UK Flood Risk Glossary
Definitions, examples, and data units for flood risk, hydrology, and monitoring terms used across the UK property, insurance, and infrastructure sectors.
75 terms across 7 categories
Risk, Governance & Resilience
Flood hazard
The likelihood and physical characteristics of flooding at a location (extent, depth, velocity), without considering impacts..
Flood risk
Flood hazard combined with the potential for harm to people, assets, operations or the environment..
Exposure
People, assets or services located in areas that could be flooded..
Vulnerability
How susceptible exposed receptors are to damage at a given flood intensity (depth/velocity/duration)..
Consequence (impact)
The resulting effects if flooding occurs (safety, financial loss, service disruption, environmental harm)..
Flood Risk Assessment (FRA)
A structured assessment of flood sources, pathways and impacts for a site/development, with mitigation measures..
Flood zone
Planning classification based on probability of river/sea flooding (e.g., UK Flood Zones 1/2/3)..
Floodplain
Low-lying land that naturally stores and conveys floodwater during high flows/tides..
Return period (recurrence interval)
Average interval between events of a given magnitude (e.g., ‘1-in-100 year’); not a schedule..
Annual exceedance probability (AEP)
Chance an event will be equalled/exceeded in any year (e.g., 1% AEP ≈ 1-in-100 year)..
Standard of protection (SoP)
Flood level a defence or system is designed to withstand (often expressed as AEP/return period)..
Residual risk
Risk remaining after defences/mitigation (overtopping, breach, blockage, operational limits)..
Freeboard
Additional height added above a design flood level to allow for uncertainty and safety margin..
Business continuity plan (BCP)
Plan for maintaining/restoring critical operations during disruption, including flood scenarios..
Flood defence
Permanent or temporary measures that reduce flood probability/impact (walls, embankments, gates, barriers)..
UK Institutions, Data & Planning
Property Flood Resilience (PFR)
Measures applied to buildings to reduce flood damage and speed recovery (resistance and resilience), such as barriers, non‑return valves, raised electrics and flood‑resistant materials..
UKCP18 & Environment Agency climate change allowances
UK Climate Projections (UKCP18) and related policy allowances used in UK flood and drainage assessments to account for future changes in rainfall, river flows and sea levels..
Strategic Flood Risk Assessment (SFRA)
A local planning evidence base produced for a Local Planning Authority area, assessing flood risk and informing land allocation, site policies and planning decisions..
Lead Local Flood Authority (LLFA)
The county or unitary authority responsible for managing local flood risk (surface water, groundwater and ordinary watercourses) in England under the Flood and Water Management Act..
Flood Estimation Handbook (FEH) & ReFH2
UK methods used to estimate design rainfall and flood hydrographs/flows for catchments (FEH rainfall and runoff methods; ReFH2 for small catchment event hydrographs)..
Risk Management Authorities (RMAs)
Organisations with statutory responsibilities for managing flood and coastal erosion risk in England and Wales (e.g., Environment Agency, LLFAs, water companies, highways authorities, internal drainage boards)..
Environment Agency (EA)
The public body responsible for flood risk management in England, including flood warnings, mapping products and regulation of main rivers and sea defences (with devolved equivalents in Wales, Scotland and Northern Ireland)..
Met Office
The UK’s national weather service, providing forecasts, weather warnings, and key data feeds used in flood monitoring (e.g., rainfall radar and numerical weather prediction)..
Floodline / Floodline Warnings Direct (FWD)
A public flood warning service used in England (and closely aligned services in the devolved nations) that issues alerts and warnings by area; Floodline is also the public information/helpline brand..
Flood Alert Area (Environment Agency)
A defined geographic area used by the Environment Agency to issue a Flood Alert in England (flooding is possible; be prepared)..
Flood Warning Area (Environment Agency)
A defined geographic area used by the Environment Agency to issue a Flood Warning in England (flooding is expected; immediate action required)..
Flood Map for Planning (Rivers and Sea)
Environment Agency planning mapping for river and sea flooding in England, including Flood Zones used in planning decisions and screening of flood risk..
