floodplain.
- Surface water flooding
- Flooding from rainfall running over the ground before it reaches a drain or watercourse, typically in intense storms. Shown on the Flood Map for Planning and increasingly a reason an FRA is required on Flood Zone 1 sites.
- Sustainable Drainage Systems (SuDS)
- Drainage designed to mimic natural processes — slowing, storing, soaking and treating run-off close to source, using features such as permeable paving, swales, rain gardens, basins and ponds — rather than piping water away as fast as possible.
- SuDS hierarchy
- The order of preference for discharging surface water: infiltration first, then to a watercourse or surface water body, then to a surface water sewer, and only lastly to a combined sewer. A drainage strategy must justify each step down.
- Swale
- A shallow, vegetated channel that conveys, slows and treats surface water run-off, often used along roads and around fields.
- TAN 15
- Technical Advice Note 15: Development and Flood Risk — the Welsh Government's flood risk planning guidance, under which a Flood Consequence Assessment is prepared.
- Urban creep
- The gradual increase in impermeable area on a development after it is built (extensions, paved gardens). An allowance — typically 10% — is added to the impermeable area when sizing surface water storage.
- Vortex flow control
- A device with no moving parts that restricts discharge from an attenuation system to the permitted rate using a swirling flow, less prone to blockage than an orifice plate.
- Vulnerability classification
- The NPPF's grouping of land uses by their vulnerability to flooding — from water-compatible to highly vulnerable — which, with the flood zone, determines whether a use is acceptable and whether the Exception Test applies.