About the future flood maps

Using the SEPA flood maps for land use planning

1. Do SEPA’s Flood Maps take into account the effects of climate change?
2. How do the Future flood maps account for climate change?
3. Viewing the future flood maps
4. What about other future changes?
5. Why aren’t future flood maps available everywhere?
6. What about Flood Protection Schemes?
7. What about future risk from small watercources?

1. Do SEPA’s Flood Maps take into account the effects of climate change?

SEPA’s Flood Maps include two sets of flood hazard maps for river, coastal and surface water flooding - ‘Present Day’ and ‘Future’ Flood Hazard maps.

Present Day flood maps – maps generated using meteorological, rainfall, river flow and sea level data that are broadly representative of present day climate conditions without taking into account potential future changes due to climate change.

Future flood maps – maps generated taking into account the projected effects of climate change for a single future climate scenario based on available predictions of river flows, rainfall intensities and sea levels for time periods towards the end of this century. More information on the time periods used is described below.

2. How do the Future flood maps account for climate change?

The future flood maps were developed using the same modelling and mapping approaches as the present day flood maps but with revisions to the peak river flow, rainfall intensity and sea levels each based on projections for one future scenario for time periods towards the end of this century. More information on the time horizons used is described below.

The impact of climate change on flood risk will depend on how much global action there is to reduce greenhouse gas emissions and on the characteristics of a local area. SEPA’s future flood maps currently represent one possible future, however there are uncertainties regarding emissions and climate response which mean changes in flood risk could be smaller or larger than those shown.

Emissions scenarios

Future greenhouse gas emissions depend on a range of social, economic and technological factors including, for example, population, economic growth, and energy use, as well as unknowns about how the climate system responds. For this reason climate projections are produced for a range of emissions scenarios which correspond to different ways the world might change over the rest of the 21st century.

The future flood maps have been developed using projections from a high emissions scenario where little or no action is taken to avoid dangerous levels of climate change, with greenhouse gases continuing to rise and leading to a greater global temperature increase.

Where do we get information on how climate change could affect the UK?

The UK Climate Projections (UKCP) provide assessments of how the climate of the UK may change over the 21st century.

The river and coastal future flood maps are generally based on the UK Climate Projections 2009 (UKCP09), which was the best available projection information for the UK at the time the national river and coastal flood modelling was carried out in 2011-2013.

Since then, the UK Climate Projections 2018 (UKCP18) was launched and is now the most up to date information on the future climate of the UK to 2100. UKCP18 projections informed:

• the future coastal flood maps for the Outer Hebrides and Loch Etive.
• the future surface water and small watercourses flood maps.

The future flood map climate change allowances will be reviewed in light of the UKCP18 information, which may be incorporated into future updates.

River future flood maps

The river future flood maps reflect projected increases in peak river flows.

SEPA commissioned a study in 2011 that provided information on future changes in peak flows for Scotland’s rivers based on UKCP09 climate projections and an assessment of the vulnerability of Scotland’s river catchments to the impacts of climate change.

For each river basin region SEPA used the projected uplifts to peak flows for the High emissions scenario 67th percentile for the 2080s.

Surface water and small watercourses future flood maps

The surface water future flood maps reflect projected changes in rainfall based on analysis of future changes in short duration rainfall published in 2021 by the FUTURE-DRAINAGE research project (PDF 394KB) (FUTURE-DRAINAGE, 2021, Chan et al., 2021). The FUTURE-DRAINAGE project used analysis of the UKCP18 high resolution UKCP Local projections for Representative Concentration Pathway 8.5 (RCP8.5).

For each river basin region for the surface water and small watercourses future flood map, mean rainfall intensity uplifts were calculated for the 100 year return period, 50th percentile and for the 2070 time horizon. These were applied to uplift rainfall for the medium likelihood (1 in 200 year) event. 2070 is the central year for the 2061-2080 time period from the UKCP Local projections used to develop the rainfall uplifts.

Uplifts from FUTURE-DRAINAGE are not available for Shetland therefore uplift factors for Orkney were applied to Shetland.

Coastal future flood maps

For most of Scotland SEPA’s coastal flood maps reflect the risk from high tides and storm surges only, the risk associated with waves is generally not included at present. However, for North East Scotland, the Orkney Islands and the Outer Hebrides SEPA has undertaken regional scale modelling studies which considered waves and wave overtopping.

The coastal future flood maps reflect projected increases in mean sea levels. Predicted changes in sea level vary around the coast of Scotland.

For most of Scotland, the UKCP09 projections of sea level rise for the High emissions scenario 95th percentile confidence limit for the year 2080 were used and applied to uplift the sea levels from the medium likelihood (1 in 200 year) event.

Sea level rise projections for a high emissions scenario have increased in the latest UK Climate Projections (UKCP18). Comparison with the UKCP18 projections suggests that, with limited global action to tackle climate change, there is a 1 in 2 chance that sea level rise by 2100 will be higher than that mapped in the future coastal flood maps.

