Updated Peak Rainfall Allowances: New Environment Agency Guidance on Climate Change Uplifts
The EA removes the standard 40% rainfall climate change uplift and implements regional variation by management catchment, with 48 catchments seeing increases.
The Environment Agency has issued updated rainfall climate change allowances for England, changing how peak rainfall allowances are applied in flood risk assessments and development planning.
Published on the 10th of May, the Environment Agency has rolled out changes to peak rainfall intensity allowances across Management Catchments in England. The update removes the standard 40% uplift in rainfall to account for climate change and instead implements a regional variation in allowances, similar to the way that peak river flow updates were introduced in July 2021.
What’s Changed?
Peak rainfall changes now vary spatially by management catchment unit, and over time — with different allowances for different epochs of climate change. There are also different allowances for rainfall intensity for different return periods or annual exceedance probability (AEP) events, even within the same epoch.
Guidance for Development Planning
The updated guidance now sees the requirement to run additional return periods and to be sized for the upper end 1% AEP scenario.
For development with a lifetime up to or beyond 2100, assess the upper end allowances for both the 1% and 3.3% annual exceedance probability events for the 2070s epoch (2061 to 2125).
There are 48 management catchments where developers will now have to account for increased climate change beyond the existing 40%, broadly impacting the South East, South West and North West of the UK.
No catchments will see a reduction in peak rainfall allowance for climate change.
Applying Rainfall Climate Change Allowances in Rural Catchments
The assessment of climate change also has to take into account the rural vs. urban nature of the catchment as well as the size of the catchment.
“Use peak rainfall allowances in small catchments (less than 5 sq km), or urbanised drainage catchments. For large rural drainage catchments use the peak river flow allowances.”
The implication is that for developments in large catchments that are predominantly rural, peak rainfall allowances are taken from the fluvial climate change allowances, which could result in significantly higher uplifts.