Guidance

Flood and coastal risk projects, schemes and strategies: climate change allowances

When and how risk management authorities should use climate change allowances for flood and coastal risk projects, schemes and strategies.

From:
Environment Agency
Published
22 July 2020
Last updated
17 May 2022 — See all updates

Applies to England

This guidance is for risk management authorities seeking flood and coastal erosion risk management (FCERM) grant-in-aid (GIA) for FCERM projects, schemes and strategies. You should use:

  • climate change allowances to assess and plan for future flood risk and coastal change
  • this guidance together with the FCERM appraisal guidance

You need to use the flood risk assessments: climate change allowances guide for:

  • flood risk assessments
  • strategic flood risk assessments
  • FCERM schemes that need planning permission, in addition to this guide

What climate change allowances are

Climate change allowances are predictions of the anticipated change for:

  • peak river flow
  • peak rainfall intensity
  • sea level rise
  • offshore wind speed and extreme wave height

They are based on UK climate change projections (UKCP). There are different allowances for different epochs or periods of time over the coming century. Climate change allowances support the National FCERM Strategy for England.

Appraising FCERM projects, schemes and strategies

The Environment Agency appraises projects, schemes and strategies that seek FCERM GIA funding. They use this guidance to compare projects, schemes and strategies in different locations. This ensures they allocate government funding where it provides the biggest benefit.

Exceptions to using climate change allowances

Sometimes, local evidence might support using other data or allowances. For example, the impact of climate change on peak river flow may not be the same in all river catchments in a river basin district. The Environment Agency may want to check how and why you used other data.

Why you need to use climate change allowances

Climate change allowances help you to understand how flood or coastal erosion risk may change over time. They enable you to develop projects, schemes and strategies that can adapt to a range of future climate change scenarios. We call this an adaptive approach. See section ‘Adaptive approach to managing flood and coastal erosion risk’ in this guide for more information.

General approach to apply climate change allowances

By applying this guidance, you will develop solutions that are more effective in the long-term and for a range of future climate change scenarios.

Proportionate climate change allowance assessments

This guidance provides an approach that will be appropriate to a wide range of projects, schemes and strategies. Application of this guidance should be proportionate to the scale of your project, scheme or strategy.

This may involve a broad assessment of climate impacts over the lifetime of the project, scheme or strategy. It could identify where you need to do more detailed analysis over interim time periods.

To minimise detailed analysis, group different options together that have common circumstances. Develop your approach based on the following 3 steps. You should apply the appropriate climate change allowances as set out in this guide. Your assessment should show how different climate change scenarios may affect the baseline option over time. This will help you understand the scale and consequences of the changing risks.

In the FCERM appraisal the baseline is usually the ‘do nothing’ option. For information about:

1. Apply the ‘design’ climate change allowances to each appraisal option

The ‘design’ climate change allowances are set out in each of the ‘which allowances to use’ sections in this guide. Apply the design climate change allowances to each appraisal option. This will help you understand the effect on each design option over time.

If you want to achieve a particular standard of protection, you’ll need to allow for suitable mitigation in your design. You’ll need to do this for each design option. This helps you to:

  • compare different options based on potential performance over time
  • identify options with the greatest potential to meet your project objectives
  • select your leading design options for further testing

For more information about:

Your design allowances must be based on the climate change allowances in this guide.

2. Apply climate change allowances for severe climate change

You should apply severe climate change allowances to your leading options. This will show how sensitive these options are to severe climate impacts. Work with your stakeholders to identify:

  • how the risk changes
  • extra resilience or mitigation measures to cope with severe impacts

Use climate change allowances over interim periods to help identify when to put these extra measures in place. See section 11 of the FCERM appraisal guidance for information about:

  • how to select your preferred option
  • using sensitivity testing to support your recommendation

Use the severe climate change allowances in this guide.

This will show how your recommended option performs under extreme conditions. Share this information with stakeholders in the appraisal process to:

  • show the impacts of extreme climate change
  • help inform their planning to achieve community-wide resilience

You must include the extreme scenario where:

  • the scale of flooding or coastal erosion risk impacts are extreme
  • management options involve very high value or low adaptability assets

You do not usually need to include the extreme scenario in:

  • the design of your recommended option
  • your appraisal’s economic and financial assessment

If management options must involve very high value or low adaptability assets you should consider:

  • when to put them in place
  • their costs
  • the benefits of extra measures to cope with the more extreme scenario

Transition arrangements

Peak rainfall intensity allowances

The transition deadlines explain when you should use the new peak rainfall intensity allowances.

Sea level rise, wind and wave allowances

These allowances have not changed. Continue to use them as set out in this guide.

Peak river flow allowances

Peak river flow allowances show the anticipated changes to peak flow by management catchment. Management catchments are sub-catchments of river basin districts. The peak river flow allowances are for management catchments.

