New Environment Agency guidance on peak river flow allowances for flood risk assessments

This week the Environment Agency have updated their guidance on climate change allowances for peak river flows in England.

The update is one of the most significant changes to the climate change guidance since its release in 2016 and provides local planning authorities, developers and their agents new guidance on how climate change allowances should be applied in strategic and site-specific flood risk assessments.

What are the main changes?

The Environment Agency have used the UKCP19 projections to update the peak river flow allowances in England. The updated guidance incorporates the following changes:

  • Peak river flow allowances are now determined on a sub-catchment level rather than per river basin district, providing a greater level of spatial variation in allowances than before.
  • The range of allowances are based on three percentiles, the central allowance, higher central and upper end. There is no longer a H++ scenario for river basin flows.
  • The guidance on how to apply the peak flow allowances for development types has changed; for all development types the central allowance is now used, except for essential infrastructure where the higher central allowance should be used.
  • When assessing off-site impacts and calculating floodplain storage compensation, the central allowances will be used in most cases, other than when the affected area contains essential infrastructure, in which case the higher central should be used.
  • For National Significant Infrastructure projects, new settlements or significant urban extension, the upper end allowance should be used to assess climate change impacts on peak river flows. This should be treated as a ‘sensitivity test’.

How much have the allowances changed?

The greater level of spatial variation in allowances has meant that in some locations allowances have increased whilst in others they have decreased from previous predictions.

For example, in the River Nene catchment (Anglian river basin district), the central allowance has decreased from 25% to 4% for the 2080s epoch. The higher central allowance for the 2080s epoch has also decreased from 35% to 13%. For the River Soar catchment (Humber river basin district), the central allowance has now increased from 20% to 28% for the 2080s epoch. The higher central allowance for the 2080s epoch has also increased from 30% to 37%.

Has there been any changes to allowances for peak rainfall or sea level rise?

Currently the update to the climate change allowances only applies to peak river flow allowances.

What does this mean for my development site?

In many ways, the new climate change guidance makes it simpler to determine which allowance to use in a site-specific flood risk assessment. For highly vulnerable, more vulnerable and less vulnerable development sites in Flood Zone 2 or 3a only the central allowance now needs to be considered rather than a range of allowances.

For new planning applications, flood risk assessments will now need to consider these updated allowances.  In some cases this may require existing hydraulic models to be re-run to determine the new flood extent generated from the updated allowances.

Want to know more?

To discuss the impacts of the new guidance to your site, please email Alice Blanchard or  contact your nearest JBA office.

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