Guidance

Flood risk assessments: climate change allowances

Find out when and how to use climate change allowances in flood risk assessments and strategic flood risk assessments.

The National Planning Policy Framework (NPPF) sets out how the planning system should help minimise vulnerability and provide resilience to the impacts of climate change. NPPF and supporting planning practice guidance on flood risk and coastal change explain when and how flood risk assessments should be used. This includes demonstrating how flood risk will be managed now and over the development’s lifetime, taking climate change into account. Local planning authorities refer to this when preparing local plans and considering planning applications.

The Environment Agency has produced this guidance as the government’s expert on flood risk.

What climate change allowances are

Making allowances for climate change in your flood risk assessment will help provide resilience to flooding and coastal change.

Climate change allowances are predictions of 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. There are different allowances for different epochs or periods of time over the next century.

When to use climate change allowances

The Environment Agency will use climate change allowances when they provide advice on flood risk assessments and strategic flood risk assessments.

Exceptions – when it might be appropriate to use other data or allowances

There may be circumstances where local evidence supports using other data or allowances. For example, the impact of climate change on peak river flow may not be even across all river catchments in a river basin district.

The Environment Agency may want to check how and why you used other data in your plans and proposals.

Types of allowances

Peak river flow allowances

Peak river flow allowances show the anticipated changes to peak flow by river basin district.

The range of allowances in table 1 is based on percentiles. A percentile is a measure used in statistics to describe the proportion of possible scenarios that fall below an allowance level. The 50th percentile is the point at which half of the possible scenarios for peak flows fall below it and half fall above it. The:

  • central allowance is based on the 50th percentile
  • higher central allowance is based on the 70th percentile
  • upper end allowance is based on the 90th 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% of the projections in the range.

For these allowances it is important you do not use a single percentile out of context. For example, while the 50th percentile is the central estimate (the average), it does not represent the full range of likely futures. Using this percentile on its own may risk under-adapting to climate change.

Decide which peak river flow allowances to use for assessment

The Environment Agency uses the following data and standards as the benchmarks for the advice it gives as a statutory consultee:

The Environment Agency will want to see if you have considered if it is appropriate to apply high++ allowances for your flood risk assessment or strategic flood risk assessment in addition to the peak river flow allowances in table 1.

Table 1: peak river flow allowances by river basin district (use 1961 to 1990 baseline)

River basin district Allowance category Total potential change anticipated for the ‘2020s’ (2015 to 2039) Total potential change anticipated for the ‘2050s’ (2040 to 2069) Total potential change anticipated for the ‘2080s’ (2070 to 2115)
Northumbria Upper end 20% 30% 50%
  Higher central 15% 20% 25%
  Central 10% 15% 20%
Humber Upper end 20% 30% 50%
  Higher central 15% 20% 30%
  Central 10% 15% 20%
Anglian Upper end 25% 35% 65%
  Higher central 15% 20% 35%
  Central 10% 15% 25%
South east Upper end 25% 50% 105%
  Higher central 15% 30% 45%
  Central 10% 20% 35%
Thames Upper end 25% 35% 70%
  Higher central 15% 25% 35%
  Central 10% 15% 25%
South west Upper end 25% 40% 85%
  Higher central 20% 30% 40%
  Central 10% 20% 30%
Severn Upper end 25% 40% 70%
  Higher central 15% 25% 35%
  Central 10% 20% 25%
Dee Upper end 20% 30% 45%
  Higher central 15% 20% 25%
  Central 10% 15% 20%
North west Upper end 20% 35% 70%
  Higher central 20% 30% 35%
  Central 15% 25% 30%
Solway Upper end 20% 30% 60%
  Higher central 15% 25% 30%
  Central 10% 20% 25%
Tweed Upper end 20% 25% 45%
  Higher central 15% 20% 25%
  Central 10% 15% 20%

The following maps will help you find out which river basin district you are in:

Using peak river flow allowances for flood risk assessments

Consider the flood risk vulnerability classification to decide which allowance applies to your development or plan. This will help you understand the range of impact. The central, higher central and upper end allowances are in table 1.

Also apply these allowances for developments and allocations in places where a local authority’s strategic flood risk assessment identifies them as likely to be at increased risk of flooding in the future. This would include locations currently in flood zone 1, but might be in flood zone 2 or 3 in the future.

In flood zones 2 or 3a for:

  • essential infrastructure – use the upper end allowance
  • highly vulnerable – use higher central and upper end allowances to assess a range of allowances (note – development should not be permitted in flood zone 3)
  • more vulnerable – use the higher central and upper end allowances to assess a range of allowances
  • less vulnerable – use the central and higher central allowances to assess a range of allowances
  • water compatible – use the central allowance

For less vulnerable development use the higher central allowance as the basis for designing safe access, escape routes and places of refuge. This will ensure the safety of people using the development.

