New climate change allowances

Our take on the EA’s peak river flow allowances

In July, policy changes were announced as part of the Planning Practice Guidance that affect the way we assess flood risk; setting out when and how local planning authorities, developers and their agents should use climate change allowances in flood risk assessments.

We hear from Alex Bearne, Head of Flood Risk & Hydrology at Calibro, as we explain these changes and look at how they may make it more complex to navigate the planning process and to get the green light for development. The new guidance introduces some interesting opportunities in certain parts of the country to increase developable area from reduced flood risk, while other areas may be faced with more burdensome results.

Climate change allowances

When appraising flood risk we must take account of the projected impacts of climate change by applying the ‘climate change allowances’ presented in the Planning Practice Guidance.

They provide allowances for different climate scenarios at different periods over the next 100 years, including worst-case estimates for extreme climate change. Ultimately, the purpose of the guidance is to boost resilience to flooding, which is crucial to sustainability.

So, what has changed?

On 20 July 2021 the Planning Practice Guidance was updated to include revised climate change allowances for peak river flows using the latest rainfall predictions from the UK Climate Projections. Whereas the previous guidance was based on just ten river basins, the new guidance takes a more refined approach, subdividing England into 94 ‘management catchments’. This is a potentially significant improvement, leading to a logical increase in spatial variability. In essence, there are some ‘winners’ and ‘losers’.

Map of the UK depicting the relative change in Central Allowance for the 2080s
Figure 1 – Relative Change in Central Allowance for the 2080s

Greater range of predicted changes than previous allowances

The guidance retains peak river flow allowances for three distinct timeframes: the 2020s, 2050s and 2080s. It also retains three probabilistic scenarios:

  • The ‘Central’ estimate (50th percentile)
  • The ‘Higher Central’ estimate (70th percentile)
  • The ‘Upper End’ estimate (90th percentile)

The range of predicted changes across the UK is greater than the previous allowances. For example, the range of allowances for the ‘central’ estimate for the 2080s has changed from 20-35% to 4-52%. While there are no universal trends apparent from comparison of the old and new allowances, the changes for the central estimate allowances for the 2080s are generally an increase in western coastal catchments, and a decrease in the Anglian and Lower Thames river basins (see Figure 1).

Changes to the application of scenarios

However, the impact on specific projects is complicated by the change in the application of scenarios. Alex Bearne explains: “Most developments will now have to consider the ‘central’ estimate, rather than both the ‘central’ and ‘higher central’ estimates.

Notable exceptions are ‘Essential Infrastructure’, Nationally Significant Infrastructure Projects, new settlements and significant urban extensions, where proposals will need to be tested against the extreme climate change scenarios.”

What will be the impact of these changes?

These changes will certainly have an impact on the assessment of future fluvial flooding for proposed developments and for strategic flood risk assessments.

For planning applications, the changes are not just limited to those sites currently affected by a 1 in 1,000 year flow (Flood Zone 2). In fact, the Planning Practice Guidance specifically includes “locations that are currently in flood zone 1, but might be in flood zone 2 or 3 in the future.”

“Existing modelling studies do not currently include the latest allowances, so it may be necessary to re-run them, or otherwise agree with the Environment Agency on an acceptable approach using the existing data,” Alex explains. “We don’t yet know how they will approach this. Based on experience, we suspect different approaches may be taken in different regions and for different scales of project – at least in the short term.”

Alex continues: “The guidance does not state a timeframe for implementation, so these changes could impact any planning application that has not yet reached the determination stage. Either way, it is clear that early and positive engagement with the EA will be even more crucial over the coming months than ever before.”

Further updates to come

Some of the allowances appear to be counterintuitive. For example, for the Nene Catchment, the central estimate allowances predict peak flows to decrease to the 2050s and then increase in the 2080s. Calibro has contacted the EA to confirm that these values are correct.

The EA is scheduled to release updated rainfall guidance later in the year, which may change the assessment of surface water flood risk and surface water drainage design. We look forward to sharing our thoughts with you on their implications for your development projects.

The insight to help you

Calibro’s expert flood risk team is ready to help you understand the impact of these revised climate change allowances on current and future developments; whether that’s modelling the latest allowances, negotiating with the EA to take a pragmatic approach using existing data, or helping you realise the additional value this may create for your project.