Risk of Flooding from Surface Water (RoFSW)
Environment Agency national dataset indicating the risk of surface water flooding in England (often expressed as likelihood bands)..
Sequential & Exception Test (NPPF planning)
Planning tests used in England to steer development away from areas at highest flood risk (Sequential Test) and, where needed, demonstrate wider sustainability benefits and safety (Exception Test)..
Ordnance Survey National Grid Reference (OSGB)
The coordinate reference system commonly used in Great Britain (British National Grid) for locating assets and linking flood data to precise site points and polygons..
National Flood Risk Assessment (NaFRA)
Environment Agency national-scale assessment for England that estimates flood risk from rivers, sea and surface water and supports national and local planning and prioritisation..
Flood Mechanisms & Drivers
Fluvial flooding
Flooding from rivers/streams when discharge exceeds channel capacity or defences are overtopped..
Pluvial flooding (surface water)
Flooding from intense rainfall when drainage/infiltration capacity is exceeded, often in urban areas..
Coastal flooding
Flooding from sea water due to high tide, storm surge, waves, overtopping or breaching..
Groundwater flooding
Flooding when groundwater rises to the surface, often after prolonged rainfall in permeable geology..
Flash flood
Rapid-onset flooding with very short lead times (minutes to hours), typically from intense rainfall..
Storm surge
Temporary rise in sea level driven by low pressure and wind, superimposed on the tide..
Tide locking
Restricted river outflow because high sea levels prevent discharge at an outfall, backing water upstream..
Compound flooding
Coinciding drivers (e.g., surge + high river flow + heavy rainfall) increasing overall hazard..
Sewer/backflow flooding
Flooding from surcharged sewers/drains or reverse flow through outfalls during storms/tides..
Debris / blockage-induced flooding
Flooding worsened by obstructions at culverts, trash screens, bridges or channels..
Climate change allowance (uplift)
Uplift factors applied in assessments to account for future increases in rainfall, flows or sea level..
Catchment Hydrology & Rainfall
Catchment (drainage basin)
Area of land draining to a common point (gauge, outfall, confluence), defining the runoff contributing area..
Runoff
Portion of rainfall that becomes flow reaching drains/rivers; depends on soils, land cover and wetness..
Rainfall intensity
Rate of rainfall over time; key driver for pluvial and flash flooding..
Antecedent conditions
Pre-event wetness (soil moisture, groundwater, storage) controlling how much rainfall becomes runoff..
Hydrograph
Time series of flow (or level) showing rise, peak and recession during an event..
Monitoring, Sensors & Telemetry
Hydrometric station
Instrumented site that measures hydrological variables (level, flow, rainfall) with logging and communications..
River gauge
Location/instrument used to observe river stage (and often derive flow)..
Stage (water level)
Height of water surface relative to a datum; primary variable for real-time monitoring..
Discharge (flow rate)
Volume of water passing a cross-section per unit time; often derived from stage via a rating curve..
Rating curve
Empirical relationship converting stage to discharge for a specific gauge cross-section..
Vertical datum (mAOD/ODN)
Reference surface for elevations and water levels; ensures consistent comparisons across gauges/models/maps..
Rain gauge (tipping bucket)
Gauge that records rainfall by counting tips of a calibrated bucket, producing high-temporal-resolution totals..
Forecasting & Early Warning
Nowcasting
Very short-term (0–6 hour) forecasting using radar/satellite extrapolation and rapid-update models..
Quantitative precipitation forecast (QPF)
Forecast of rainfall amounts over space/time from numerical weather prediction models..
Ensemble forecast
Set of model runs with varied initial conditions/physics to represent uncertainty..
Probabilistic forecast
Forecast expressed as a probability of exceeding thresholds (e.g., 30% chance stage > trigger)..
Lead time
Time between alert issuance and expected threshold exceedance/impact; drives response options..
Threshold / trigger level
Pre-defined value that initiates a notification or action (stage, rainfall, tide, probability)..
Flood warning levels (UK)
Standard categories (e.g., Flood Alert, Flood Warning, Severe Flood Warning) used to communicate urgency..