In flood zone 3b for:

  • essential infrastructure – use the upper end allowance
  • highly vulnerable – development should not be permitted
  • more vulnerable – development should not be permitted
  • less vulnerable – development should not be permitted
  • water compatible – use the central allowance

If development is considered appropriate when not in accordance with flood zone vulnerability categories, use the upper end allowance.

Floodplain storage compensation

The appropriate allowance to assess off-site impacts and calculate floodplain storage compensation depends on the land uses in affected areas. In the majority of cases use the higher central allowance to calculate floodplain storage compensation.

Use the upper end allowance to calculate floodplain storage compensation when the:

  • catchment is particularly sensitive to small changes in volume, causing significant increases in flood depth or hazard
  • affected area contains essential infrastructure or vulnerable uses such as primary schools, caravans, bungalows or basement dwellings

Use the central allowance for floodplain storage compensation if you can demonstrate that the affected area contains only low vulnerability uses such as water compatible development. When you apply this principle you should also consider likely future land uses, as indicated by local plan allocations or submitted in planning applications. The Environment Agency will want to see evidence from the developer to demonstrate this is the case.

If you are unsure which allowance to use for flood storage compensation contact the Environment Agency.

Peak rainfall intensity allowance

Increased rainfall affects river levels and land and urban drainage systems.

How to use the peak rainfall intensity allowance

Table 2 shows anticipated changes in peak rainfall intensity in small catchments (less than 5 km2) and urban catchments. For large rural catchments use the allowances in table 1.

For flood risk assessments and strategic flood risk assessments, assess both the central and upper end allowances to understand the range of impact.

Table 2: peak rainfall intensity allowance in small and urban catchments (use 1961 to 1990 baseline)

Applies across all of England Total potential change anticipated for the ‘2020s’ (2015 to 2039) Total potential change anticipated for the ‘2050s’ (2040 to 2069) Total potential change anticipated for the ‘2080s’ (2070 to 2115)
Upper end 10% 20% 40%
Central 5% 10% 20%

Design your drainage system to make sure there is no increase in the rate of runoff discharged from the site for the upper end allowance.

Where on-site flooding for the upper end allowance presents a significant flood hazard (for example, depths and velocities of surface water runoff cause a significant danger to people), you will need to take further mitigation measures to protect people and property (for example, raising finished floor levels). As a minimum, there should be no significant flood hazard to people from on-site flooding for the central allowance.

In areas with critical drainage problems contact the Environment Agency if you are unsure. In all other locations contact your lead local flood authority through your local council.

Sea level allowances

There is a range of allowances for each region and epoch or time frame for sea level rise in table 3.

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. The maps are based on the best available data. They show the shoreline management plan policy for each stretch of coast and erosion predictions where there is no policy to maintain defences.

The Environment Agency will want to see if you have considered if it is appropriate to apply high++ allowances for your flood risk assessment or strategic flood risk assessment in addition to the sea level rise allowances in table 3.

Table 3: sea level allowance for each epoch in millimetres (mm) per year, with total sea level rise for each epoch in brackets (use 1981 to 2000 baseline) by river basin district

Area of England Allowance 2000 to 2035 (mm) 2036 to 2065 (mm) 2066 to 2095 (mm) 2096 to 2125 (mm) Cumulative rise 2000 to 2125 (metres)
Anglian Higher central 5.8 (203) 8.7 (261) 11.6 (348) 13 (390) 1.2
Anglian Upper end 7 (245) 11.3 (339) 15.8 (474) 18.1 (543) 1.6
South east Higher central 5.7 (200) 8.7 (261) 11.6 (348) 13.1 (393) 1.2
South east Upper end 6.9 (242) 11.3 (339) 15.8 (474) 18.2 (546) 1.6
South west Higher central 5.8 (203) 8.8 (264) 11.7 (351) 13.1 (393) 1.21
South west Upper end 7 (245) 11.4 (342) 16 (480) 18.4 (552) 1.62
Northumbria Higher central 4.6 (161) 7.5 (225) 10.1 (303) 11.2 (336) 1.03
Northumbria Upper end 5.8 (203) 10 (300) 14.3 (429) 16.5 (495) 1.43
Humber Higher central 5.5 (193) 8.4 (252) 11.1 (333) 12.4 (372) 1.15
Humber Upper end 6.7 (235) 11 (330) 15.3 (459) 17.6 (528) 1.55
North west Higher central 4.5 (158) 7.3 (219) 10 (300) 11.2 (336) 1.01
North west Upper end 5.7 (200) 9.9 (297) 14.2 (426) 16.3 (489) 1.41

The following maps will help you find out which river basin district you are in:

For places in:

  • Thames river basin district use ‘south east’ sea level rise allowance
  • Severn river basin district use ‘south west’ sea level rise allowance
  • Solway Tweed river basin district on the west coast, and in England only and Dee river basin district, use ‘north west’ sea level rise allowance
  • Solway Tweed river basin district on the east coast, and in England only, use ‘Northumbria’ sea level rise allowance

These allowances account for slow land movement. This is due to ‘glacial isostatic adjustment’ resulting from the release of pressure at the end of the last ice age. The northern part of the country is slowly rising and the southern part is slowly sinking. This is why net sea level rise is less for the north west and north east than the rest of the country.

