Climate Change Allowances
Percentage uplifts applied to peak river flow, rainfall intensity, sea level rise, and offshore wind speed projections to account for the future impacts of climate change on flood risk.
Climate change allowances are nationally defined percentage increases published by the Environment Agency that must be applied when assessing future flood risk. They ensure that development is designed to remain safe over its planned lifetime — typically 100 years for residential development — as climate change alters rainfall patterns, river flows, and sea levels.
The allowances cover four main categories:
- Peak river flow allowances: Percentage increases to fluvial design flows, varying by river basin district and epoch (2020s, 2050s, 2080s). The upper end and H++ scenarios are used for higher-risk applications.
- Peak rainfall intensity allowances: Uplifts to rainfall volumes used in surface water drainage design, typically between 20% and 40% depending on the epoch.
- Sea level rise: Absolute increases (in millimetres per year) applied to tidal flood levels, varying by region.
- Offshore wind speed and wave height: Used for coastal flood risk assessments.
When preparing a Flood Risk Assessment, the appropriate allowances must be selected based on the development’s vulnerability classification, its design lifetime, and the relevant river basin district. Hydraulic models are then run with these uplifted flows to determine future flood extents and levels.
The current allowances were updated by the Environment Agency in 2022 using UKCP18 climate projections. They are subject to periodic revision, so it is important to use the latest published guidance at the time of assessment. Our climate change allowances guide provides a detailed breakdown by region.
Aegaea applies the correct allowances across all flood risk and modelling projects, ensuring compliance with current EA guidance.