The coastal future flood maps for the Outer Hebrides used projected changes in sea levels based on UKCP18 projections for RCP8.5 scenario 95th percentile for the year 2100. These were applied to uplift the sea levels from the medium likelihood (1 in 200 year) event.

Additional sea level rise beyond the current projected ranges cannot be ruled out as there is uncertainty regarding the Antarctic ice sheet contribution to sea level rise. (Fung, et al., 2018).

More information on the projections used in the future flood maps can be found on SEPA’s Developing Our Knowledge web page.

Please note, there are differences between the scenarios used within the future flood maps and the allowances in SEPA’s “Climate change allowances for flood risk assessment in land use planning guidance”. More information can be found on SEPA’s Land Use Planning pages.

3. Viewing the future flood maps

The future flood maps for coastal, river and surface water flooding can be viewed on
SEPA's Flood Maps pages.

Future and present day medium likelihood flood maps showing areas at risk of flooding can be compared by switching on both layers.

• Large differences between the two can indicate that flood hazard is more sensitive to climate change in these areas. Communities not at risk of flooding now may become at risk of flooding in the future.


• Where the area potentially at risk of flooding looks similar between the future and present day medium likelihood maps, communities may see an increase in the severity of flooding, such as higher depths of floodwater, or flood more often in the future.

4. What about other future changes?

The future flood maps do not reflect other potential future changes which are likely to influence future flood risk, such as:

• Changes in river channel or floodplain geomorphology
• Changes in land use
• Population changes
• Condition of defences or other infrastructure
• Construction of new Flood Protection Schemes, bridges or culverts.
• Changes in capacity of drainage networks
• Coastal erosion. Coastal erosion may also increase with rising sea levels. For more information on coastal erosion in Scotland see
  Dynamic Coast: Scotland's Coastal Change Assessment.
• Changes in tidal range
• For most of Scotland, changes in wave overtopping or wave run-up at the coast. In many locations the size of waves at the coast is limited by the depth of water, so higher sea levels may also lead to larger waves at the coast which may lead to an additional increase in flood risk due to wave overtopping. In North East Scotland, Orkney and the Outer Hebrides, changes in wave overtopping and wave runup with increased still water levels are taken into account.

5. Why aren’t future flood maps available everywhere?

There are a small number of locations for which suitable future flood map data is not available. These locations are identified using the Future Flood Data Not Available layer. The lack of future flood map data in these locations does not mean there is no future flood risk.

As part of our ongoing flood map improvement programme, we will look to address these areas and work with our flood risk management partners to improve the flood maps.

Watercourses with catchment areas between 3-10km² are represented in both the river and surface water flood maps. For areas where future river flood map data is not available, if small watercourses of this catchment size and below are present, users are recommended to view the future surface water flood map to understand the potential future flood risk from small watercourses.

6. What about Flood Protection Schemes?

SEPA’s present day river flood maps have been improved in some locations by incorporating modelling from studies for flood protection or alleviation schemes.

In some of these locations the flood study modelling has not been incorporated into the future flood maps and any residual benefit from flood protection schemes above their standard of protection is not shown.

Flood protection schemes are generally designed to withstand a flood of a specified likelihood or chance of occurring in any given year, sometimes referred to as the ‘design flood event’. The standard of protection that defences offer can change over time though due to a number of factors, including climate change.

The future flood maps do not reflect any potential changes in condition of defences or construction of new Flood Protection Schemes.

7. What about future risk from small watercourses?

For understanding the future flood risk from small watercourses it is recommended that users consult both the river and surface water future flood maps as:

• The surface water future flood map includes risk from small watercourses which have a catchment area smaller than 10km².
• SEPA’s river future flood map shows flooding from rivers with catchment areas greater than 3km². Therefore, watercourses with catchment areas between 3- 10km² will be represented in both the river and surface water future flood maps.



Different modelling approaches and input datasets were used to develop each flood map which means there may be differences in the outputs.

Further information on both the river and the surface water flood map methodologies can be found in the flood map summaries on SEPA’s Developing Our Knowledge web page.

References

Chan, S.C.; Dale, M.; Fowler, H.J.; Kendon, E.J. (2021): Extreme precipitation return level changes at 1, 3, 6, 12, 24 hours for 2050 and 2070, derived from UKCP Local Projections on a 5km grid for the FUTURE-DRAINAGE Project. NERC EDS Centre for Environmental Data Analysis. https://dx.doi.org/10.5285/18f83caf9bdf4cb4803484d8dce19eef

Fung, F., Lowe, J., Mitchell, J., Murphy, J., Bernie, D., Gohar, L., . . . Sexton, D. (2018). UKCP18 Guidance: Caveats and Limitations. Exeter: Met Office Hadley Centre.

FUTURE DRAINAGE, 2021. FUTURE-DRAINAGE: Guidance for Water and Sewerage Companies and Flood Risk Management Authorities: Recommended uplifts for applying to design storms. Prepared by Murray Dale, JBA Consulting

Kay, A., Crooks, S., Davies, H., & Reynard, N. (2011). An assessment of the vulnerability of Scotland's river catchments and coasts to the impacts of climate change. Wallingford: Centre for Ecology and Hydrology.