To calculate sea level, add the allowances for the appropriate one of the 3 geographical areas to the 1981 to 2000 base sea level year. For example, to get sea levels:

  • up to 2035, use the mm per year rates or the cumulative totals for the appropriate geographical area
  • from 2036 to 2065, you get the increase in sea level in this period by adding the number of years on from 2035 (to 2065), multiplied by the respective rate shown in table 3 for the appropriate geographical area – if the whole time period applies use the cumulative total
  • treat subsequent time periods 2066 to 2095 and 2096 to 2125 as you would 2036 to 2065

For flood risk assessments and strategic flood risk assessments, assess both the central and upper end allowances to understand the range of impact.

Offshore wind speed and extreme wave height allowance

Wave heights may change because of increased water depths. The frequency, duration and severity of storms could also change.

Environment Agency coastal models may already include the wind speed and wave height allowance. Ask the Environment Agency if the wind speed and wave height allowance has been included when you get the model.

The Environment Agency will want to see if you have used the appropriate allowance for wind speed and wave height in table 4 if these allowances are not included in the coastal model.

Use the epoch the lifetime of your development falls into to choose the appropriate allowance. If your development lifetime is beyond 2056, only use the allowance for the 2056 to 2125 epoch.

The allowances are not cumulative across the epochs. Use the sensitivity test allowances in assessments where the high++ allowances apply in addition to the offshore wind speed and extreme wave height allowances.

Table 4: offshore wind speed and extreme wave height allowance (use 1990 baseline)

Applies around all the English coast 2000 to 2055 2065 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%

Storm surge

The majority of Environment Agency coastal models include an allowance for storm surge – ask the Environment Agency when you get the model. If your coastal model does not include an allowance for storm surge, refer to allowances and advice for storm surge in Adapting to climate change: guidance for risk management authorities.

High++ allowances

High++ allowances only apply in assessments for developments that are very sensitive to flood risk, and with lifetimes beyond the end of the century. For example, infrastructure projects or developments that significantly change existing settlement patterns. This includes urban extensions and new settlements.

High++ allowances are in the Environment Agency guidance Adapting to climate change: guidance for risk management authorities.

How to use a range of allowances to assess flood risk

To help you decide which allowances to use to address flood risk for a development or development plan allocation, consider the:

  • likely depth, extent, speed of onset, velocity and duration of flooding for each allowance of climate change over time – consider the allowances for the relevant development lifetime
  • vulnerability of the proposed development types or land use allocations to flooding
  • ‘built in’ measures used to address flood risk, for example, raised floor levels
  • capacity or space in the development to include measures to manage flood risk in the future, using a managed adaptive approach

Future flood risk management

Some measures to manage flood risk are not necessary now but may be in the future. This is a managed adaptive approach, for example, setting a development away from a river so it is easier to improve flood defences in the future.

The Environment Agency will consider whether a managed adaptive approach might be appropriate when reviewing your plans or proposals.

Get planning advice on your proposals

The Environment Agency can give a free preliminary opinion to applicants who consult us on their proposals outside the statutory consultation process. This will include advice on what allowances to apply and the appropriate approach to incorporating the allowances into assessments.

There is a charge for more detailed pre-application planning advice and reviews of flood risk modelling.

Contact your lead local flood authority (through your local council) for advice on flood risk from local watercourses, surface or groundwater.

Contact the Environment Agency

General enquiries

National Customer Contact Centre
PO Box 544
Rotherham
S60 1BY

Monday to Friday, 10am to 4pm.

Due to the current health emergency we have reduced the times our phone lines are open. This is under review and we hope to be able to extend them soon. If possible please contact us by email at enquiries@environment-agency.gov.uk.

The impact of COVID-19 on our teams means you may experience some delays in responses as most of our staff will be working from home. We are currently unable to deal with post sent to our offices in the usual way.

Published 19 February 2016
Last updated 16 March 2020 + show all updates
  1. Correction to example 3, to get sea levels: treat subsequent time periods 2066 to 2095 and 2096 to 2125 as you would 2036 to 2065.

  2. We have made the following updates: 1) Updated the sea level rise allowances using UKCP18 projections. 2) Added guidance on how to a) calculate flood storage compensation, b) use peak rainfall allowances to help design drainage systems, c) account for the impact of climate change on storm surge, d) assess and design access and escape routes for less vulnerable development. 3) Changed the guidance on how to apply peak river flow allowances so the approach is the same for both flood zones 2 and 3.

  3. Added: This guidance is being revised in line with the UK Climate Projections 2018. Please contact the Environment Agency for interim guidance if you are preparing a flood risk assessment for a development or local plan affected by tidal flooding.

  4. Temporary exceptions sections removed and information for flood zone 1 added

  5. Epochs in tables 2,3 and 4 updated.

  6. First published.