Updates to Climate Change Allowances in England, July 2021
The Environment Agency revised climate change guidance for river flows, replacing broad regional approaches with granular management catchment boundaries based on UKCP18.
On 20 July 2021, the Environment Agency revised its climate change guidance for river flows, replacing regional approaches with more granular management catchment boundaries.
Key Changes
Previous system: The old framework used broad regions (Thames, South East, South West, etc.) with uniform climate change scenarios applied across each area.
New system: The revised guidance implements “management catchment boundary” designations, with specific allowances that vary significantly within regions. For example, the Humber region now contains 15 sub-catchments, while Thames contains 17.
These boundaries align with Water Framework Directive surface water management catchment designations, accessible via the Environment Agency’s catchment data explorer.
Impact
The guidance affects all flood modelling studies, flood risk assessments, and developments in England from the effective date. Importantly, climate change factors did not increase uniformly — some regions experienced reductions, potentially enabling previously unviable development sites to move forward.
For instance, in South East England, residential dwellings now require allowances of either 45% or 105%, compared to the previous approaches. These changes can affect “flood depths, flood extents and sometimes both,” making pre-development climate impact assessments crucial.
Climate Change Allowance Framework
Allowances are determined using percentiles:
- Central allowance: 50th percentile
- Higher central allowance: 70th percentile
- Upper end allowance: 90th percentile
Each application requires individual evaluation, as “each assessment is therefore unique especially the approach and methods applied to each development.”