Flood assessment is only as good as the terrain data
A 10 cm error in ground elevation across a flat floodplain can be the difference between Flood Zone A and Flood Zone B. That’s the difference between getting planning permission or not, between affordable insurance or uninsurable.
The OPW Guidelines on Flood Risk Management require site-specific assessment for developments near flood zones. Drone LiDAR provides the terrain accuracy these assessments need, especially in the vegetated river corridors where it matters most.
Why LiDAR specifically
Vegetation is the problem
Floodplains and river corridors are among the most heavily vegetated places in Ireland – riparian woodland, rushes, reed beds, scrub. A standard photogrammetric survey maps the top of all that vegetation, not the ground. For flood modelling, it’s the ground surface that determines where water goes. LiDAR punches through vegetation to map the ground.
Accuracy that matters
Drone LiDAR hits plus or minus 2 to 3 cm vertical on bare earth. National airborne LiDAR datasets, where they exist, are typically plus or minus 15 to 25 cm. For a site-specific FRA where precise flood levels determine whether a development can proceed, that difference is significant.
Detail that national data misses
National LiDAR is captured at 2 to 4 points per square metre. Drone LiDAR gives you 50 to 300. At that resolution, you pick up drainage ditches, field drains, low embankments, and culvert inverts – features that control local flood pathways but don’t show up in coarser national data.
Flood risk assessment applications
Site-specific FRA
For planning applications needing Justification Test compliance under OPW Guidelines, the drone LiDAR DTM is the terrain foundation for hydraulic modelling. It feeds into:
- 1D/2D hydraulic models (MIKE, HEC-RAS, Tuflow) to simulate flood extents and depths
- Flood level comparison with proposed finished floor levels
- Flood extent mapping overlaid on the site layout
- Climate change scenario modelling (Mid-Range and High-End Future Scenarios per OPW guidance)
Strategic flood risk assessment
Local authorities updating SFRAs for Development Plans benefit from drone LiDAR for critical urban and peri-urban areas where national data isn’t detailed enough. Particularly relevant for towns where small watercourses and culverted sections create localised flood risk that coarse terrain data misses entirely.
Catchment analysis
For greenfield sites, understanding surface water flow paths is essential for SuDS design. A LiDAR DTM lets you run hydrological analysis – flow direction, accumulation, watershed delineation – at a resolution that picks up natural drainage features and concentration points across the site.
Drainage design
Surface water
Drone LiDAR DTMs work well as the topographic base for surface water drainage design:
- Identifying natural drainage paths and low points
- Designing attenuation storage with accurate volume calculations
- Verifying gravity drainage feasibility (fall calculations)
- SuDS feature design – swales, detention basins, filter strips – where you need accurate existing levels
Agricultural drainage
For land drainage projects, LiDAR reveals the subtle terrain variations controlling surface water movement. Open drain condition, field drain outfalls, and wet spots all show up in the DTM and imagery.
Road and infrastructure drainage
For road corridor work, LiDAR captures verge levels, ditch profiles, culvert positions, and cross-fall. The data goes straight into Civil 3D for detailed design.
What we deliver
- Classified point cloud (.LAS): Ground, vegetation, buildings, and water separated for selective surface generation
- Bare-earth DTM: As TIN surface (.DWG/.XML), raster grid (.TIFF/.ASC), or both
- Digital Surface Model: Including vegetation and structures, for overland flow assessment
- Contour drawing (.DWG): Typically 0.25 m or 0.5 m intervals for detailed drainage design
- Cross-sections: At specified chainages along watercourses or drainage routes
- Hillshade visualisation: For picking out subtle features, drainage lines, and old channel positions
Everything in ITM with Malin Head heights – the standard for flood modelling in Ireland.
We work with hydraulic modellers regularly
We supply LiDAR terrain data to modelling consultancies working on FRAs for local authorities, OPW, and private developers. We know what MIKE, Tuflow, HEC-RAS, and JFLOW need in terms of format and resolution, and we make sure the deliverables are model-ready.
To discuss a flood risk or drainage project, get in touch. We’ll advise on survey timing, expected data quality, and formats. More on our LiDAR survey service.