On the level: how climate change could impact development


On the level: how climate change could impact development

By Paul Jenkin, Partner – Water Management

Last week, the UK government released new climate change guidance for flood risk assessments, outlining the allowances for the impact of climate change on peak river flows, peak rainfall intensities, sea level rise, offshore wind speeds and extreme wave height.

Any project that involves a flood risk assessment which has not yet been submitted for planning will probably need to be updated to cater for these allowances. If a development is perceived as ‘particularly sensitive to flood risk or in a vulnerable location’, the EA will base its advice on the new allowances, even when a planning application has already been submitted.

The peak river flow and rainfall intensities for the next 100 years have increased significantly – from 20% to as much as 105% for peak river flow, and from 30% to 40% for peak rainfall. This could have a significant impact on development proposals.

The guidance allows room for interpretation, so it is essential that the relevant approving bodies (the Environment Agency, local planning authorities and lead local flood authorities) are engaged at the earliest opportunity to agree the correct approach.

The new river allowances are complex, varying with geographical location, Flood Zone, development lifetime and type of development. The change in allowances will impact upon the design of finished floor levels and watercourse crossings, the assessment of safe access and egress and the assessment of the impact of proposals on flood risk elsewhere. The EA are not proposing to update their models to reflect this guidance. It will be the responsibility of the applicant to assess the future risk, based on the new climate change allowances. This could have significant time and cost implications.


It is our understanding that the EA flood maps for planning, which present the Flood Zones but which do not include climate change effects, will remain unchanged. We therefore assume that the changes will not be used to inform the sequential test – and that there will be no direct impact on the allowable development types within a given Flood Zone. However, the increase in flood levels used in the design process could impact the deliverability and economic viability of proposed developments.

The revised rainfall allowances will need to be accounted for in surface water drainage design. It is expected that lead local flood authorities (LLFAs) will request use of the 40% increase in rainfall for the purpose of surface water drainage design and assessment of exceedance flow pathways. This will cause an increase in the storage volumes required to meet planning policy, and greater consideration will need to be given to overland flow pathways.

While this guidance is in its infancy, there will be a lot of uncertainty with regards to its potential impacts. Although it is ostensibly for flood risk assessments, the guidance is likely to extend into all areas where flood risk is considered and may affect the design of bridges and other structures through the Flood Defence Consent process.

Peter Brett Associates is now working with each of the approving bodies to clarify how this guidance will need to be applied. For more information, please contact Paul Jenkin or Simon Darch.

Paul Jenkin

Paul Jenkin

Director – Water Management

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