Flood Risk Assessment in Scotland: The Future Fluvial Flood Risk Area
SEPA's online flood maps underestimate the true Flood Risk Area due to outdated climate change allowances and missing structure blockage modelling.
During the early stages of your project, it is vitally important that consideration is given to flood risk and understanding the Flood Risk Area.
Generally, the first port of call is to review your site against SEPA’s online fluvial flood maps. However, you must be made aware that SEPA’s fluvial flood mapping has been produced at the national scale using national datasets and a consistent methodology, and should not be used for site specific assessment.
SEPA’s “Future Flood Maps” currently provide the closest freely available representation of the Flood Risk Area from Fluvial sources. However, there are two key factors which have not been included but are now frequently required to be addressed by both SEPA and the Local Authority:
- The updated modelled climate change allowance
- Structure Blockage potential
Climate Change Allowance
The modelled climate change allowances used for SEPA’s online mapping have now been superseded by those values presented in their Climate Change Allowances for Flood Risk Planning, updated in August 2024.
For nearly all River Basins across Scotland, SEPA’s Future Flood Risk Mapping underestimates the impact of climate change in relation to current predictions. In particular for larger rivers within the Tweed catchment, current predictions are 26% greater than those modelled as part of SEPA’s dataset.
Blockage
Our recent experience has shown that SEPA now considers the Flood Risk Area as the 1 in 200 Year Event, plus climate change, plus an appropriate blockage of nearby structures. SEPA recommend that a 75% blockage is applied at the critical structures to demonstrate the Flood Risk Area. However, on occasions, a 100% blockage may be requested.
SEPA do not model blockages within their online mapping dataset (with the exception of culverts larger than 50m in length). So if your site is shown to be located close to SEPA’s Future Fluvial Flood Risk Extent, and/or close to a structure, then there could well be areas within your site that might fall within the Flood Risk Area following a detailed FRA.
For smaller catchments, SEPA’s modelled extents are displayed as “pluvial” flooding rather than “fluvial” (as SEPA do not present fluvial flood extents for catchments smaller than 3.0km2). SEPA’s pluvial extents currently include no uplift for climate change.