Guidance

Flood risk assessments: climate change allowances

When and how local planning authorities, developers and their agents should use climate change allowances in flood risk assessments.

Applies to England

This guidance is for:

  • local planning authorities preparing strategic flood risk assessments
  • developers and their agents preparing flood risk assessments for planning applications, and development consent orders for nationally significant infrastructure projects

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

The Environment Agency will check that you have used climate change allowances when they provide advice on flood risk assessments and strategic flood risk assessments.

For guidance on designing flood and coastal risk projects, schemes and strategies see Flood and coastal risk projects, schemes and strategies: climate change allowances.

For guidance on how the planning system will help minimise vulnerability and provide resilience to the impacts of climate change, see the:

Local planning authorities should refer to these when they prepare local plans and consider planning applications.

What climate change allowances are

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

To increase resilience to flooding and coastal change, you should make allowances for climate change in your flood risk assessment.

There are allowances for different climate scenarios over different epochs, or periods of time, over the coming century. They include figures for extreme climate change scenarios.

When to use climate change allowances

The Environment Agency uses 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 the same for all rivers in a management catchment.

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

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 range of allowances is based on percentiles. A percentile describes 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 flow 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 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%.

Select the peak river flow allowances to use for your assessment

For flood risk assessments and strategic flood risk assessments, the Environment Agency, as a statutory consultee, uses the management catchment climate change allowances from the peak river flow map as benchmarks.

To work out which management catchment allowances to use, you need to:

  • access the climate change allowances for peak river flow map
  • search for your location by postcode, national grid reference or town in the ‘find address or place’ search box
  • select the management catchment for your location – the allowances appear in a pop-up box

In some locations the dominant source of flooding will be from a neighbouring management catchment. If so, use the allowances from the neighbouring management catchment to assess the risk for your development or site allocation. Contact the Environment Agency if you are unsure which allowance 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.

You also need to know the flood zone your development is located in.

Using peak river flow allowances for flood risk assessments

Use the flood risk vulnerability classification to classify the vulnerability of your development.

Assess both the central and higher central allowances for strategic flood risk assessments.

In flood zones 2 or 3a for:

  • essential infrastructure – use the higher central allowance
  • highly vulnerable – use central allowance (development should not be permitted in flood zone 3a)
  • more vulnerable – use the central allowance
  • less vulnerable – use the central allowance
  • water compatible – use the central allowance

In flood zone 3b for:

  • essential infrastructure – use the higher central 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

Apply the peak river flow allowances to developments and allocations where the strategic flood risk assessment shows an increased risk of flooding in the future. This includes locations that are currently in flood zone 1, but might be in flood zone 2 or 3 in the future.

To ensure the safety of people using the development when designing safe access, escape routes and places of refuge, use the central allowance for all development types except for essential infrastructure. Use the higher central allowance for this.

If the local planning authority consider the development is appropriate, even though it will not follow the flood zone compatibility categories for flood zones 2, 3a or 3b, use the higher central allowance.

Where it is appropriate to apply a credible maximum scenario, use the upper end allowance.

Assess off-site impacts and calculate floodplain storage compensation

The appropriate allowance to assess off-site impacts and calculate floodplain storage compensation depends on land uses in affected areas. Use the:

  • central allowance for most cases
  • higher central allowance when the affected area contains essential infrastructure

You should also consider likely future land uses shown by local plan allocations or unimplemented extant planning permissions. The Environment Agency will want to see evidence from the developer to prove they have done this.

Contact the Environment Agency if you are unsure which allowance to use for flood storage compensation.

Peak rainfall intensity allowance

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

How to use the peak rainfall intensity allowance

Table 1 shows anticipated changes in peak rainfall intensity in small catchments (less than 5km2), or urbanised drainage catchments. For large rural drainage catchments use the peak river flow allowances.

For flood risk assessments and strategic flood risk assessments, assess both the central and upper end allowances.

Table 1: peak rainfall intensity allowance in small catchments (less than 5km2) or urban drainage catchments (based on a 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, where depths and velocities of surface water runoff cause a significant danger to people), you will need to take measures to protect people and property. This could include, for example, raising floor levels. As a minimum, there should be no significant flood hazard to people from on-site flooding for the central allowance.

Contact the Environment Agency if you are unsure about applying peak rainfall allowances in areas with critical drainage problems.

Contact your lead local flood authority (through your local council) if you are unsure about applying peak rainfall allowances in areas without critical drainage problems.

Sea level allowances

There are a range of allowances for each river basin district and epoch for sea level rise. They are set out in table 2 and are based on percentiles. A percentile describes the proportion of possible scenarios that fall below an allowance level.

The:

  • higher central allowance is based on the 70th percentile
  • upper end allowance is based on the 95th percentile

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.

For flood risk assessments and strategic flood risk assessments, assess both the higher central and upper end allowances.

The Environment Agency will want to see if you have considered whether it is appropriate to apply the H++ allowances for your flood risk assessment or strategic flood risk assessment. Where applicable you should do H++ allowance assessments as well as assessing the sea level rise allowances in table 2.