The allowances are based on percentiles from UKCP18 data. A percentile is a measure used in statistics. They describe the proportion of possible scenarios that fall below an allowance level. The:

  • central allowance is based on the 50th percentile
  • higher central allowance is based on the 70th percentile
  • extreme allowance is based on the 95th percentile

An allowance based on the 50th percentile is exceeded by 50% of the projections in the range. At the 70th percentile it is exceeded by 30%. At the 95th percentile it is exceeded by 5%.

Which allowances to use for peak river flow

To find out which allowances to use:

The Environment Agency also provide these allowances in the peak river flow climate change allowances by management catchment table. You have to know your management catchment to get the information you need.

To manage river flooding in your project, scheme or strategy you should use the:

  • central allowance as your design allowance
  • higher central allowance to test the impacts of higher scenarios of climate change and any extra mitigation
  • extreme allowance to test your option under more extreme climate change and exceedance events
  • 2080s epoch allowances for changes beyond the 2080s epoch and up to 2115

Apply your allowances

You should:

  • follow the general approach to apply climate change allowances section
  • interpolate (adjust) the peak river flow allowance between the current year and 2030
  • apply the peak river flow allowance to the hydrological estimate for the 2050s and 2080s

Figure 1 shows how the climate change allowances for peak river flows should be applied.

Figure 1: application of peak river flow allowances

peak river flow allowances graph

Non-stationarity

Hydrological estimates usually assume extreme flood events in the past are ‘stationary’. This is where we assume past flood events can represent future flood events.

In some rivers, we can observe an increasing number of extreme flood events. This has prompted hydrologists to consider how peak river flows change over time. Hydrologists term this phenomenon as non-stationarity.

The Environment Agency has published guidance on non-stationary fluvial flood frequency estimation. You should apply the methods in section 5.7 of the guidance if using non-stationary peak river flows. Where a river flow record is non-stationary, you may be able to adjust the peak river flow allowance in the 2020s. This would account for observed changes in peak river flows during the 2020s epoch (2015 to 2039).

Peak rainfall intensity allowances

Increased rainfall affects river levels and land and urban drainage systems. Peak rainfall intensity allowances show the anticipated changes to peak rainfall by management catchment. The allowances are based on percentiles from UKCP18 data. A percentile is a measure used in statistics. They describe the proportion of possible scenarios that fall below an allowance level. The:

  • central allowance is based on the 50th percentile
  • upper end allowance is based on the 95th percentile

An allowance based on the 50th percentile is exceeded by 50% of the projections in the range. At the 95th percentile it is exceeded by 5%.

Which allowance to use for peak rainfall intensity

To find out which allowances to use:

The Environment Agency also provide these allowances in the climate change allowances by management catchment table. You need to know your management catchment to get the information you need. You should use the peak rainfall intensity ranges in the peak river flow and rainfall intensity allowances for management catchment map to assess and design projects, schemes and strategies in:

  • small catchments of less than 5km2
  • urban locations, for example inner city locations with lots of impermeable surfaces

You should use the:

  • central allowance as your design allowance
  • upper end allowance to test the sensitivity of your option to more severe climate change and extra mitigation

For large rural river catchments over 5km2 you should use the peak river flow allowances.

Apply your allowances

You should follow the general approach to apply climate change allowances section.

Sea level allowances

UKCP18 provide projections of relative mean sea level rise around the whole UK coast. You can find out more about how the marine projections were developed on the UKCP18 website.

You should use site specific sea level rise values from the UKCP18 user interface. The user interface provides a range of allowances based on percentiles.

UKCP18 Representative Concentration Pathway (RCP) 8.5 use the:

  • 70th percentile (higher central) as the design allowance
  • 95th percentile (upper end) allowance in planning for more severe climate impacts

An allowance based on the 70th percentile is exceeded by 30% of the projections in the range. At the 95th percentile it is exceeded by 5% of the projections in the range.

Calculating sea level

To calculate sea level, add together the:

Present day extreme sea levels account for the astronomical tide level and the surge.

The UKCP18 dataset projects to 2100. To calculate epochs up to 2125, use the average incremental increase from the last 5 years of the dataset – 2094 to 2099 from RCP 8.5.

To perform an extra assessment for the lifetime of the scheme after 2125, use the sea level rise projections to 2300 for RCP 8.5. You can get sea level rise projections for the appropriate location and year from the UKCP18 user interface.

The data in table 1 is from the UKCP18 user interface. It is provided for illustrative purposes only. tIt presents change using RCP8.5 at the 70th and 95th percentile, based on a 1981 to 2000 baseline. You should use the climate change allowances from the interface in your assessment.

Table 1: time-mean sea level rise in metres in 2099 in 4 locations across the UK

Location Mean sea level rise (metres) in 2099 (70th percentile) Mean sea level rise (metres) in 2099 (95th percentile)
Bristol 0.85 1.11
London 0.86 1.13
Newcastle 0.73 1.00
Liverpool 0.76 1.02

Credible maximum (H++) allowances

National credible maximum sea level rise estimates (also referred to H++) are provided for the UK. The Met Office has not produced new H++ allowances as part of UKCP18. They consider it is still appropriate to use the UKCP09 H++ allowances.

