On the 12th of March 2025 a significant update to the Scottish Environment Protection Agency (SEPA)’s mapping was introduced in the form of the publication of a new surface water and small watercourses flood mapping dataset to accompany the existing fluvial (rivers) and coastal mapping extents.

Key Takeaways

• New SEPA mapping was released on 12 March 2025
• Coverage now includes smaller catchments (<3.0 km²)
• Map resolution has improved
• A future medium flood risk layer is included, accounting for climate change

SEPA’s current fluvial dataset utilised Digital Terrain Data with varying grid resolution, which, in rural areas in north-western Scotland, can be up to 20m (Figure 1). This generally results in pixellated fluvial extents with a lower degree of confidence for all modelled likelihood events (high, medium, low, and future risk).

The new small watercourse and pluvial dataset incorporates a resampled 2.0m spatial resolution DTM, developed using the already available Light Detection and Ranging (LiDAR) and photogrammetry data from 25 data sources (figure 2). This increased resolution results in greater confidence when analysing the surface water and small watercourses flood maps and could mean that watercourses with catchments between 3- 10km2 are likely better represented in this new dataset and not the existing fluvial (river) mapping. SEPA do make it clear that users consult both the river and surface water flood maps to understand flood risk from small watercourses.

Figure 1

Figure 2

Rainfall and Climate Change

Although this new dataset is welcomed, this combined dataset raises a few questions, not least with regards to understanding if the risk presented is from rainfall / pluvial sources or if it is out-of-bank flooding from a small watercourse.

Across Scotland, flooding from pluvial / surface water sources could be managed on site provided that flood risk is not increased elsewhere, using various soft and hard engineering techniques to manage, attenuate and divert runoff when required.

Flooding from watercourses is treated differently, however and in accordance with NPF4 Policy 22, the fluvial flood risk area must be retained, and most new developments need to be situated outside of this extent.

Now that SEPA have combined flood extents from both surface and fluvial sources as part of this new dataset, it is currently unclear whether they will need to be treated differently or if this could potentially see the rise of Blue Green infrastructure and methods to manage flooding from smaller watercourses.