How Climate Change Updates Affect Peak River Flows and Flood Planning
The Environment Agency moved from a blanket 20% climate change allowance to a River Basin Districts approach, with allowances varying significantly by location and development type.
Note: The climate change allowances were updated on 20 July 2021. See our updated post addressing the new guidance.
Overview
Under the National Planning Policy Framework (February 2019), developments in Flood Zones 2 and 3 must evaluate climate change scenarios. The Environment Agency shifted from a “20% blanket policy” to a more nuanced River Basin Districts approach, incorporating UK climate change projections from 2009.
Key Changes in Climate Change Allowances
The updated methodology varies significantly by development type and location. For instance, in South East England, residential dwellings now require allowances of either 45% or 105%, compared to the previous uniform 20% approach. These increases can affect “flood depths, flood extents and sometimes both,” making pre-development climate impact assessments crucial.
River Basin Districts
England has been divided into 10 river basin districts. A river basin district constitutes “the area of land and sea, made up of one or more neighbouring river basins together with their associated groundwaters and coastal waters.”
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.”