Where it is appropriate to apply H++ allowances you should use the allowance use a figure of 1.9 metres by 2100 (based on 1990 baseline). Contact the Environment Agency if you need to consider plausible, high end scenarios for other time periods.

Which allowance to use for coastal flooding

To manage coastal flooding in your project, scheme or strategy you should use the:

  • higher central (70th percentile from UKCP18 RCP 8.5) as your design allowance
  • upper end allowance (95th percentile from UKCP18 RCP 8.5) to test sensitivity to severe climate change and any required mitigation
  • H++ to test your option under more extreme climate change and exceedance events

Apply your allowances

You should follow the approach in the general approach to apply climate change allowances section.

The Environment Agency expects sea level rise to increase the rate of coastal erosion. Use the coastal erosion risk maps to plan for any changes in the position of the coastline. They show:

  • the shoreline management plan policy for each stretch of coast
  • erosion predictions where there is no policy to maintain defences

The coastal flooding allowance may help inform your coastal erosion scenarios.

Offshore wind speed and extreme wave height allowance

Wave heights may change because of:

  • increased water depths
  • changes to the frequency, duration and severity of storms

Use the appropriate allowance for wind speed and wave height in table 2 if they are not included in the coastal model. The allowances are not cumulative across the epochs. Use the:

  • sensitivity test in addition to offshore wind speed
  • extreme wave height allowances in assessments where the H++allowances apply

Table 2: offshore wind speed and extreme wave height allowance (based on a 1990 baseline)

Applies around all the English coast 2000 to 2055 2056 to 2125
Offshore wind speed allowance 5% 10%
Offshore wind speed sensitivity test 10% 10%
Extreme wave height allowance 5% 10%
Extreme wave height sensitivity test 10% 10%

Which allowance to use offshore wind speed and extreme wave height

You should:

  • apply the allowances as standard for any coastal hydraulic modelling of climate change impacts
  • use the sensitivity test in addition to the allowances where flooding impacts could be extreme

Do not apply the allowances or sensitivity tests across epochs on a cumulative basis.

Storm surge

Present day levels in the Coastal design sea levels - coastal flood boundary extreme sea levels (2018) account for storm surge.

Most Environment Agency coastal models use these extreme sea levels. Check whether this is the case with the Environment Agency when you receive the model. New coastal modelling must use the coastal flood boundary extreme sea levels (2018) to account for storm surge. You should then apply the sea level allowance in this guide.

You should test your designs and plans against the H++ scenario where flooding impacts could be extreme. This is based on an assessment of the research in the UKCP18 Marine Report (2018). A surge increase of +2mm per year should be applied for all annual exceedance probabilities. You should apply this from a baseline year of 2017 to align with extreme sea levels in the coastal flood boundary extreme sea levels (2018).

Change to storm surge

The H++ estimate is +2mm per year from 2017.

This ensures a rigorous assessment of current coastal extreme water levels. The assessment is done by using the Coastal design sea levels - coastal flood boundary extreme sea levels (2018).

Wave period

Increases in wave period are caused by changes in storm characteristics that drive swell conditions. No specific allowances are recommended as more research is needed in this area.

You should consider sensitivity testing increases to wave period where flooding impacts are extreme. If you need support on this please contact the Environment Agency.

Adaptive approach to managing flood and coastal erosion risk

An adaptive approach involves implementing planned actions when particular thresholds are triggered. This avoids over-engineering assets and provides flexibility to manage future climate change uncertainties. This is likely to be appropriate where the ongoing responsibility is assigned to an operating authority for:

  • monitoring the change in risk and thresholds over time
  • implementing the actions

An adaptive approach may not always be possible or economically efficient. In these cases, it will be more appropriate to take a precautionary approach. This is where you pursue ‘no regrets’ or ‘low regret’ options. For example, a one-off option that uses an extreme allowance to mitigate future climate impacts. Adaptive approaches help to make:

  • economically optimal investment decisions
  • more flexible use of GIA funding

For example, you can make decisions where and when they are most needed, rather than making them up front. It reduces the risk of making decisions that are inflexible to future climate change.

You need to justify your approach to managing climate change impacts in your FCERM appraisal and any GIA application.

Find more information about the adaptive approach in the government’s FCERM policy statement.

Updates to this page

Published 22 July 2020
Last updated 17 May 2022 + show all updates

  1. Updates to climate change allowances page.

  2. Added clarity on how to apply the climate change allowances following user feedback. Also added the definition for non-stationarity.

  3. The guidance for peak river flows is now provided at a management catchment level rather than by river basin district. The new guidance will apply to flood and coastal risk projects, schemes and strategies from 20 July 2021. The updated guidance now takes account of the latest climate science from the 2018 UK Climate Impacts Programme.

  4. First published.

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