Trigger Action Response Plan (TARP)
Pre-agreed actions linked to triggers and escalation levels to ensure consistent response..
Data latency
Delay between measurement and availability in the platform; affects situational awareness..
Quality control (QC)
Automated/manual checks for sensor drift, spikes, dropouts and unrealistic values before use..
Modelling, Mapping & Integration
Hydrological model
Simulates rainfall–runoff processes to produce flows at points of interest from rainfall and catchment data..
Hydraulic model
Simulates water movement in rivers/floodplains/drainage networks to estimate levels and inundation..
1D/2D hydraulic modelling
Numerical modelling of water movement in channels, floodplains and drainage networks to estimate water levels, depths, velocities and extents.
Flood extent (inundation map)
Mapped boundary of flooded area for a scenario or event, from models or observations..
Flood depth
Depth of water above ground level; key for estimating damage and safe access decisions..
Flood velocity
Speed/direction of floodwater; critical for hazard to people/vehicles and structural loading..
Digital elevation model (DEM)
Gridded ground elevation dataset used for flood mapping/modelling; accuracy drives outputs..
LiDAR
Remote sensing producing high-resolution elevation data (point clouds) used to build DEM/DTM..
Culvert
Enclosed conduit (pipe/box) conveying water under infrastructure; capacity/blockage controls local flooding..
SuDS (Sustainable Drainage Systems)
Approach to manage surface water at source using infiltration/attenuation/green infrastructure to reduce peaks..
REST API
Web interface to query/push flood data and alerts programmatically between systems..
Frequently Asked Questions
What is Flood risk?
Flood hazard combined with the potential for harm to people, assets, operations or the environment.
Example: The insurer priced the policy using site flood risk, not just proximity to a river.
Read full definition →What is SuDS (Sustainable Drainage Systems)?
Approach to manage surface water at source using infiltration/attenuation/green infrastructure to reduce peaks.
Example: The retrofit SuDS reduced runoff and frequency of yard flooding.
Read full definition →What is Annual exceedance probability (AEP)?
Chance an event will be equalled/exceeded in any year (e.g., 1% AEP ≈ 1-in-100 year).
Example: We report thresholds in both AEP and return period to avoid misinterpretation.
Read full definition →What is Flood zone?
Planning classification based on probability of river/sea flooding (e.g., UK Flood Zones 1/2/3).
Example: The warehouse sits in Flood Zone 3, so we required an FRA and finished-floor levels.
Read full definition →What is Catchment (drainage basin)?
Area of land draining to a common point (gauge, outfall, confluence), defining the runoff contributing area.
Example: We delineated the catchment upstream of the site gauge to prioritise rainfall monitoring.
Read full definition →What is Return period (recurrence interval)?
Average interval between events of a given magnitude (e.g., ‘1-in-100 year’); not a schedule.
Example: Design checks were run for the 1-in-200 year return period plus climate uplift.
Read full definition →What is Lead Local Flood Authority (LLFA)?
The county or unitary authority responsible for managing local flood risk (surface water, groundwater and ordinary watercourses) in England under the Flood and Water Management Act.
Example: We route surface water enquiries and culvert consent issues to the relevant LLFA for the site location.
Read full definition →What is Strategic Flood Risk Assessment (SFRA)?
A local planning evidence base produced for a Local Planning Authority area, assessing flood risk and informing land allocation, site policies and planning decisions.
Example: The SFRA identifies functional floodplain constraints and preferred mitigation approaches for developments in the district.
Read full definition →What is Floodplain?
Low-lying land that naturally stores and conveys floodwater during high flows/tides.
Example: We avoid placing critical equipment in the active floodplain even with defences.
Read full definition →What is Property Flood Resilience (PFR)?
Measures applied to buildings to reduce flood damage and speed recovery (resistance and resilience), such as barriers, non‑return valves, raised electrics and flood‑resistant materials.
Example: For ground‑floor retail units, PFR upgrades reduce reinstatement time and business interruption following shallow flooding.
Read full definition →Monitor Flood Risk Across Your Portfolio
From catchment hydrology to real-time alerts — FloodWatch turns these concepts into actionable intelligence for your sites.