The impact of climate uncertainty on

flood risk assessment

The challenges surrounding flood risk have never been greater. We need to find a way to balance the growing frequency and severity of floods, predicted as a result of climate change, with the ever-increasing demand for new houses, offices and commercial premises.

Either our towns and cities require increased expenditure on flood defences or there must be an improved approach to planning and design in flood risk areas.

Legislation and guidance is rapidly evolving in this field. In February 2016, the Environment Agency released new guidance on how to assess the impact of climate change on new development and flood defences (primarily assessing the risk from rivers).

Since the launch of the Government’s Planning Policy Statement 25 on development and flood risk in 2001 we have used a fixed allowance, increasing the peak flow rate for the 1% annual exceedance event (the 1-in-100-year flood) by 20% to allow for the impact of climate change over the next century.

The Environment Agency’s revised guidance takes a more geographically distributed approach, devising allowances based on regions or catchment boundaries. It also adopts a more probabilistic approach where different allowances are promoted based on the potential use and existing risk to allow for inherent uncertainty.

While this may be a more robust and scientific approach, it does raise significant issues in how to apply the policy to new development.

 

Larger potential allowances

The potential allowances are much greater in the new guidance. In the worst case, increasing from the previous allowance of 20% to a revised allowance of 105% (120% if the development is considered very sensitive). This represents a considerable increase in the level to which the development should be designed. The guidance does present ranges, and a lower allowance can be considered but this would need to be agreed with the Environment Agency before the assessment. It could also increase the uncertainty and time taken to assess the
risk posed to new development.

 

Lack of readily available data

Since 2001 the Environment Agency has been modelling and mapping the English river network to provide high quality, consistent estimates of flood levels, including the 20% allowance for climate change. This has allowed planners and developers to rapidly assess the potential impacts of climate change within their flood risk assessments.

 

 We’re in uncharted territory, where the projected climate change flood levels are unknown.

 

However, flood level data for all the Environment Agency’s new projections is currently not available. We’re therefore in uncharted territory, where the projected climate change flood levels are unknown and a detailed assessment must be made on each occasion after the correct allowance has been agreed with the Environment Agency. All of which adds to the cost and time spent on establishing the design constraints.

 

Taking early advice is the best policy

While this can be a source of frustration for all parties involved in new development, the challenge is not insurmountable. As practitioners, we are gradually building on our knowledge and data as the new guidance begins to bed in. The future may be uncertain, but – as always – taking early advice can minimise the issues that may appear during the planning process.

 

20%

The current fixed allowance which represents the increase in the peak flow rate to allow for the impact of climate change over the next century

105%

The revised potential allowance in the new guidance, representing the considerable increase in the level to which the development should be designed 

 

 

 

 

 

 


Paul Jenkin

Partner, Peter Brett Associates

t: 01189 520349

e: pjenkin@peterbrett.com


 

Help us find an exact match for your search. Do you mean?

    or close this panel to search again