Table 2: sea level allowances by river basin district for each epoch in mm for each year (based on a 1981 to 2000 baseline) – the total sea level rise for each epoch is in brackets

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) 1.20
Anglian Upper end 7 (245) 11.3 (339) 15.8 (474) 18.1 (543) 1.60
South east Higher central 5.7 (200) 8.7 (261) 11.6 (348) 13.1 (393) 1.20
South east Upper end 6.9 (242) 11.3 (339) 15.8 (474) 18.2 (546) 1.60
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 allowances
  • Severn river basin district use ‘south west’ sea level rise allowances
  • parts of Solway Tweed river basin district on the west coast and Dee river basin district that are in England, use ‘north west’ sea level rise allowances
  • parts of Solway Tweed river basin district on the east coast that are in England, use ‘Northumbria’ sea level rise allowances

The allowances in table 2 account for slow land movement. This is due to ‘glacial isostatic adjustment’ from the release of pressure at the end of the last ice age. The northern part of the UK 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 using table 2, add the allowances for the appropriate one of the 6 geographical areas:

  • up to 2035, use the mm for each year rates for the appropriate geographical area, starting from the present day extreme sea levels from Coastal design sea levels – coastal flood boundary extreme sea levels (2018)
  • from 2036 to 2065, get the increase in sea level by adding the number of years on from 2035 (to 2065), multiplied by the respective rate shown in table 2 for the appropriate geographical area – if the whole time period applies use the cumulative total
  • treat time periods 2066 to 2095 and 2096 to 2125 as you would 2036 to 2065

Where it is appropriate to apply a credible maximum scenario, use the H++ allowance. There is no H++ value for sea level rise beyond 2100.

H++ sea level rise allowance

For the change to relative mean sea level use the H++ scenario of 1.9m for the total sea level rise to 2100.

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
  • 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 an adaptive approach

Coastal erosion

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, together with any designated coastal change management areas and relevant policies in local plans.

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

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

Environment Agency coastal models may already include wind speed and wave height allowance. Ask the Environment Agency if they have included wind speed and wave height allowance in your model.

The Environment Agency will want to see if you have used the appropriate allowance for wind speed and wave height in table 3 if these allowances are not included in your coastal model.

Use the epoch that the lifetime of your development falls into to choose the appropriate allowance. If your development lifetime is beyond 2056, use the allowance for the 2056 to 2125 epoch. The allowances are not cumulative across the epochs.

Use the sensitivity test allowances in addition to the offshore wind speed and extreme wave height allowances for assessments where it is appropriate to apply a credible maximum scenario.

Table 3: offshore wind speed and extreme wave height allowance (based on a 1990 baseline)

Applies all around 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%

Storm surge

The present day extreme sea levels in 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. Ask the Environment Agency when you get the model.

If your coastal model does not include an allowance for storm surge, or you are creating a new model, use the Coastal design sea levels – coastal flood boundary extreme sea levels (2018) for your extreme sea levels. Then apply the climate change sea level rise values in table 2.

Where it is appropriate to assess a credible maximum scenario, add 2mm for each year starting from 2017.

Assessing credible maximum scenarios for nationally significant infrastructure projects, new settlements or urban extensions

Nationally significant infrastructure projects (NSIPs) are major infrastructure projects such as new harbours, roads, power stations and power lines. If you develop NSIPs you may need to assess the flood risk from a credible maximum climate change scenario. Check the relevant national policy statement.

In other cases, such as new settlements or significant urban extensions, you may also need to assess the flood risk from a high impact climate change scenario. In these circumstances you should use:

  • the H++ climate change allowances for sea level rise
  • the upper end allowance for peak river flow
  • the sensitivity test allowances for offshore wind speed and extreme wave height
  • an additional 2mm for each year on top of sea level rise allowances from 2017 for storm surge

You should treat this as a ‘sensitivity test’. It will help you assess how sensitive your proposal is to changes in the climate for different future scenarios. This will help to ensure your development can be adapted to large-scale climate change over its lifetime.

Adaptive approach

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

The Environment Agency will consider whether an adaptive approach is appropriate when they review 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

Email enquiries@environment-agency.gov.uk

Telephone 03708 506 506

Telephone from outside the UK (Monday to Friday, 8am to 6pm GMT) +44 (0) 114 282 5312

Monday to Friday, 8am to 6pm.

The impact of COVID-19 means you may experience some delays in responses.

Published 19 February 2016
Last updated 6 October 2021 + show all updates
  1. Edited the section 'Peak river flow allowances': In some locations the dominant source of flooding will be from a neighbouring management catchment. If so, use the allowances from the neighbouring management catchment to assess the risk for your development or site allocation.

  2. There is an error in the 20 July 2021 update. We used UKCP18 projections, not UKCP19 to update the peak river flow allowances.

  3. We have used the UKCP19 projections to update the peak river flow allowances and we have based them on management catchments instead of river basin districts. We have also changed the guidance on how to apply peak river flow allowances. You now use (a) the central allowance for all assessments except for essential infrastructure, where you use the higher central allowance, (b) the upper end for ‘credible maximum scenario’ assessments, and (c) the central allowance to calculate flood storage compensation, except for where essential infrastructure is affected, where you use the higher central allowance.

  4. We have edited the sections on peak river flow, sea level rise, wind speed, wave height and storm surge to include guidance on how to use High ++ allowances in developments where they need to be assessed.

  5. 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.

  6. 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.

  7. 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.

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

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

  10. First published.